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# If 2 = elec waterheaters are connected in each of the 4 possible ways?...

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### If 2 = elec waterheaters are connected in each of the 4 possible ways?...

... Sorry for the incomprehensible title above but this site offers only 75 characters for it! The complete title follows:
"How the water temperature would change if 2 equal electric water heaters are connected in the 4 possible arrangements?"

The said water heaters are of the electrical type, go right inline on top of the shower and they don't have a thermostat. They're activated by just opening the valve due to the flow of water; so, if it's not opened sufficiently the heater won't react with its mechanical-pressure relay/switch. If it's opened fully, due to the water flow level, it won't heat the water as much. The conversion of electricity into heat is a constant as it's with an electric stove or clothes iron, only these contain a control (thermostat) for limiting the amount of heat concentrated within the confines of the unit.

You can put them in a series or parallel configuration in each category: electrical & mechanical. Have made 4 simple diagrams to exactly show what I mean by configuration but I don't see here how to post them along with this thread, yet.

Does somebody around knows how to deal with these two different but simultaneous events that aren't fully independent? Have searched the web for some tips/info without luck. Pointers or links to suitable sites are welcome.

### Not Enough Info

The electrical connection should make no difference. In parallel they each get half the current but the full voltage, while in series they have the same current, but half the voltage. Since power is the product of voltage and current, the two arrangements give the same power.

The water is the hard part. If, for instance, they raise the temperature of the water by 50 degrees C, and that the water enters at say 10 degrees, in series the first one would heat it to 60C. But, the second can't raise it another 59C as that is over the boiling point of the water, and it takes more energy to actually boil it.

The effectiveness is also dependant on the water temp entering the unit. The heat flow is dependant on the difference in temperature between the heating element and the water. As the difference goes down, so does the rate of increase.

So, you need to know, at least, the temperature of the water at the start and the maximum temperature of the heating element in the heater. You would also need some more information about the efficiency of the heating elements and the rate of heat flow from them to the water as a function of the rate of flow.

No that's not correct Electrical wise in Parallel they consume twice the current as the mains is applied directly to both Water heaters.Full voltage to both heaters.
Connected in series they have twice the resistance so half the current flow. as half the mains voltage across each heater. Cant see how you would control temperature in Cylinder this way as when 1 thermostat cut out both water heaters would turn off due to series connection.
You would have to have water heaters electrically connected in parallel but could have water passing from one cylinder to the other and the first cylinder preheating it but that would take time to respond.
The first cylinder then could used as preheat only and the second one set to final delivery temperature.. But being water wised connected in series also restricts water flow as more restriction.

### I might be missing somethingm, but

If the heaters are "equal"
There are only two combinations, series and parallel
In series Heater 1 is going to do it's stuff then heater 2, bearing in mind heater2 is going to work on heater 1's output. There will be some change in flow, don't know whether you can discount non-linear effects either. It effectively takes more energy to raise a volume of water from 30 - 40 degress than 20 to 30.
In parallel, each one is is going to get half the flow...

### What "4" combinations?

As Tony said, there are really only two combinations, series or parallel for the water.

If you're thinking about mucking with the electrical supply - don't! It should be connected the way the manufacturer specifies. So don't even think about trying to hook the electric supplies in series; you'll create a serious risk and it's completely avoidable. Ground Fault protection as specified is essential too. You don't want someone in the shower to get zapped. Have an electrician do the electrical connections for you, so they're done properly.

Your comment "... and they don't have a thermostat ..." also is a concern. Are these heaters approved for residential use, or is it some industrial unit? Without a thermostat is there the possibility that they will produce scalding-hot water that may be dangerous in the shower? And as dogknees mentioned, if they're in series the water may be dangerously hot all of the time. Usually, putting industrial equipment in a residential application is a no-no.

All of that being said, parallel will give you more flow rate, at a lower temperature. But if the shower is on low, the parallel configuration might not have enough flow rate in each unit for the sensor to turn the heat on.
Series will give you a higher temperature, at a lower flow rate. But "higher" might be dangerously so.

Given your question, it sounds like you should bring professionals in to review and make recommendations for you, not just post vague questions on a blog. Someone might get hurt if this is done improperly.

### Thermo ...

homework or is this a real world application?

You might try

http://www.gdn.edu/PT_Faculty/cmckeithan/1011_6thru8prob.htm

Which has a bunch of fluidic and thermo problems/solutions. Hope this helps! :)

### This is most certainly a homework problem

In home plumbing, there are at least a dozen reasons you don't put things like heaters in parallel or in series, nor do you typically have more than one. If your goal is to heat the water, then it would be logical to have a recirculating pump of some sort.

A typical 'on demand' electric hot water heater is not suitable for use for a shower, as it does not provide enough hot water fast enough. The only tankless models that can do this are gas-fired units.....

### OK maybe I'm missing something here

But when you say in Series or Parallel are you talking about the Electrical Connections, The Water Connections or both?

Frankly I can see no benefit for feeding one heater with the output of another so here I'm assuming that they are feeding different showers and you want to know how to wire them in.

This is a basic Electrical Examination question and if you can not answer it you are not qualified to be considering doing the work.

For a complete answer read and learn the contents of your Text Books that is the best answer that I can give you so you will learn these things.

Col

### orfice upstream of switch.

for size on your own. jg

### To difuse all the smog

First of all, lets say thank you all for responding! Now for the specifics...

I- To dogknees: If you consider that in parallel the current is half you're wrong. 120 Vac is what's coming from the grid & if in parallel they both receive that voltage. When in series they both experience half of the previous current since they're equal in resistance & power rating (wattage). Think of it as if they were 2 equal bulbs: in parallel they both shine with the expected luminosity; but if in series they might even fail to give any apreciable light (half of the current of any of the previous bulbs & half the voltage, so one fourth of the power for each).
In your 2nd paragraph you really gave me a valuable tip referring to the specific heat of water, something that I hadn't considered yet. The last remark is very tied to your 3rd paragraph. It's clear for me that you're right I need more specifics on the circumstances...

II- To Tony Hopkinson: The following link may serve as a clarifier:

Also, the amount of heat in a certain quantity of water will affect heat transfer ratio (a valuable tip)...

III- To oldbaritone: The following links are intended for clarification:

http://tankless-water-heaters.ecrater.com/p/1450643/marey-aquamatic-2-electric-shower-tankless
http://www.inspectapedia.com/plumbing/Electric_Shower_Heaters.htm

The last one contain a picture of the exact type I'm referring to. Sorry for the lack of info in my original post but, as I mentioned right there, this site doesn't offer the possibility for uploading attachements/diagrams. I did mention this there & also that I had the diagrams already prepared so you had the possibility to ask for them instead on insisting in judging what, for you, was already blurred...

