I have a 3000 sq ft house on 3 levels. We have 3 full bathrooms and run out of hot water. No Gas available, everything is electric and I live in VA. Can I run a small, 3GPM, Tankless in series with a small, 15-20 Gal Tank water heater so we have an endless supply of hot water on demand? A whole house Electric Tankless Water Heater would require a new panel and wiring at a cost that is prohibitive. How about another "Hybrid" type system?

I appreciate your insights.

  • Even a 3 GPM tankless will use a lot of power. A quick search finds that would be on the order of 60 Amps! That's a lot of power. It is physically small, but not electrically. Commented Nov 14, 2021 at 14:51
  • "Point of Sale" Tankless? What is POS, in this context?
    – FreeMan
    Commented Nov 14, 2021 at 15:45
  • @FreeMan -- Point of Service (Point of Use) Commented Nov 14, 2021 at 17:04
  • Thank you, @ThreePhaseEel. POS has a non-polite meaning that might be appropriate to describe the OPs water heating situation, but I was hoping that wasn't it.
    – FreeMan
    Commented Nov 14, 2021 at 17:36

5 Answers 5


There is no magic to this.

Your most straightforward solution (without excessively costly upgrades to your electric power service) to an absurdly undersized storage tank water heater is a correctly sized storage tank water heater for your needs. In the current era, that would likely be a "hybrid heat pump" system for efficiency and because you can't buy a large tank that isn't one of those these days.

An inadequate tankless heater will not provide "endless" hot water (particularly for 3 bathrooms.) Pairing it with an absurdly small storage tank heater will not make a particularly notable difference after the small capacity tank is run out, since the small tank heater's heating element is of distinctly limited capacity.

I would bet that propane gas is available to you, which is another way to get to an "on demand" water heating system that's not depending on massive electric service for the massive amount of heat needed to heat large amounts of water "on demand." However, in a climate with as much cooling demand on a yearly basis as is typical for Virginia, a hybrid heat pump tanked electric will probably cost less to run. With 3 bathrooms and concerns about running out of hot water, choose 80 or 120 gallons.


You'll want to get a good (read: split system) heat pump water heater

There's really only one way to go for mass quantity hot water in an all-electric house like yours, and that's a split system heat pump water heater. These use a mini-split-sized outdoor unit to heat water and pump it over to a storage tank located inside the house, and provide much greater capacity (80+ gallons first hour recovery on a 43 gallon tank for the SANCO2 heaters discussed below) than a "hybrid" water heater for the same size tank as a result.

Unfortunately, they're a bit scarce/expensive in North America: the ECO2 Systems SANCO2 (made by Sanden in Japan) is really the only option you have, and runs between $4500 and $5000 depending on the size of tank you get. However, it does come with a stainless steel tank with a 15 year warranty, which means that the only parts you really have to worry about are the outdoor unit that does the heating. It also uses CO2 as its refrigerant in a transcritical cycle, which is good for both performance (it can put out 160-180°F hot water without the aid of resistance heat, unlike most other heat pump water heaters which need to use their elements to lift that high) and the environment (HFC refrigerants have extremely high global warming potentials, while CO2 has a GWP of 1).

  • I looked into these. They are so prohibitively expensive I'd rather roll my own CO2 (R744) based heat pump than buy one. More than $5K and no warranty at all unless installed by a licensed plumber (waste another couple of kilobucks.)
    – Ecnerwal
    Commented Nov 14, 2021 at 18:04
  • @Ecnerwal -- they used to be more like $3k (I blame the supply chain omnishambles), but yeah, the issue with the warranty requiring a licensed plumber for the install is a bit of a nuisance Commented Nov 14, 2021 at 18:16

You didn't state the size of the existing WH but like others have said, if it's small, like 40 gallons, upsize to the largest tank type you can find...hopefully at least 80 ga.

The other thing you could try that's not very expensive is to install a mixing valve and turn up the temp on the WH quite high, like 150 or so. The mixing valve would reduce the delivered HW to a safe level. This approach effectively increases the size of the existing water heater by storing hot water at a higher temp.

Lastly, while probably not a popular move, would be to install water saving shower heads. I don't like them, but it sounds like you have limited options.

BTW, I hate on demand electric tankless water heaters, They take a tremendous amount of power which is difficult to supply in a typical residential setting.


Your little series heater would need to pass two tests:

  1. if your existing tank becomes fully cold, and your existing heater is nevertheless able to heat the water "instantly" by even say ten degrees, your new second heater would need to further heat the water enough to be useful. It seems very unlikely, I suspect your existing heater would do virtually nothing and your second "series" heater would be a lone soldier in that situation.

  2. If your existing tank is fully hot, your new "series" heater would need to accept the hot input temperature without failing, exploding, or popping a safety valve. It will have a maximum input temperature, that you can look up.

I think you should look to other solutions and I have a couple. Both of them require some retraining of user behaviors and expectations.

  1. Buy two small, single-fixture water heaters. One for ONE shower, and one for the kitchen sink. If you have an electric tea kettle it can serve as the kitchen one. Teach your family that when the hot water runs out, they need to use only those two appliances.

  2. Buy a much larger storage tank, replace your existing storage heater with the largest one your electric supply will accommodate, and set the stored water temp a little higher than usual. If the heater has enough time to heat the whole tank, your problems should be over. Almost. You just need to train your family to understand the limitations of the larger tank, and behave accordingly. Say you get a 70 gallon tank. It will easily be fully hot every morning, and you'll have enough for say 6 people to each have a 10 minute shower and still be able to wash the dishes. But if one person takes an hour-long shower, or if the kitchen sink is left running hot for half an hour for no reason .... all bets are off. The solution has to be part tech part human.


Even a 3 GPM tankless will use a lot of power. A quick search finds that would be on the order of 60 Amps! That's a lot of power. It is physically small, but not electrically.

In addition, dasiy-chaining tanked and tankless is technically easy but in practice may not work so well for a bunch of reasons.

A typical house will have a minimum 40 gallon heater. Larger house = more usage = bigger tank. I'd go for either 2 typical tanks, assuming your plumbing can be arranged appropriately, or one really big tank. The good thing about tanks is they reheat over a longer period of time, so their peak demand is much lower. Typical is 30 Amp circuit.

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