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My house is old and has radiator heat, which is heated by a gas furnace from which the hot water is pumped through 7 radiators and back in. Hypothetically, and by no means would I ever want to do this to my system, if gas became unavailable in my area, would it be possible for me to adapt a regular electric water heater and mount a pump nearby to circulate water through the system like the present one does with gas?

Another way to ask is, what if I were designing a radiant hydronic floor heating system and my only source of energy were electric. I'm sure there are specialized heaters for that purpose that have a pump built in but, if I wanted to make my own (for experiment's sake), could I do it with a regular electric water heater and a pump?

At first thought, I don't see why not and I think the most complicated part would be designing a wiring control that works with the thermostat and connects the heater and the pump. I'm asking this primarily to understand the visceral concepts of heating and associated devices. Of course, unlike with a residential water heater that has open inlet for new water to come in, in this case it would have to be on a closed system which itself has a fill valve and a backflow adapter.

  • Is electricity cheaper in your area, or the only energy source available? A tank water heater is likely not going to be efficient used in this way, and might not even be capable of heating enough water fast enough. Maybe a tankless heater. – Tester101 Feb 2 '16 at 5:26
  • The price of energy is not an issue, I'm curious about technical feasibility. But I think you're right about tankless since my gas heater for radiators is tankless. – amphibient Feb 2 '16 at 5:37
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It's actually not uncommon to use a water heater for heating a well-insulated building (though an electric water heater is very UN-common, due to operating cost.) But....

This is not commonly done with radiators, which are typically designed to operate on 180F water. It is much more common with radiant floor heating, which can function quite well down at 100F water temperature in many cases.

The building must be well-insulated - the typical reason for using a hot water heater is the inability to get a "boiler" that's SMALL enough for the load of a well-insulated building. As designed, my shop will take about 30K BTUs/Hr to heat to 68F on a -20F "design day." (the minimum temperature used for the sizing the heating system.) A 100K BTU/Hr boiler would be massively oversized.

You need an expansion tank for a closed system, and you may need to use stainless steel pumps (rather than cast iron) if any part of the system allows oxygen to get in (non-oxygen barrier tubing, or an open system where the SAME water heater heats the floor AND the domestic hot water - in which case the floor tubing needs to be potable-water rated - not actually difficult to find in PEX tubing.)

Finally, if there is gas in your area now, it's stunningly unlikely to "become unavailable" in a future where you still have electricity. And if you are "heating with electricity" you really should be using a heat pump (in cold climates, a cold-climate air-to-air heat pump or a water-source/geothermal heat pump) since you get about 3 times the heat (depending on conditions and specific unit) for a given amount of electricity purchased. Heat pump water-heaters won't work without some other source of heat, as they are designed to operate from air that's 50F or more, typically.

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Let me preface this answer with this is the setup we use in our house. Our electricity and gas bill is about 125 a month for a 2400 SQ foot house. It is extremely efficient. I will go through the pros and cons and when it should and should not be used and how it works.

First off, we live in northern California and temperatures only get down to single digits a handful of times each winter. Average temps area typically 50 degrees during the day and high 20s at night. Our primary source of heat is a wood burning stove so on a typical day we do not heat the house at all until we get home from work.

The way our system works is when we turn the thermostat on a 1.5hp pump circulates water from our hot water heater through a radiator located within our forced air central heating system. The cold air blows through the radiator, gets warm, then blows out through several registers throughout the house. The warm air rises and to the top of the house and a return pulls the air back down. The air continues this loop as it gradually gets warmer each cycle through.

Same goes for the water. The water leaves the 55 gallon hot water heater at approximately 120 degrees and when it is returned to the hot water heater it is about 95 degrees. as the system runs the return water gradually has a higher average temperature, So the longer it runs the more efficient it is.

The air coming out of the registers is approximately 80 degrees depending on the location and the distance. The hot water heater is keeping water hot either way so might as well use it right.