IV- To Charles Bundy: Those links were very good & interesting. By the way, this isn't any homework even though I have had those heaters around for quite a few years now; but in a sense, it's a home 'work' but not any assignment from any school or university. It's a personal thing/idea/project to be dealt with any time in the future. That time might be right now, though!

V- To robo_dev: I'm sorry to tell you but you're wrong twice: isn't a homework (if I'm understanding 'homework' correctly, it's what you bring home in order to work it out & presenting your results to the teacher/professor). I'm not a student, I'm curious!
Next, we have had in this house for more than 30 years those kind of water heaters (electrical). I'm not sure where they come from but all my life suppossed that it was something in the category 'Universal knowledge'. I
mean that I always suppossed that everybody else new about them...

VI- To OH Smeg: Yes, you're right! You're missing a few things. Have a look at the diagrams...

Basically all conneecting either water and or electricity will do is chnage the size of the pipes / wires required to carry the load. Both in parallel you get half the water and half the current, Both in series you get (discounting losses) the full current and flow, but you'll need that because you only get half the time to apply it...
Get somebody qualified to do this for you before you become the star of the next Darwin awards, or at best end up spending a lot of money for a shower that gives you a scalding dribble or a cold blast.

My mistake. I must have been thinking about the long weekend when I wrote that.

### More fog diffusion needed...

Thanks, Tony Hopkinson, for responding!

...Going from a general sense to the more specific one.

This whole thing of starting this discussion was aimed at ending with a clear understanding of the effect of the different configurations, in a purely theoretical sense, of the output water temperature.

In relation to your second sentence/first paragraph, Tony, I guess you're referring to, say, current (I) or water flow, each of the units receives half of the system total. Next sentence, 'full current flow' (for both water & electrical current) of the whole system; but, in relation to the previous configuration it makes little sense. It would have been helpful to use a standard way of measuring electric current & water flow, for comparison purposes (i.e., Amps = I(total), I1 & I2 & I1+I2 = I(total) & GPM = Gallons Per Min.).
The end of that sentence seems to be a contradiction: ...'get half of the time to apply it' is referring to what, precisely? If the flow is for all configurations equal at the input stage then, when in series, the water will have double the time within the confines of the heating enclosures.

Seems to me that your last post is difficult to grasp, sorry.

### Nope

If split one flow into two, you get half in each, same as if you split one current into two you get half in each.

If your heaters had a capacity of one liter and you had a flow of a liter per seond.
In series each heater has each litre of water for one second to work on it, in parallel two...
Taking your measurements at the wrong point there, when you join them back together you get 2 * half a litre per second.

If you connect to a regular house outlet a hair blower & a tv unit you expect that both be served, say, 120 VAC & both will demand what its respective impedance/resistance established at manufacturing facility. If you disconnect one of them then you have no longer anything in parallel, but the unit still demands whatever it needs. The reverse is also true.
Have you seen the referred diagrams above in my post?

### The real questions

The real questions here are:

What is your lowest average water temperature available at the inlet tube of the heater?
What temperature do you consider to be proper for a shower?
How much flow do you need?

Per the article from one of your links, if you plumb the 2 heaters so that they are in series, you can get up to a 30 degree F rise in temperature at a flow rate of between 1 and 2 GPM.
Not very useful in northern climates where the winter water temperature can drop below 40 degree F.

If you have water that just needs to be warmed a little, and you want between 2 and 4 GPM of flow, you can plumb the 2 heaters in parallel.

An alternative for a moderate climate that does get cool in the winter would be to plumb both heaters in series with a contactor wired to one of the heaters. When the water is colder, run both heaters. When the water is warmer, run just 1.
Using a contactor, you could even add a thermostat so that the second heater runs automatically when needed.

It all depends on your water source, your needs, and what the local electrical and plumbing codes will allow.

Keep in mind that if your system does not meet local codes, you may need to remove or replace it if you ever sell your home.

Chas

Thank you Chas for responding!
The base water-temp is between 65-70 F-deg for the whole year. At the shower, cold & warm weather seasons, it would be ok if water temp is 110 & 105 F-deg, respectively. What our utility serves is about 1-2 GPM, if measured at the shower (not fully opened).
Your tips are indeed very clever!

### Buy a more powerful unit

If the flow or the temperature are not right.... buy a bigger heater.

If you've had the heater for 30 years, try descaling them... will improve efficiency and flow rate.

If all else fails, learn to love cooler slower showers and conserve electricity and water at the same time.

Your tip, Desertjim, is wise; thanks!

Did you read my 2nd post above? I'm looking for some theoretical means to dissect the issue...

For practical use just get a tanked unit of the right size but if this is just a physics problem electrically parallel gives hotter water while up to the rated flow rate for the heater serial plumbing gives hotter water and past this you are better off parallel as you get to double the rated flow as the units may have flow limmiters.
In real life I have never seen two instantaneous heaters in line but sometimes one is used to boost water temperature at the end of of a hot water line instead of a loop and pump back to a tanked unit.

Thanks, delf20k@, for responding with some new insight!

You're right, it's a physics problem. The mathematical tools that I need to solve the issue fail to appear anywhere around...but I'll keep trying a little bit longer.

### Tools - Neat JAVA applet for circuit simulation

This is pretty cool and you can substitue water flow for current and voltage for pressure if you want to treat it as a physical versus electrical sim.

Thank you for the link def20k@, I'm pretty sure it's a very good site. Unfortunately, I'm using Firefox 3.6.16 as my browser & since version 3.6.14 it no longer supports java, which is required for using the site. I'm going to try Chrome to see if it fits the site's requirements.

### My assumption is that we are talking about two identical instant haters

Im not sure I entirely understand the question either but Ill take a shot at this.
My assumption is that we are talking about two identical instant haters or the type that dont store water in a tank because you mention a pressure sensor.
The performance factor for these types of heaters is the flow rate or volume of water they can heat to the desired temperature. This being said they have an endless supply up to their maximum flow rate. I believe the standard shower head is about 2.2 gallons per minute in the United States.
So if you exceed the maximum flow rate the water temperature will drop even with an instant heater.
If you look at some of the manufactures you will see that there are several models designed for different voltages to accommodate both commercial and residential power systems. Such as 120 / 240 or single phase residential then 208 and 277 for the more common poly phase commercial systems. You will notice the higher voltage units have a higher temperature increase rating. These things have coils which can basically be considered low resistive devices. Remember another way to say power is watts which can be converted to heat or BTUs.
These devices were designed to work at specific voltages and this is why they have these operating voltage options. The coils within the instant heaters are just big resistors from an electrical standpoint but the pressure sensors and relay or contactor that applies voltage to the coils may not even work at half the voltage. And if you actually hooked two of these in a series circuit you would probably damage both of them.
As far as hooking the hot water output up in series this does seem to defeat the purpose as well. Though I couldnt find the data online for you due to lack of time I can tell you these units have a high temperature limiter. I have installed a few and had to make this adjustment for the client. In other words it will only get so hot. And there is an assumption of certain cold water in temperature range. Remember they will raise the temperature NOT attain a certain temperature. So connecting in series might make sense if the incoming water was too cold to rise to the desired temperature when passing through a single unit. In this case you might consider feeding one into another to get the desired output temp.
Assuming we have a normal supply temperature and we have two units parallel on the water supply seems like the best solution. Ideally you could pressure regulate water coming in so that the flow through both units is equal.
If this were the case and both units could heat the water to the desired temp at 3 GPM you would then have 6 GPM available. So now both showers and maybe the dishwasher can all run together.
Two Instant-Flow Micro heaters are connected

