I installed 5.5kW photovoltaic solar plant and I have a gas instantaneous heater for hot water. During the day, I have an excess of energy production and I would like to use it to heat water. However, if I use an ordinary water heater, this seems to me not to be green at all! Since it mixes hot water and cold water and cover the delta of the temperature even in hours when the solar energy is not available.

Ideally, in my opinion, the system should work in this way: warm up the water when the sun is available (e.g, from noon to 4pm). Then, consume this water without refilling by excluding the gas heater bypassing its flowmeter. In the event the water terminates in the tank, exclude it and use the gas heater. At noon of the next time, the cycle restarts.

Although it seems to me that this is the green way of solving the problem because you always use the most sustainable energy source, it seems that the water heaters do not allow this while the electronics to regulate the valves and bypass is ok. For example, the water heaters seem not work properly if they are half empty since they take the hot water from the top.

What am I missing? I'd like to say that I am ok with home automation but less with hydraulic skills.

Thanks a lot.

  • So buy the hybrid panels that do electricity and hot water.
    – Solar Mike
    Oct 24, 2022 at 10:26
  • What kind of solar installation do you have? I know that many off-grid systems have a "dump load" terminal where excess energy can be drawn from when the batteries are full. I assume yours is grid-tied though. Oct 24, 2022 at 15:40

4 Answers 4


I believe this is a little bit over engineered.

If you buy a normal electrical water heater they are internally desinged to return hot water from the top of the column and push cold new water from the bottom. Hotter water stays on top due to being lighter than cold water for the same volume (it's expanded), in a sense hot water "float" on top of cold water. It does mix a bit, especially when hot water is ending, but you can always use your faucet to mix it.

Also, if your instantaneous gas water heater have a thermostat I'd pipe the output of the electrical water heater into the input of the gas heater. This way the gas water heater won't turn on when the input water temperature is above the desired temp and will turn on if it is below that temp.

  • I agree that making the instantaneous production work only if the buffer is empty is not a problem. I disagree about the observation that the system is over-engineered. The idea of buffering the energy in terms of hot-water only when and if the green energy is available while otherwise produce hot water on demand should be the correct behavior in the next future!
    – haran76
    Oct 24, 2022 at 10:23
  • 2
    Hot water rising to the top is called stratification
    – Solar Mike
    Oct 24, 2022 at 10:25
  • 1
    Don't get me wrong, I agree with you, getting the energy from the greenest available source should be the future, I agree. See Fronius solution on this field: fronius.com/en/solar-energy/home-owners/products-and-solutions/… What I believe is a little bit over engineered is the system to totally empty the buffer and thus switch the flow with valves etc. Stratification (thanks @SolarMike) do the work for you.
    – mCasamento
    Oct 24, 2022 at 10:28
  • Thanks a lot! I'll study stratification then. Thanks for the pointer!
    – haran76
    Oct 24, 2022 at 10:33
  • 1
    I don't know how to do the math, but yes, it seems obvious that "some" heat will be mixing when cold water enter the boiler. It's however how most commercial electrical boiler works.
    – mCasamento
    Oct 24, 2022 at 14:20

I'd like to say that I am ok with home automation but less with hydraulic skills.

If all you have is a hammer, everything looks like a nail. You are good at home automation so you are attempting a home automation solution. See also Martin's Marble Machine design lessons.

Don't make it harder than it is

STOP and consider how gas on-demand continuous water heaters work. They take the inlet water (at whatever inlet temperature it happens to be), and add just enough heat to raise it to a target temperature. No more than that because they don't want to scald you. Get it? The gas heater adjusts its gas use based on a thermostat and inlet temperature.

So what if your cold water line to the heater went across an attic? Summer afternoons that inlet water is actually quite warm. Does the heater use less gas to heat it? You bet it does.

Getting the picture? Think.

By golly, you got it!

Put the tanked heater immediately before the continuous/tankless heater, and you don't need to do anything else. If the tanked heater does nothing, the continuous heater works same as before. If the tanked heater raises the temperature somewhat, there is a corresponding savings in gas usage.

And that's it.

Isn't that slick?

Notice how a whole bunch of complex design requirements just flew right out the window. If you clicked that video above, this is just like Elon Musk's stowable grid fins and battery mats. Also no more need that the tank heater raise the temperature high enough to be usable. Which is an issue you had not even really started thinking through.

How to split power

The simplest way, and the way most people do this, is to use a battery charge controller which has "Dump" terminals. The charge controller routes power to "Dump" that it cannot fit in the battery due to charge rate limits (which occur as the battery approaches top of charge). This feature is pre-built into solar charge controllers, and you just need to buy one that provides it.