Now the cons. Using this system changes the dynamic of a shared hot water supply system. When the heat pump is running you are sharing hot water with the heat pump and a faucet, so the ratio of hot water is reduced when the system goes on. so your showers are not hot... just warm. In addition if you take a long hot shower and use all the hot water while the system runs you also create the risk of sending cold water to your heat pump effectively making it an air conditioner instead of a heater.

We have found that with this setup it is a great supplemental source of heat that compliments our wood burner. Some nights we use it for convenience instead of starting a fire, but in extremely cold climates I don't think it would be sufficient. We adjust how we take showers and when we run the system which mitigates the problems I discussed earlier. As others mentioned it would work great with radiant floors but radiant floors operate best with constant heat applied so if you plan on sharing the system and reducing the hot water (taking a shower) you lose the benefits of the efficiency.

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Hydronic heaters are typically pressurized to 15psi, and use thinner copper tubing; there's a pressure reducing valve. Hot water heaters operate at city water pressure which can be more than double that.

So while you could theoretically pump water from the HW heater to heating system, you'd have to worry that there were iron pipes and pumps, and that you'd over-pressurize the system and possibly produce leaks.

Also, as per the other answers above, it wouldn't work very well except on spring/fall days; it would also generate a lot of wear and tear on the pumps, as they'd be on nearly all of the time.

Finally, assuming that your house has a programmable thermostat, you'd also find your morning shower to be deeply unsatisfying, as the water would be cool from having circulated through the house.

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Sure, it would work in general principle. The "currency" here is hot water. Any way you can heat it, is fair game.

You'd have a practical issue in actually doing it though: sizing. You wouldn't find a tanked hot water heater with a large enough heating element to heat a whole house. So at normal pumping rates, your radiators would be tepid at best. So you'd need to use several hot water heaters, or a commercial model, or a pretty big tankless.

But costwise, it's way too expensive to heat water. A 2000-watt baseboard heater is fifty bucks at Cabelas, and you can't beat that. Dog simple, easy to "pipe", and individually controllable - you could do zone heating by literally putting a thermostat in every room. If you must heat via electric, you might as well fully exploit its versatility.

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Your proposal seems sound in principle. But then, in principle, you could heat your house with hair dryers if that's all you had. Some things to consider:

A typical home heating furnace is about five times as powerful as a typical water heater. For example, my furnace is 150,000 BTU/hr and my water heater is 40,000 BTU/hr.

So a re-purposed water heater wouldn't be big enough to heat your house, although it could probably keep your pipes from freezing in an emergency. You would need about five water heaters to heat the average home.

The controller shouldn't be too difficult. Using standard parts, your thermostat will demand heat by using relays to run the circulating pump and energize the water heaters. The low and high range thermostats built into the water heaters will control the heating elements to maintain a toasty water temp without boiling. In other words, whenever the thermostat senses that the room is cold, the circulating pump runs and heating elements cycle on and off.

A furnace costs about $8,000 to $13,000 (with installation), and will last 20 years. A water heater costs $180 to $250, so five of them would cost $900 to $1,250. (Installation cost is irrelevant as no sane plumber will do it.) Used as furnaces, the water heaters will probably last about 5 years, so that's $4,500 to $6,250 over the same 20 years. I don't know why people are not doing this.

Electricity is the most expensive source of energy compared to gas and oil. (Gas is about as expensive as oil where I live, in New England.)

Gas as a utility is quite unlikely to fail or become unavailable.

Heating with electrically heated circulating hot water seems silly since you can heat more simply with electric baseboards or space heaters.

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The largest water heater coils I could find are 4.5 KW that translates to less than 16,000 BTU/hr. So, if your heater has two of those at 9 KW you could provide 32,000 BTU/hr. That would be enough to heat a small well-insulated house.

However, electric water heaters would be a 1 to 1 ratio of KW's to BTU's.

Now, a geothermal heat pump could get you up to a 4 to 1 coefficent of production so for the same 9 KW you could get 124,000 BTU's/HR which should be plenty to heat a mid-sized home even if it is older and not so well insulated.

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