Thanks mpgraham for your response! In the final part of the 3rd paragraph you make reference to the 'relay' or 'contactor in a way that makes me think you believe they're electrically controlled which isn't the case: the ones Ive seen are controlled via pressure difference whenever there's water flow. Imagine a paint gun... the same principle.
Near the end of your post you mention that 'pressure regulate water coming in so that the flow through both units is equal'; hum, I was thinking of controlling both, a main supply regulator as well as, at least, the feed for one of the units to lower/rise the flow through that one only convinced that it would give me some regulating capacity.
Precisely, the link you provide is a perfect example of what I call Electrically-Parallel & Mechanically-Parallel configuration.

### Sorry double post

removed double post

### Back to basics

I thought I would have a go at representing the problem with an asci diagram, to give the four possible combinations Benny has described. Apologies if it doesn't display well. I'm not sure I can help too much with the physics of the problem, but I have put my best guess in as well, Which is that water supply is constrained by flow rate (current) whereas electricity supply is constrained by voltage(pressure). This means that when water is provided with a parallel line over a short distance, flow rate halves through each branch. In the analogous electrical system, flow rate through each branch remains the same, therefore total current doubles.

LEGEND
/\/\/\/ Water Heater
------- Water Supply
##### Electricity Supply

Water in Series: electricity in series
Baseline

..-----/\/\/\/---/\/\/\/-----
.......#....#..#....#
.......#....#..#....#
.......#.............#
#####.............#
#############

Water in parallel: electricity in parallel
water flow through heaters halved, Electricity flow through heaters doubled

.........--/\/\/\/--
.........|.#.....#.|
..-------|.#.....#.|--------
.........|.#.....#.|
......... --/\/\/\/--
..........#......#..
######.......#..
###########

Water in Parallel: electricity in series
Water flow through heaters is halved
heating effect doubled

.......
. .
..........--/\/\/\/--
.......#.|.......#.|
...-------|.......#.|--------
.......#.|.......#.|
..........--/\/\/\/--
#####.........#
###########

Water in series: electricity in parallel
Electricity flow through heaters doubled
heating effect doubled

.. ..############
....#.........#........#
....#.........#........#
..-------/\/\/\/---/\/\/\/-----
....#...#........#.......
###....#........#........
############

the Water in Parallel: electricity in series diagram didn't turn out right. And the system won't let me edit it. Try this:

........######
........#.......#
........#...----/\/\/\/----
........#...|.........#...|
-------------|.........#...|--------
........#...|.........#...|
........#...----/\/\/\/----
#####.......#
##########

A very interesting way of putting it that never occurred to me, pevans56!
It would make your explanation more understandable if, when referencing by way of the words DOUBLE & HALF, it could be clear with reference to what.
PS. I'm going to experiment with your symbolism.

### OK I've looked at the images

But I only see a Water Connection no electrical connections.

Where I'm having an issue is trying to understand why you would want to heat already heated water to begin with.

Ideally if the ability to heat the water passing through the first heater is insufficient to heat the water enough to be usable adding a second heater may increase the heat but it will certainly use more electricity than just moving to a system with the ability to increase the heat of the water.

Where I have a problem is with the unregulated nature of the way that the supposed heaters work. They can only increase the heat by a upper factor and as they have no thermostat will continue to heat the water if there is enough flow through the unit. So if the first heater was to increase the heat by 30 F from 65-70 F that would increase the temp to 95-105 F and then provided that the Temperature Increase is constant in the second heater another 30 F rise would take the water to 125-130 F which is above the comfortable/safe limit.

This would require the water connections in Series and the electrical connections in Parallel as having the electrical connections in Series would limit the ability of the heating element to function correctly and maybe even cause it's early failure.

Heat rise is a factor of the element which consumes the electricity so if you have a water flow greater than the elements ability to rise the temperature sufficiently you need a bigger element not another heater to which only has a form of heat regulation by the water flow.

Adding more heaters could very easily drive the water above boiling point which defeats the purpose of Shower/Bath Water Heaters.

As stated above the standard accepted flow rate is around 9 Liters Per Minute which is close to 2 Gallons per minute and with a base temp of 65-70 F how hot do you want to make the water?

What am I missing here?

As I understand it Benny asked a theoretical question about the impact of connecting two water heaters in the four possible ways. Not necessarily which was best or safest or most efficient, just a "thought experiment" on the possible combinations.

Try this matrix of my take on the answer.

WATER _____Series______Parallel
ELECTRICITY
Series_________1___________2x__
Parallel________2x__________4x__

|| <--- wire #2 Notes: At the top left it's to be
|| ### understood that either a single
|| #####---> water plug or 2 separate plugs are
| ----###########---> out #2 going to a 120 Vac receptacle.
------#########
|| ### In this case the water flows in
|| ### <--- Heater #2 series but the current & voltage
|| ### are in parallel.
|| ^
|| | Sorry, OH Smeg, that you didn't
|| |---------- see the electrical connections at
|| <--- wire #2 | the supplied link. I revisited the
|| | site to check it out & had no
|| ### problem seeing the wires, that's
|| #####---> water why I'm including this diag. here.
| ----###########---> out #1
------#########
^ ###
| ### < Heater #1
wire ###
#1 ^
|
|
^
Cold >----|
Water in

### Water heater issue

There are not 4 possible arrangements, only two. For electric heaters - assuming this is in the US - you'll need a separate power feed of 240V for each unit. Breaker and wire size is determined by the power requirements. On a typical whole house 50 gallon electric heater, it will typically require 10 gauge wire fused at 30A. A 50 will have at least two 4500W elements in it. There is no series/parallel here - each unit gets a separate circuit. I can't quote exact, but it'll be a code requirement that they get their own runs. If the breaker panel doesn't have the space or allow some breaker stacking to make more room for a new circuit, then game over.

As for the heat - you'll be limited on flow by the pipe size. Stacking one in front of the other won't change much of anything and hooking them up in parallel will half the flow though each unit and -if I read the Q right - only make your flow issue worse or nto change it at all.

