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This is hypothetical, but based on a real case.

A landlord has an apartment building with 20 units. There is a single 150-gal gas water heater in the building that serves all the units. The landlord pays for the hot water in the building.

The landlord installs a solar array on the apartment roof, but the buildings's electric bill is minuscule, compared with the gas bill to provide hot water. Because of net metering by the electric company, the landlord has a huge surplus of electric energy to use.

Given the described situation, would it make sense to install a high-capacity electric tankless water heater upstream of the 150-gal? The logic being to preheat the water with "free" electricity before it goes into the tank, and thereby lower the usage of gas to heat the water in the tank.

If not, how would you change the water heating setup in the building to get the most use out of the solar surplus?

Edit: A few more details:

The solar array is 50 kW, and the monthly amount of water that passes through the water heater is 18K gallons (actual observation). Assuming that the temperature gain of the water passing through the heater is 70F, on average, that will require 11GJ of energy.

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    The numbers just don't add up. 30 gallons is typical for a home with 3 - 5 people. Even if your apartments have 2 people each, that's 40 people - 150 gallons doesn't seem like nearly enough. Plus with typical apartment buildings - let's say 4-story apartments, so 5 per floor x 4 floors - the roof area is much smaller relative to the inhabited space compared to a typical single family home, so the idea of solar electricity being more than actual needs is really unlikely. In any case, tankless water heating uses so much electricity that your solar will never be able to handle it. What might Aug 22, 2022 at 3:55
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    make sense is to use solar to heat the water directly - which is commonly done in Israel and other places. But that is a different solar (water going through tubes, not solar panels generating electricity). Using the electricity, what makes sense is another tank heater because that can run continuously to use the electricity, at least until the tank is fully heated. Aug 22, 2022 at 3:57
  • Just something to consider ... heated water in tanks has to be heated to (and kept at) much higher temperatures than "on-demand" heat [in order to kill bugs, prevent bacteria growing, etc.]... Talking something like 85-90C vs 40-50C for on-demand. That 150gal is also perhaps to minimise pressure drops when everyone turns their taps on ...
    – Mr R
    Aug 22, 2022 at 4:38
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    Clarifying my question. Most of the electric load in the building is paid by the individual unit tenants, not the landlord. Landlord's electric just covers lights in the hallways, and a few other things like that. Solar array generates about 50 KW peak. So, basically, there is a huge imbalance between what the building generates in solar and the landlord's electric consumption. Because of net metering, it's a use-it-or-lose-it situation. Since the only other energy-related landlord responsibility in the building is water heating, it is logical to apply the excess energy to it.
    – tavr
    Aug 22, 2022 at 5:20
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    @tavr clarifying a question is done by editing the question, NOT putting relevant info in a comment.
    – Solar Mike
    Aug 22, 2022 at 5:49

2 Answers 2

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Given the described situation, would it make sense to install a high-capacity electric tankless water heater upstream of the 150-gal?

All schemes which place a tankless heater in series with a tanked heater are instant heinous fails. You don't need to think or justify any further. It will not work.

However, the normal way to do that same thing is with an immersion heating element inside the 150 gallon tank. There should be an extra fitting or two to allow its installation. This is the same way electric water heaters heat.

These immersion heating elements are tiny, and are designed to go into a threaded opening in the side or top of the tank. Kit as special as a 150 gallon tank is sure to have a few spares for sensors or indeed, auxiliary heating elements.

Consider solar thermal

They already make solar panels specifically for heating water. They circulate water or antifreeze instead of electrons. Thermal is significantly more efficient but they have some problems. First, they are not commodities, and are fairly custom, so they are more costly (but this can be offset by DIY work). Second they involve complex piping. Third, solar thermal performance is degraded by cold air, whereas PV performance is enhanced by cold air.

Here is a field-test video. https://www.youtube.com/watch?v=KVeGummoXS0

The other wrinkle is heat pump water heaters. They can increase PV electric to beyond that of solar thermal heaters - with the wrinkle that you need an appropriate place to dump the chill (or obtain the heat, if you prefer to be accurate). Stealing the heat from utility room air is problematic.

All that to say, using PV to heat water isn't so unreasonable.

Make it so you can vary the storage temperature

The heater's own thermostat, which controls the gas, is aiming to turn their heater on when the water falls 1 or 2 degrees below setpoint, and turn the heater off 1 or 2 degrees above setpoint. Their thermostat's goal is to hold water at a constant temperature.

