Our current shower is a tankless mains pressure type and is instantaneously heated by a gas combi boiler. Eventually we intend to build our own house and I am looking for non fossil fuel heating for space and water. What alternatives are available for providing mains pressure heating for shower? I have tried a mains pressure tank fed shower in the past but the problem is they run out of water too quickly even with just two people using. So I am reluctant to use tank systems. We want to keep the same pressure we get off mains as we do not like the low flow rate with electric showers.

Is there an alternative instantaneous heating system?

Thanks in advance for your replies

  • Presumably you will be using solar electricity, right? So get a 50 gallon electric tank.
    – MonkeyZeus
    Commented Sep 13, 2021 at 16:32
  • We have a 40 (or 50) gallon tanked gas-fired water heater and unless we have 4-5 people take short showers in quick succession or someone takes a long shower, we don't run out of hot water, and we don't have any pressure issues.
    – FreeMan
    Commented Sep 13, 2021 at 17:19
  • Have you looked at the recovery ratings for the tank heaters you had, and compared them with what's available out there? Commented Sep 13, 2021 at 23:40

3 Answers 3


From "boiler", "mains" and "electric shower" I gather you are in UK-influenced areas.

I am looking for non fossil fuel heating for space and water.

We want to keep the same pressure we get off mains as we do not like the low flow rate with electric showers.

OK, seems like a bit of a values conflict, environmentalism without sacrifice, but let's see if a triumph of technology can prevail.

What went wrong with the electric showers?

A British style electric shower is 8500 or 9500 watts (VA). And the basic unit of measure of heat energy is the kilocalorie (which is the energy needed to heat 1 kilogram/1 litre of water 1 degree C). Now, let's do some computation.

It's winter and you're taking in water at 5C. You want it at 40C. That means you need a 35C heat increase.

1 watt is 0.860 kilocalories per hour. So a 9500 watt heater can make 8170 kcal/hr. How much water can it raise 1 degree C per hour? 8170 litres. How much can it raise 35C per hour? 233 litres.

Or 3.89 litres/minute. Or 1 gallon per minute.

That's a coarse rule of thumb: 10,000 watts gives 4 litres per minute.

Well, no wonder.

Let's step it up.

A modern California "low flow" shower head is 1.5 gallons/min. You probably want double that or 3 gallons/minute. Or 11.36 litres/min. That is 700 litres/hour.

700 litres/hour x 35 degree lift = 24,500 kilocalories/hour.

So to sustain that rate, you need 28,488 watts.

At 230V, that will be 124 amps.

Assuming 3-phase power, you'll be at 41 amps per phase.

The minimum electric service they offer in the USA these days is 200A @ 240V (48,000 watt). So tankless heaters as large as 36,000 watts are within the reach of US customers provided not too much else in the house is electric. Most of them use 3 circuits of 240V each, which is necessary due to the eccentric way the USA gets to 240V, but it's also designed to be compatible with European "230V" 3-phase power.

In Europe they don't bring you 200A of a single phase, they have all 3 phases up at the pole, so they simply bring you down all 3 phases at 65 amps each. At that point you just need to make sure your chosen heater is designed for 3-phase operation, which it will be if it's marketed in Europe.

  • 1
    "environmentalism without sacrifice ... a triumph of technology" ... I vote this answer moved to PoetrySE. ! :)
    – jay613
    Commented Sep 14, 2021 at 12:35
  • 1
    "UK-influenced area" also strongly implied by username "kiwibloke". ;)
    – FreeMan
    Commented Sep 14, 2021 at 14:55
  • If OP is in NZ where 74% of utility power is hydro/geo/wind this answer is pretty close to a bullseye.
    – jay613
    Commented Sep 14, 2021 at 18:21
  • 1
    @Freeman I missed that? I really backslid on that one. Normally my geolocation skills are uncanny. Commented Sep 14, 2021 at 18:57

Heating of water requires energy, and instantaneous heating of water requires instantaneous energy. In a residential setting usually gas and commercial electricity are the only available sources of instantaneous and "endless" energy to support indefinite/continuous water heating.

Eliminating use of fossil fuel can be easy: use a heater fueled by electricity. Note however that it takes a high electric current to heat many liters per minute of water instantaneously. This could require upgrades of the electric supply system, a higher-cost tier of service from the electrical supplier, etc.

An electric-fuel tank heater eliminates the problems related to high current consumption. The tank(s) have to be sized so that they can provide sufficient volume of hot water according to the usage cycle.

Of course electric power may be derived from fossil sources, so one may be led toward solar electric, solar thermal, or wind powered heating. Those are fine, but they're intermittent, so one has to store the energy somehow. This road leads back to tank storage.

  • Move to an area served by a nuclear power station and your electric instant water heater will be "non fossil fuel". Probably not what you're looking for.
  • Move to an area with an abundant supply of geothermal energy. I don't know if there are home geothermal systems that can do instant hot water. I don't think so. But it's worth a look.
  • More realistically, only a utility can provide enough power for instant hot water to a shower when you also demand high pressure and high flow. Without a utility you need storage. The most obvious and simplest solution is a solar hot water system that heats the water gradually and stores it for later use. Or you could use batteries, or piles of bricks or other ways to store energy but if the objective is hot water, it seems simplest to just store that, whether the energy comes from solar, geothermal, or anything else.

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There just isn't any common source of home-derived power sufficient to instantly heat water for a really nice shower, that doesn't burn fuel. Bear in mind .... fuel is itself storage for power.

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