If the bottom line is that the shower goes cold rather quick, then welcome to the fact that tankless heaters are not really designed to work for a shower. That, along with the fact that you can buy 3 50 gallon water heaters for what you can buy one tankless says you need to think about another solution. Tankless heaters are great for spot heating at remote sinks. For shower or other large volume requirements, you need a regular house size water heater. You can use a smaller heater if you have natural gas or propane. If you are all electric then a 50 is about the minimum due to capacity and recovery capabilities.

Thanks ckelly for responding! Check the supplied diagrams above, that alone should dissipate much of the fog.

### well

I had this problem in real life.
I used a holding tank between the water heaters, basically a 4 feet long 3 inches pipe with both capped ends and fittings.
Also, I modified the flow restrictors on both heaters and modified the heating elements so they had more surface area.
The electric bill was killing me so I then used one but with a bigger holding tank and dual, smaller heating elements.
Both heaters installed in paralell with the "pipe tank" in between.

Appreciate your response, CListo, but that's not my case. Sorry!

### Sorry OH Smeg for the posted diagram being dismantled

I prepared it in a small word processor but at posting time this site reformatted it in such a way that it has become useless.

### To all that care to see a better diagram...

I didn't know that Google Docs had a better (simpler) drawing api there until yesterday, so I took the time to re-draw everything again, but this time in just one diagram you can see all four combinations. Hope this help a little. The link:

Thanks indeed to all that have responded!

### I might be missing somethingm, but

If the heaters are "equal"
There are only two combinations, series and parallel
In series Heater 1 is going to do it's stuff then heater 2, bearing in mind heater2 is going to work on heater 1's output. There will be some change in flow, don't know whether you can discount non-linear effects either. It effectively takes more energy to raise a volume of water from 30 - 40 degress than 20 to 30.
In parallel, each one is is going to get half the flow...

### What "4" combinations?

As Tony said, there are really only two combinations, series or parallel for the water.

If you're thinking about mucking with the electrical supply - don't! It should be connected the way the manufacturer specifies. So don't even think about trying to hook the electric supplies in series; you'll create a serious risk and it's completely avoidable. Ground Fault protection as specified is essential too. You don't want someone in the shower to get zapped. Have an electrician do the electrical connections for you, so they're done properly.

Your comment "... and they don't have a thermostat ..." also is a concern. Are these heaters approved for residential use, or is it some industrial unit? Without a thermostat is there the possibility that they will produce scalding-hot water that may be dangerous in the shower? And as dogknees mentioned, if they're in series the water may be dangerously hot all of the time. Usually, putting industrial equipment in a residential application is a no-no.

All of that being said, parallel will give you more flow rate, at a lower temperature. But if the shower is on low, the parallel configuration might not have enough flow rate in each unit for the sensor to turn the heat on.
Series will give you a higher temperature, at a lower flow rate. But "higher" might be dangerously so.

Given your question, it sounds like you should bring professionals in to review and make recommendations for you, not just post vague questions on a blog. Someone might get hurt if this is done improperly.

### Thermo ...

homework or is this a real world application?

You might try

http://www.gdn.edu/PT_Faculty/cmckeithan/1011_6thru8prob.htm

Which has a bunch of fluidic and thermo problems/solutions. Hope this helps! :)

### This is most certainly a homework problem

In home plumbing, there are at least a dozen reasons you don't put things like heaters in parallel or in series, nor do you typically have more than one. If your goal is to heat the water, then it would be logical to have a recirculating pump of some sort.

A typical 'on demand' electric hot water heater is not suitable for use for a shower, as it does not provide enough hot water fast enough. The only tankless models that can do this are gas-fired units.....

### OK maybe I'm missing something here

But when you say in Series or Parallel are you talking about the Electrical Connections, The Water Connections or both?

Frankly I can see no benefit for feeding one heater with the output of another so here I'm assuming that they are feeding different showers and you want to know how to wire them in.

This is a basic Electrical Examination question and if you can not answer it you are not qualified to be considering doing the work.

For a complete answer read and learn the contents of your Text Books that is the best answer that I can give you so you will learn these things.

Col

### orfice upstream of switch.

for size on your own. jg

### To difuse all the smog

First of all, lets say thank you all for responding! Now for the specifics...

I- To dogknees: If you consider that in parallel the current is half you're wrong. 120 Vac is what's coming from the grid & if in parallel they both receive that voltage. When in series they both experience half of the previous current since they're equal in resistance & power rating (wattage). Think of it as if they were 2 equal bulbs: in parallel they both shine with the expected luminosity; but if in series they might even fail to give any apreciable light (half of the current of any of the previous bulbs & half the voltage, so one fourth of the power for each).
In your 2nd paragraph you really gave me a valuable tip referring to the specific heat of water, something that I hadn't considered yet. The last remark is very tied to your 3rd paragraph. It's clear for me that you're right I need more specifics on the circumstances...

II- To Tony Hopkinson: The following link may serve as a clarifier:

Also, the amount of heat in a certain quantity of water will affect heat transfer ratio (a valuable tip)...

III- To oldbaritone: The following links are intended for clarification:

http://tankless-water-heaters.ecrater.com/p/1450643/marey-aquamatic-2-electric-shower-tankless
http://www.inspectapedia.com/plumbing/Electric_Shower_Heaters.htm

The last one contain a picture of the exact type I'm referring to. Sorry for the lack of info in my original post but, as I mentioned right there, this site doesn't offer the possibility for uploading attachements/diagrams. I did mention this there & also that I had the diagrams already prepared so you had the possibility to ask for them instead on insisting in judging what, for you, was already blurred...

IV- To Charles Bundy: Those links were very good & interesting. By the way, this isn't any homework even though I have had those heaters around for quite a few years now; but in a sense, it's a home 'work' but not any assignment from any school or university. It's a personal thing/idea/project to be dealt with any time in the future. That time might be right now, though!

V- To robo_dev: I'm sorry to tell you but you're wrong twice: isn't a homework (if I'm understanding 'homework' correctly, it's what you bring home in order to work it out & presenting your results to the teacher/professor). I'm not a student, I'm curious!
Next, we have had in this house for more than 30 years those kind of water heaters (electrical). I'm not sure where they come from but all my life suppossed that it was something in the category 'Universal knowledge'. I
mean that I always suppossed that everybody else new about them...

VI- To OH Smeg: Yes, you're right! You're missing a few things. Have a look at the diagrams...

### More fog diffusion needed...

Thanks, Tony Hopkinson, for responding!

...Going from a general sense to the more specific one.

This whole thing of starting this discussion was aimed at ending with a clear understanding of the effect of the different configurations, in a purely theoretical sense, of the output water temperature.

