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

By Benny7440 ·
... 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.

Thanks in advanced!

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All Answers

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Not Enough Info

by dogknees In reply to If 2 = elec waterheaters ...

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.

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Reponse To Answer

by twinjenz In reply to Not Enough Info

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.

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I might be missing somethingm, but

by Tony Hopkinson In reply to If 2 = elec waterheaters ...

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...

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What "4" combinations?

by oldbaritone In reply to If 2 = elec waterheaters ...

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.

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Thermo ...

by Charles Bundy In reply to If 2 = elec waterheaters ...

homework or is this a real world application?

You might try

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

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This is most certainly a homework problem

by robo_dev In reply to If 2 = elec waterheaters ...

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.....

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OK maybe I'm missing something here

by OH Smeg Moderator In reply to If 2 = elec waterheaters ...

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.


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orfice upstream of switch.

by goodsellus In reply to If 2 = elec waterheaters ...

for size on your own. jg

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To difuse all the smog

by Benny7440 In reply to If 2 = elec waterheaters ...

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:

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...

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Reponse To Answer

by Tony Hopkinson In reply to To difuse all the smog

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.

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