If you want to store surplus electrical energy, you should probably install an auxiliary electric heating element in an appropriate fitting on the main tank. This will lift the water temperature above the set point, causing the thermostat to keep the gas off for awhile.

However, this could result in water becoming quite hotter than expected. So you will need a thermostatic mixing valve, either at the output of the tank, or at each user's individual faucet.

You will also need a high temperature cutoff to turn off this electric heat system off before temperature exceeds a point which could risk the water heater itself, such as 180F (80C). Don't rely on the system not being strong enough to ever do that. It can happen.

Your storage capacity is 2.5 watt-hours per gallon per degree fahrenheit of temperature flex you can use. Or 1.15 watt-hours per liter per degree C. For instance if you work the tank between 140 and 180F, and have a 150 gallon tank, then you have about 15,000 watt-hours of usable storage. If you want more, use a bigger tank.

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  • Can you explain what you mean by an auxiliary electric heating element fitting? Where and how would it attach, given that this is a gas-powered water heater. I googled, but was not able to find anything that made sense in the context of this question.
    – tavr
    Aug 22, 2022 at 14:21
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    @tavr google "replace water heater heating element". On an electric water heater, the actual heating element is tiny - like 1 inch (25mm) x 12 inch (300mm). It just screws into a threaded hole (bung) in the tank. On a tank that large, I would expect the designer provided at least a few extra bungs for just such applications. Aug 22, 2022 at 16:39
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Net-metered PV is a special kind of energy "storage." One presumes that a significant part of the demand for hot water occurs early in the morning when people bathe at the start of the day, or in the evening in connection with bathing and cleaning. A PV array does not produce much power at either of those times, of course, and the net metering arrangement provides a simple way of banking the daytime-generated electricity for later use.

Water heat in the building is probably using gas fuel now because normally, natural gas is cheaper than electricity. This PV electricity case is special because the PV electricity is "free" (when the cost of the solar array is ignored) or, at least one hopes it's still cheaper than the gas even when the amortized cost of the array is considered.

One can imagine a system in which an electric-fired tankless water heater, in series with the existing gas-fired tank heater, provides some pre-heat to the water. The key to making it economical, though, would be in the controls. If the electric-fired heater is enabled to run only when there's a surplus in the net-metering account then the landlord can get maximum value from the PV, while minimizing the usage of higher-cost grid electricity.

In some areas the net metering is balanced on an annual basis: there's a certain date each year on which the outstanding net-metered surplus is zeroed out. In this case the "control algorithm" for the system could be very simple. Assuming that the zero-out date occurs in mid to late spring, consider this strategy:

  1. Let net-metered surplus accrue during the spring, summer, and early fall
  2. Manually enable the electric tankless pre-heat late in the fall (can one speculate that there's increased hot water consumption during the winter when incoming water is cooler and long, hot showers are more enjoyable?)
  3. Watch the electric bill each month through the winter. When the net-metered surplus has been consumed, manually disable the electric tankless heater until the next fall.
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  • Thank you, this answer is where I was going with my question. Assuming this setup, I see two complications. The first is that during periods of peak water use, the tankless would not be able to keep up, which means that the a lot of gas would still have to be burned to get the water temp to the thermostatic set point. The solution would seem to be two tankless running in parallel upstream of the tank. The second was pointed out in one of the comments: tankless only raise the water temp by 40F, not the 70F required to satisfy the gas water heater's thermostat.
    – tavr
    Aug 23, 2022 at 3:18
  • Given that there is a large energy surplus (a 50 kW array only needs to run for 60 hours to generate the 11GJ of monthly energy required for water heating), what would be the most economical way to configure the tankless to provide almost the entirety of those 11GJ, and minimize the gas burning?
    – tavr
    Aug 23, 2022 at 3:18
  • @tavr I had assumed annual PV generation would be less than the annual energy need for water heating. In that case, whatever amount of pre-heat is added upstream of the gas heater leads to a reduction in gas consumption. Doesn't matter that the tankless heater couldn't carry the full load; the gas heater can provide whatever additional heat may be needed above and beyond what the electric tankless could contribute.
    – Greg Hill
    Aug 23, 2022 at 4:31
  • If the PV generation is so far above and beyond the gas consumption for heating water then just rip out the gas heater and install multiple electric tankless heaters. Maybe one per each apartment, or a bank of them in parallel. Since it's net-metered it doesn't matter whether the electricity is actually consumed during the day while the PV is generating. Maybe add a bitcoin facility to burn up the remainder of the PV generation?
    – Greg Hill
    Aug 23, 2022 at 4:32

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