In relation to your second sentence/first paragraph, Tony, I guess you're referring to, say, current (I) or water flow, each of the units receives half of the system total. Next sentence, 'full current flow' (for both water & electrical current) of the whole system; but, in relation to the previous configuration it makes little sense. It would have been helpful to use a standard way of measuring electric current & water flow, for comparison purposes (i.e., Amps = I(total), I1 & I2 & I1+I2 = I(total) & GPM = Gallons Per Min.).
The end of that sentence seems to be a contradiction: ...'get half of the time to apply it' is referring to what, precisely? If the flow is for all configurations equal at the input stage then, when in series, the water will have double the time within the confines of the heating enclosures.

Seems to me that your last post is difficult to grasp, sorry.

### Nope

If split one flow into two, you get half in each, same as if you split one current into two you get half in each.

If your heaters had a capacity of one liter and you had a flow of a liter per seond.
In series each heater has each litre of water for one second to work on it, in parallel two...
Taking your measurements at the wrong point there, when you join them back together you get 2 * half a litre per second.

### Not Enough Info

The electrical connection should make no difference. In parallel they each get half the current but the full voltage, while in series they have the same current, but half the voltage. Since power is the product of voltage and current, the two arrangements give the same power.

The water is the hard part. If, for instance, they raise the temperature of the water by 50 degrees C, and that the water enters at say 10 degrees, in series the first one would heat it to 60C. But, the second can't raise it another 59C as that is over the boiling point of the water, and it takes more energy to actually boil it.

The effectiveness is also dependant on the water temp entering the unit. The heat flow is dependant on the difference in temperature between the heating element and the water. As the difference goes down, so does the rate of increase.

So, you need to know, at least, the temperature of the water at the start and the maximum temperature of the heating element in the heater. You would also need some more information about the efficiency of the heating elements and the rate of heat flow from them to the water as a function of the rate of flow.

No that's not correct Electrical wise in Parallel they consume twice the current as the mains is applied directly to both Water heaters.Full voltage to both heaters.
Connected in series they have twice the resistance so half the current flow. as half the mains voltage across each heater. Cant see how you would control temperature in Cylinder this way as when 1 thermostat cut out both water heaters would turn off due to series connection.
You would have to have water heaters electrically connected in parallel but could have water passing from one cylinder to the other and the first cylinder preheating it but that would take time to respond.
The first cylinder then could used as preheat only and the second one set to final delivery temperature.. But being water wised connected in series also restricts water flow as more restriction.

### I might be missing somethingm, but

If the heaters are "equal"
There are only two combinations, series and parallel
In series Heater 1 is going to do it's stuff then heater 2, bearing in mind heater2 is going to work on heater 1's output. There will be some change in flow, don't know whether you can discount non-linear effects either. It effectively takes more energy to raise a volume of water from 30 - 40 degress than 20 to 30.
In parallel, each one is is going to get half the flow...

### What "4" combinations?

As Tony said, there are really only two combinations, series or parallel for the water.

If you're thinking about mucking with the electrical supply - don't! It should be connected the way the manufacturer specifies. So don't even think about trying to hook the electric supplies in series; you'll create a serious risk and it's completely avoidable. Ground Fault protection as specified is essential too. You don't want someone in the shower to get zapped. Have an electrician do the electrical connections for you, so they're done properly.

Your comment "... and they don't have a thermostat ..." also is a concern. Are these heaters approved for residential use, or is it some industrial unit? Without a thermostat is there the possibility that they will produce scalding-hot water that may be dangerous in the shower? And as dogknees mentioned, if they're in series the water may be dangerously hot all of the time. Usually, putting industrial equipment in a residential application is a no-no.

All of that being said, parallel will give you more flow rate, at a lower temperature. But if the shower is on low, the parallel configuration might not have enough flow rate in each unit for the sensor to turn the heat on.
Series will give you a higher temperature, at a lower flow rate. But "higher" might be dangerously so.

Given your question, it sounds like you should bring professionals in to review and make recommendations for you, not just post vague questions on a blog. Someone might get hurt if this is done improperly.

### Thermo ...

homework or is this a real world application?

You might try

http://www.gdn.edu/PT_Faculty/cmckeithan/1011_6thru8prob.htm

Which has a bunch of fluidic and thermo problems/solutions. Hope this helps! :)

### This is most certainly a homework problem

In home plumbing, there are at least a dozen reasons you don't put things like heaters in parallel or in series, nor do you typically have more than one. If your goal is to heat the water, then it would be logical to have a recirculating pump of some sort.

A typical 'on demand' electric hot water heater is not suitable for use for a shower, as it does not provide enough hot water fast enough. The only tankless models that can do this are gas-fired units.....

### OK maybe I'm missing something here

But when you say in Series or Parallel are you talking about the Electrical Connections, The Water Connections or both?

Frankly I can see no benefit for feeding one heater with the output of another so here I'm assuming that they are feeding different showers and you want to know how to wire them in.

This is a basic Electrical Examination question and if you can not answer it you are not qualified to be considering doing the work.

For a complete answer read and learn the contents of your Text Books that is the best answer that I can give you so you will learn these things.

Col

### orfice upstream of switch.

for size on your own. jg

### To difuse all the smog

First of all, lets say thank you all for responding! Now for the specifics...

I- To dogknees: If you consider that in parallel the current is half you're wrong. 120 Vac is what's coming from the grid & if in parallel they both receive that voltage. When in series they both experience half of the previous current since they're equal in resistance & power rating (wattage). Think of it as if they were 2 equal bulbs: in parallel they both shine with the expected luminosity; but if in series they might even fail to give any apreciable light (half of the current of any of the previous bulbs & half the voltage, so one fourth of the power for each).
In your 2nd paragraph you really gave me a valuable tip referring to the specific heat of water, something that I hadn't considered yet. The last remark is very tied to your 3rd paragraph. It's clear for me that you're right I need more specifics on the circumstances...

II- To Tony Hopkinson: The following link may serve as a clarifier:

Also, the amount of heat in a certain quantity of water will affect heat transfer ratio (a valuable tip)...

III- To oldbaritone: The following links are intended for clarification:

http://tankless-water-heaters.ecrater.com/p/1450643/marey-aquamatic-2-electric-shower-tankless
http://www.inspectapedia.com/plumbing/Electric_Shower_Heaters.htm

The last one contain a picture of the exact type I'm referring to. Sorry for the lack of info in my original post but, as I mentioned right there, this site doesn't offer the possibility for uploading attachements/diagrams. I did mention this there & also that I had the diagrams already prepared so you had the possibility to ask for them instead on insisting in judging what, for you, was already blurred...

IV- To Charles Bundy: Those links were very good & interesting. By the way, this isn't any homework even though I have had those heaters around for quite a few years now; but in a sense, it's a home 'work' but not any assignment from any school or university. It's a personal thing/idea/project to be dealt with any time in the future. That time might be right now, though!

V- To robo_dev: I'm sorry to tell you but you're wrong twice: isn't a homework (if I'm understanding 'homework' correctly, it's what you bring home in order to work it out & presenting your results to the teacher/professor). I'm not a student, I'm curious!
Next, we have had in this house for more than 30 years those kind of water heaters (electrical). I'm not sure where they come from but all my life suppossed that it was something in the category 'Universal knowledge'. I
mean that I always suppossed that everybody else new about them...