The smarter "grid tie" controllers also have auxiliary terminals like this for times when it's more advantageous to dump to a heater rather than sell back to grid.

You are not the first person who dreamed up "use a water heater to capture otherwise wasted energy". And you're also not the first to notice that the power company pays peanuts for morning solar. And the market has provided with ready products. If you committed to an un-ready product, sell it and get one that is ready. That is the least bad use of your time.

Don't get scalded... or legionella

These are opposing goals you must account for. First, (a science lesson of Flint, Michigan) - water heaters held in a certain temperature range become breeding grounds for bacteria. 60C (140F) will assure the bacteria is killed. But 60C/140F water also scalds, which in turn requires "thermostatic mixing valves" at all faucets. This is not a big deal; most "joystick style" faucets and modern 1-knob tub spigots are thermostatic mixing valves.

So you don't want to simply have the tanked heater have a cutoff of 45C (113F) and declare victory; that will be a bacteria factory. You also don't want to turn the continuous heater up to 60C: most will not go that high (it scalds), but also a momentary flash at 60C does not kill the bacteria.

On the other hand, if the mixing valves are in place, there's no reason not to store water higher than 60C (140F). However over 90C/195F you run the risk of destroying the tanked heater (if no check valves are installed) or a BLEVE (Boiling Liquid Vapor Explosion) if check valves are installed.

Note that energy losses through the water heater's insulation are proportional to the difference in temperature.

How much energy are we talking?

For this we have a marvelous unit called the BTU. 1 BTU raises 1 pound of water 1 degree F, or 1 litre of water 0.25 degrees C. There are 3.41 BTUs in a watt-hour.

So coarsely, 5 watt-hours raises 1 litre of water 1 degree C.


It sounds to me like what you need is an insulated holding tank not a tanked water heater. Fortunately, you already have an insulated holding tank, you're just using it as a hot water heater.

During the day, heat water and feed it into the water heater/holding tank. Feed the output of that into the instant water heater, then your your faucets. This way, you'll be using/storing solar water heat during the day, supplementing it with the instant heater when necessary (2 showers back to back? washing dishes then clothes?), but storing when hot isn't in use.

At night, you'll have as much solar hot water as you were able to store up from late afternoon/early evening. You'll draw that down as the evening rolls on, and into the next morning (someone up and showering before there's enough sun to produce hot water?), but should you run out, or the temp drops too much, you're supplementing with the instant heater.

The only "advanced" feature you may want to include would be a temperature controlled inlet outlet to the holding tank that shuts off the flow of water into it if the tank outlet temp drops below a certain temperature. That way, you're maintaining as much heat in the holding tank as possible while you're drawing from it, and not feeding in colder, un-solar-heated water, causing it to cool even more rapidly. If you're familiar with home automation, I'm sure you can whip something up with water temp sensors and a smart valve to open/close it as necessary. TBH, though, this automation on the output side isn't really necessary since the water will run through the instant water heater anyway.

  • 1
    Not so sure about that temperature-controlled inlet idea. If you're drawing hot water from the tank, then you have to replace it with something - otherwise you'll end up trying to draw a vacuum in your tank... If you let air in, then you'll need some additional mechanism to let the air out again once the solar gets back up to temperature.
    – brhans
    Oct 24, 2022 at 15:50
  • Ah, excellent point, @brhans! It should be a temp controlled outlet. Updated.
    – FreeMan
    Oct 24, 2022 at 16:09

Your current hydraulic layout isn't clear to me.

You say that you have an instantaneous heater, but you want to save the heated water. Normally, an instantaneous heater doesn't have a water tank, it just heats the water as it flows from inlet to outlet.

That means you need a buffer (basically a large isolated water tank) with electrical heating elements and lots of rework on your plumbing.

The final state would be that you get your warm-water from the buffer. The buffer is fed from different sources (your gas heater, electrical heaters or perhaps an solar heater)

Not only does this require extensive rework, you also need the space for the buffer and the plumbing.

If you want to go this route then I'd add a dedicated solar water heater, ideally you can position it under the buffer, so convection alone will move the heated water and you will save yourself the circulating pump and the electricity costs.

Or you ditch all this together, and just sell the generated electricity. Also, an instantaneous water heater, while still emitting CO2, isn't the worst alternative. Especially since it only burns when you actually need the heated water. In contrast, the buffer needs to be constantly fed with heat.

Also 5.5kW isn't that much, so feeding your entire energy gain into your warm-water might be a net loss for you.

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