VI- To OH Smeg: Yes, you're right! You're missing a few things. Have a look at the diagrams...

Basically all conneecting either water and or electricity will do is chnage the size of the pipes / wires required to carry the load. Both in parallel you get half the water and half the current, Both in series you get (discounting losses) the full current and flow, but you'll need that because you only get half the time to apply it...
Get somebody qualified to do this for you before you become the star of the next Darwin awards, or at best end up spending a lot of money for a shower that gives you a scalding dribble or a cold blast.

My mistake. I must have been thinking about the long weekend when I wrote that.

### More fog diffusion needed...

Thanks, Tony Hopkinson, for responding!

...Going from a general sense to the more specific one.

This whole thing of starting this discussion was aimed at ending with a clear understanding of the effect of the different configurations, in a purely theoretical sense, of the output water temperature.

In relation to your second sentence/first paragraph, Tony, I guess you're referring to, say, current (I) or water flow, each of the units receives half of the system total. Next sentence, 'full current flow' (for both water & electrical current) of the whole system; but, in relation to the previous configuration it makes little sense. It would have been helpful to use a standard way of measuring electric current & water flow, for comparison purposes (i.e., Amps = I(total), I1 & I2 & I1+I2 = I(total) & GPM = Gallons Per Min.).
The end of that sentence seems to be a contradiction: ...'get half of the time to apply it' is referring to what, precisely? If the flow is for all configurations equal at the input stage then, when in series, the water will have double the time within the confines of the heating enclosures.

Seems to me that your last post is difficult to grasp, sorry.

### Nope

If split one flow into two, you get half in each, same as if you split one current into two you get half in each.

If your heaters had a capacity of one liter and you had a flow of a liter per seond.
In series each heater has each litre of water for one second to work on it, in parallel two...
Taking your measurements at the wrong point there, when you join them back together you get 2 * half a litre per second.

If you connect to a regular house outlet a hair blower & a tv unit you expect that both be served, say, 120 VAC & both will demand what its respective impedance/resistance established at manufacturing facility. If you disconnect one of them then you have no longer anything in parallel, but the unit still demands whatever it needs. The reverse is also true.
Have you seen the referred diagrams above in my post?

### The real questions

The real questions here are:

What is your lowest average water temperature available at the inlet tube of the heater?
What temperature do you consider to be proper for a shower?
How much flow do you need?

Per the article from one of your links, if you plumb the 2 heaters so that they are in series, you can get up to a 30 degree F rise in temperature at a flow rate of between 1 and 2 GPM.
Not very useful in northern climates where the winter water temperature can drop below 40 degree F.

If you have water that just needs to be warmed a little, and you want between 2 and 4 GPM of flow, you can plumb the 2 heaters in parallel.

An alternative for a moderate climate that does get cool in the winter would be to plumb both heaters in series with a contactor wired to one of the heaters. When the water is colder, run both heaters. When the water is warmer, run just 1.
Using a contactor, you could even add a thermostat so that the second heater runs automatically when needed.

It all depends on your water source, your needs, and what the local electrical and plumbing codes will allow.

Keep in mind that if your system does not meet local codes, you may need to remove or replace it if you ever sell your home.

Chas

Thank you Chas for responding!
The base water-temp is between 65-70 F-deg for the whole year. At the shower, cold & warm weather seasons, it would be ok if water temp is 110 & 105 F-deg, respectively. What our utility serves is about 1-2 GPM, if measured at the shower (not fully opened).
Your tips are indeed very clever!

### Buy a more powerful unit

If the flow or the temperature are not right.... buy a bigger heater.

If you've had the heater for 30 years, try descaling them... will improve efficiency and flow rate.

If all else fails, learn to love cooler slower showers and conserve electricity and water at the same time.

Your tip, Desertjim, is wise; thanks!

Did you read my 2nd post above? I'm looking for some theoretical means to dissect the issue...

For practical use just get a tanked unit of the right size but if this is just a physics problem electrically parallel gives hotter water while up to the rated flow rate for the heater serial plumbing gives hotter water and past this you are better off parallel as you get to double the rated flow as the units may have flow limmiters.
In real life I have never seen two instantaneous heaters in line but sometimes one is used to boost water temperature at the end of of a hot water line instead of a loop and pump back to a tanked unit.

Thanks, delf20k@, for responding with some new insight!

You're right, it's a physics problem. The mathematical tools that I need to solve the issue fail to appear anywhere around...but I'll keep trying a little bit longer.

### Tools - Neat JAVA applet for circuit simulation

This is pretty cool and you can substitue water flow for current and voltage for pressure if you want to treat it as a physical versus electrical sim.

Thank you for the link def20k@, I'm pretty sure it's a very good site. Unfortunately, I'm using Firefox 3.6.16 as my browser & since version 3.6.14 it no longer supports java, which is required for using the site. I'm going to try Chrome to see if it fits the site's requirements.

### My assumption is that we are talking about two identical instant haters

Im not sure I entirely understand the question either but Ill take a shot at this.
My assumption is that we are talking about two identical instant haters or the type that dont store water in a tank because you mention a pressure sensor.
The performance factor for these types of heaters is the flow rate or volume of water they can heat to the desired temperature. This being said they have an endless supply up to their maximum flow rate. I believe the standard shower head is about 2.2 gallons per minute in the United States.
So if you exceed the maximum flow rate the water temperature will drop even with an instant heater.
If you look at some of the manufactures you will see that there are several models designed for different voltages to accommodate both commercial and residential power systems. Such as 120 / 240 or single phase residential then 208 and 277 for the more common poly phase commercial systems. You will notice the higher voltage units have a higher temperature increase rating. These things have coils which can basically be considered low resistive devices. Remember another way to say power is watts which can be converted to heat or BTUs.
These devices were designed to work at specific voltages and this is why they have these operating voltage options. The coils within the instant heaters are just big resistors from an electrical standpoint but the pressure sensors and relay or contactor that applies voltage to the coils may not even work at half the voltage. And if you actually hooked two of these in a series circuit you would probably damage both of them.
As far as hooking the hot water output up in series this does seem to defeat the purpose as well. Though I couldnt find the data online for you due to lack of time I can tell you these units have a high temperature limiter. I have installed a few and had to make this adjustment for the client. In other words it will only get so hot. And there is an assumption of certain cold water in temperature range. Remember they will raise the temperature NOT attain a certain temperature. So connecting in series might make sense if the incoming water was too cold to rise to the desired temperature when passing through a single unit. In this case you might consider feeding one into another to get the desired output temp.
Assuming we have a normal supply temperature and we have two units parallel on the water supply seems like the best solution. Ideally you could pressure regulate water coming in so that the flow through both units is equal.
If this were the case and both units could heat the water to the desired temp at 3 GPM you would then have 6 GPM available. So now both showers and maybe the dishwasher can all run together.
Two Instant-Flow Micro heaters are connected

Thanks mpgraham for your response! In the final part of the 3rd paragraph you make reference to the 'relay' or 'contactor in a way that makes me think you believe they're electrically controlled which isn't the case: the ones Ive seen are controlled via pressure difference whenever there's water flow. Imagine a paint gun... the same principle.
Near the end of your post you mention that 'pressure regulate water coming in so that the flow through both units is equal'; hum, I was thinking of controlling both, a main supply regulator as well as, at least, the feed for one of the units to lower/rise the flow through that one only convinced that it would give me some regulating capacity.
Precisely, the link you provide is a perfect example of what I call Electrically-Parallel & Mechanically-Parallel configuration.

### Sorry double post

removed double post

### Back to basics

I thought I would have a go at representing the problem with an asci diagram, to give the four possible combinations Benny has described. Apologies if it doesn't display well. I'm not sure I can help too much with the physics of the problem, but I have put my best guess in as well, Which is that water supply is constrained by flow rate (current) whereas electricity supply is constrained by voltage(pressure). This means that when water is provided with a parallel line over a short distance, flow rate halves through each branch. In the analogous electrical system, flow rate through each branch remains the same, therefore total current doubles.

LEGEND
/\/\/\/ Water Heater
------- Water Supply
##### Electricity Supply

Water in Series: electricity in series
Baseline

..-----/\/\/\/---/\/\/\/-----
.......#....#..#....#
.......#....#..#....#
.......#.............#
#####.............#
#############

Water in parallel: electricity in parallel
water flow through heaters halved, Electricity flow through heaters doubled

.........--/\/\/\/--
.........|.#.....#.|
..-------|.#.....#.|--------
.........|.#.....#.|
......... --/\/\/\/--
..........#......#..
######.......#..
###########

Water in Parallel: electricity in series
Water flow through heaters is halved
heating effect doubled

.......
. .
..........--/\/\/\/--
.......#.|.......#.|
...-------|.......#.|--------
.......#.|.......#.|
..........--/\/\/\/--
#####.........#
###########

Water in series: electricity in parallel
Electricity flow through heaters doubled
heating effect doubled

.. ..############
....#.........#........#
....#.........#........#
..-------/\/\/\/---/\/\/\/-----
....#...#........#.......
###....#........#........
############

the Water in Parallel: electricity in series diagram didn't turn out right. And the system won't let me edit it. Try this:

........######
........#.......#
........#...----/\/\/\/----
........#...|.........#...|
-------------|.........#...|--------
........#...|.........#...|
........#...----/\/\/\/----
#####.......#
##########

A very interesting way of putting it that never occurred to me, pevans56!
It would make your explanation more understandable if, when referencing by way of the words DOUBLE & HALF, it could be clear with reference to what.
PS. I'm going to experiment with your symbolism.

### OK I've looked at the images

But I only see a Water Connection no electrical connections.

Where I'm having an issue is trying to understand why you would want to heat already heated water to begin with.

Ideally if the ability to heat the water passing through the first heater is insufficient to heat the water enough to be usable adding a second heater may increase the heat but it will certainly use more electricity than just moving to a system with the ability to increase the heat of the water.

Where I have a problem is with the unregulated nature of the way that the supposed heaters work. They can only increase the heat by a upper factor and as they have no thermostat will continue to heat the water if there is enough flow through the unit. So if the first heater was to increase the heat by 30 F from 65-70 F that would increase the temp to 95-105 F and then provided that the Temperature Increase is constant in the second heater another 30 F rise would take the water to 125-130 F which is above the comfortable/safe limit.

This would require the water connections in Series and the electrical connections in Parallel as having the electrical connections in Series would limit the ability of the heating element to function correctly and maybe even cause it's early failure.

Heat rise is a factor of the element which consumes the electricity so if you have a water flow greater than the elements ability to rise the temperature sufficiently you need a bigger element not another heater to which only has a form of heat regulation by the water flow.

Adding more heaters could very easily drive the water above boiling point which defeats the purpose of Shower/Bath Water Heaters.

As stated above the standard accepted flow rate is around 9 Liters Per Minute which is close to 2 Gallons per minute and with a base temp of 65-70 F how hot do you want to make the water?

What am I missing here?

As I understand it Benny asked a theoretical question about the impact of connecting two water heaters in the four possible ways. Not necessarily which was best or safest or most efficient, just a "thought experiment" on the possible combinations.

Try this matrix of my take on the answer.

WATER _____Series______Parallel
ELECTRICITY
Series_________1___________2x__
Parallel________2x__________4x__

|| <--- wire #2 Notes: At the top left it's to be
|| ### understood that either a single
|| #####---> water plug or 2 separate plugs are
| ----###########---> out #2 going to a 120 Vac receptacle.
------#########
|| ### In this case the water flows in
|| ### <--- Heater #2 series but the current & voltage
|| ### are in parallel.
|| ^
|| | Sorry, OH Smeg, that you didn't
|| |---------- see the electrical connections at
|| <--- wire #2 | the supplied link. I revisited the
|| | site to check it out & had no
|| ### problem seeing the wires, that's
|| #####---> water why I'm including this diag. here.
| ----###########---> out #1
------#########
^ ###
| ### < Heater #1
wire ###
#1 ^
|
|
^
Cold >----|
Water in

### Water heater issue

There are not 4 possible arrangements, only two. For electric heaters - assuming this is in the US - you'll need a separate power feed of 240V for each unit. Breaker and wire size is determined by the power requirements. On a typical whole house 50 gallon electric heater, it will typically require 10 gauge wire fused at 30A. A 50 will have at least two 4500W elements in it. There is no series/parallel here - each unit gets a separate circuit. I can't quote exact, but it'll be a code requirement that they get their own runs. If the breaker panel doesn't have the space or allow some breaker stacking to make more room for a new circuit, then game over.

As for the heat - you'll be limited on flow by the pipe size. Stacking one in front of the other won't change much of anything and hooking them up in parallel will half the flow though each unit and -if I read the Q right - only make your flow issue worse or nto change it at all.

If the bottom line is that the shower goes cold rather quick, then welcome to the fact that tankless heaters are not really designed to work for a shower. That, along with the fact that you can buy 3 50 gallon water heaters for what you can buy one tankless says you need to think about another solution. Tankless heaters are great for spot heating at remote sinks. For shower or other large volume requirements, you need a regular house size water heater. You can use a smaller heater if you have natural gas or propane. If you are all electric then a 50 is about the minimum due to capacity and recovery capabilities.

Thanks ckelly for responding! Check the supplied diagrams above, that alone should dissipate much of the fog.

### well

I had this problem in real life.
I used a holding tank between the water heaters, basically a 4 feet long 3 inches pipe with both capped ends and fittings.
Also, I modified the flow restrictors on both heaters and modified the heating elements so they had more surface area.
The electric bill was killing me so I then used one but with a bigger holding tank and dual, smaller heating elements.
Both heaters installed in paralell with the "pipe tank" in between.

Appreciate your response, CListo, but that's not my case. Sorry!

### Sorry OH Smeg for the posted diagram being dismantled

I prepared it in a small word processor but at posting time this site reformatted it in such a way that it has become useless.

### To all that care to see a better diagram...

I didn't know that Google Docs had a better (simpler) drawing api there until yesterday, so I took the time to re-draw everything again, but this time in just one diagram you can see all four combinations. Hope this help a little. The link:

Thanks indeed to all that have responded!

### I might be missing somethingm, but

If the heaters are "equal"
There are only two combinations, series and parallel
In series Heater 1 is going to do it's stuff then heater 2, bearing in mind heater2 is going to work on heater 1's output. There will be some change in flow, don't know whether you can discount non-linear effects either. It effectively takes more energy to raise a volume of water from 30 - 40 degress than 20 to 30.
In parallel, each one is is going to get half the flow...

### What "4" combinations?

As Tony said, there are really only two combinations, series or parallel for the water.

If you're thinking about mucking with the electrical supply - don't! It should be connected the way the manufacturer specifies. So don't even think about trying to hook the electric supplies in series; you'll create a serious risk and it's completely avoidable. Ground Fault protection as specified is essential too. You don't want someone in the shower to get zapped. Have an electrician do the electrical connections for you, so they're done properly.

Your comment "... and they don't have a thermostat ..." also is a concern. Are these heaters approved for residential use, or is it some industrial unit? Without a thermostat is there the possibility that they will produce scalding-hot water that may be dangerous in the shower? And as dogknees mentioned, if they're in series the water may be dangerously hot all of the time. Usually, putting industrial equipment in a residential application is a no-no.

All of that being said, parallel will give you more flow rate, at a lower temperature. But if the shower is on low, the parallel configuration might not have enough flow rate in each unit for the sensor to turn the heat on.
Series will give you a higher temperature, at a lower flow rate. But "higher" might be dangerously so.

Given your question, it sounds like you should bring professionals in to review and make recommendations for you, not just post vague questions on a blog. Someone might get hurt if this is done improperly.

### Thermo ...

homework or is this a real world application?

You might try

http://www.gdn.edu/PT_Faculty/cmckeithan/1011_6thru8prob.htm

Which has a bunch of fluidic and thermo problems/solutions. Hope this helps! :)

### This is most certainly a homework problem

In home plumbing, there are at least a dozen reasons you don't put things like heaters in parallel or in series, nor do you typically have more than one. If your goal is to heat the water, then it would be logical to have a recirculating pump of some sort.

A typical 'on demand' electric hot water heater is not suitable for use for a shower, as it does not provide enough hot water fast enough. The only tankless models that can do this are gas-fired units.....

### OK maybe I'm missing something here

But when you say in Series or Parallel are you talking about the Electrical Connections, The Water Connections or both?

Frankly I can see no benefit for feeding one heater with the output of another so here I'm assuming that they are feeding different showers and you want to know how to wire them in.

This is a basic Electrical Examination question and if you can not answer it you are not qualified to be considering doing the work.

For a complete answer read and learn the contents of your Text Books that is the best answer that I can give you so you will learn these things.

Col

### orfice upstream of switch.

for size on your own. jg

### To difuse all the smog

First of all, lets say thank you all for responding! Now for the specifics...

I- To dogknees: If you consider that in parallel the current is half you're wrong. 120 Vac is what's coming from the grid & if in parallel they both receive that voltage. When in series they both experience half of the previous current since they're equal in resistance & power rating (wattage). Think of it as if they were 2 equal bulbs: in parallel they both shine with the expected luminosity; but if in series they might even fail to give any apreciable light (half of the current of any of the previous bulbs & half the voltage, so one fourth of the power for each).
In your 2nd paragraph you really gave me a valuable tip referring to the specific heat of water, something that I hadn't considered yet. The last remark is very tied to your 3rd paragraph. It's clear for me that you're right I need more specifics on the circumstances...

II- To Tony Hopkinson: The following link may serve as a clarifier:

Also, the amount of heat in a certain quantity of water will affect heat transfer ratio (a valuable tip)...

III- To oldbaritone: The following links are intended for clarification:

http://tankless-water-heaters.ecrater.com/p/1450643/marey-aquamatic-2-electric-shower-tankless
http://www.inspectapedia.com/plumbing/Electric_Shower_Heaters.htm

The last one contain a picture of the exact type I'm referring to. Sorry for the lack of info in my original post but, as I mentioned right there, this site doesn't offer the possibility for uploading attachements/diagrams. I did mention this there & also that I had the diagrams already prepared so you had the possibility to ask for them instead on insisting in judging what, for you, was already blurred...

IV- To Charles Bundy: Those links were very good & interesting. By the way, this isn't any homework even though I have had those heaters around for quite a few years now; but in a sense, it's a home 'work' but not any assignment from any school or university. It's a personal thing/idea/project to be dealt with any time in the future. That time might be right now, though!

V- To robo_dev: I'm sorry to tell you but you're wrong twice: isn't a homework (if I'm understanding 'homework' correctly, it's what you bring home in order to work it out & presenting your results to the teacher/professor). I'm not a student, I'm curious!
Next, we have had in this house for more than 30 years those kind of water heaters (electrical). I'm not sure where they come from but all my life suppossed that it was something in the category 'Universal knowledge'. I
mean that I always suppossed that everybody else new about them...

VI- To OH Smeg: Yes, you're right! You're missing a few things. Have a look at the diagrams...

### More fog diffusion needed...

Thanks, Tony Hopkinson, for responding!

...Going from a general sense to the more specific one.

This whole thing of starting this discussion was aimed at ending with a clear understanding of the effect of the different configurations, in a purely theoretical sense, of the output water temperature.

In relation to your second sentence/first paragraph, Tony, I guess you're referring to, say, current (I) or water flow, each of the units receives half of the system total. Next sentence, 'full current flow' (for both water & electrical current) of the whole system; but, in relation to the previous configuration it makes little sense. It would have been helpful to use a standard way of measuring electric current & water flow, for comparison purposes (i.e., Amps = I(total), I1 & I2 & I1+I2 = I(total) & GPM = Gallons Per Min.).
The end of that sentence seems to be a contradiction: ...'get half of the time to apply it' is referring to what, precisely? If the flow is for all configurations equal at the input stage then, when in series, the water will have double the time within the confines of the heating enclosures.

Seems to me that your last post is difficult to grasp, sorry.