Guide to Using an Electric Hot Water Heater For Hydronic or Home Heating
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Can I use an electric hot water heater to heat my home
Efficiency and life expectancy of electric water heaters when used for hydronic heating or building space heating
Questions & answers about using electric water heaters as home heating systems

This article explains using an electric water heater for heating a building occupied space, connecting an electric water heater to heating baseboards or radiators.

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Thanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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Advice on Using an Electric Water Heater for Home Heating by Baseboards or Radiators?

Question: How Can I Improve the Efficiency and Reduce the Operating Cost of my Electric Water Heater Used to Heat my Entire Home?

I'm looking for information on boilers and hydronic heat. I have read the 39 steps in the operation of a boiler and my problem is this: my "boiler" is a 60 gal hot water storage tank heated by 2 4500-watt elements each controlled by a tank thermostat set at 140 F. Each stat is hooked up to a 240 V 20 A breaker/ There are 5 loops. 4 of them have TRVs and one runs wide open.

The installer was a plumber who loved this system and took out an oil-fired hot air furnace to install this system.

When I bought the house he did not give me much information on how to operate it and now he is dead.

The living-room has a loop with a TRV set at 4 (70 F according to a spare one he left me) AND a stat on the wall that turns on or off a GRUNDFOS UPS 20-42 3-speed circulator pump.

I am wondering how to make this [water-heater based home heating] system work efficiently. None of the heating contractors I have contacted seem to understand this system and think that this water heater is a boiler and should be run at 180 F and my stats go up to 177 F. I am unable to find a discussion of this system anywhere on the internet with the use of electricity to heat the water.

Any ideas? Thank you very much. -- Ben V, Canada.

Answer: Electric Water Heaters Designed for Producing Domestic Hot Water

We cannot imagine a more costly, short-lived heating system than using an electric water heater to heat an entire building, especially in Canada where winters can get pretty cold and a heater has to work hard.

Also, take a look at your water heater's warranty and you may see that using the water heater to heat the home either voids the heater warranty or reduces the warranty period. (Also see APPLIANCE EFFICIENCY RATINGS).

Electric water heaters can be the slowest to recover when cool - so the system may not be very responsive in cold weather unless it is staying on nearly all the time (even more costly). Carson Dunlop's sketch (left) explains recovery rate of water heaters.

Also, water heaters are not expected to maintain internal water temperature much above 140 degF - domestic hot water over 120 degF is dangerously scalding hot. Of course, your heater may be capable of reaching the 177 degF. you mention, but it's going to have a still shorter life and higher operating cost at that level.

We guess that your plumber/designer who loved using an electric water heater for heating the entire home expected that (as is common in Canada) the circulator pump(s) would run continuously during the heating season. Circulating water continuously at a lower temperature may be comfortable and may actually heat the home adequately. That's where his 70 degF. set point may have come from.

But unfortunately, circulating cooler water (70 degF.) than would be produced by an ordinary hydronic heating boiler (operating at around 180 degF), is less efficient in both theory and practice for a less obvious reason that we learned from a heat transfer engineer who explained that "The thermal conductivity of finned copper baseboard, or of cast iron radiators, is exponentially greater at higher temperatures."

In other words, the hotter the heating water you are circulating, the more efficiently heat is transferred into the living area. So dropping the temperature to 70 degF. may have helped the water heater life, but it probably increased the building heating cost still further.

We wouldn't rule out using an electric (or other type) of domestic water heater to heat a small space that is not served by the main building heating system, but using any water heater as the permanent and main source of building heating in a cold climate is generally a bad idea.

WATER HEATER PROPERTIES discusses water heater types and their efficiencies; there we also note that using a water heater at a high duty cycle (such as heating a home) will shorten its life. We discuss the role of water heater use (for heating a home) and its effect on warranties at AGE of WATER HEATERS.

Electricity is commonly used to heat water for hydronic heating systems as a backup heat source, such as in a heat pump system (see BACKUP HEAT for HEAT PUMPS), or in super-insulated buildings that use a tiny electric boiler and perhaps a radiant floor slab. ELECTRIC HEAT discusses typical electric heating systems

In the water-heater based home heating system described in the question above, we would guess that the thermostats simply cause the hot water to circulate, and the built-in thermostat turns on or off the electric elements in the water heater tank.

We doubt that there is any way to make this an efficient home heating system other than by removing it and installing a more economical heating source; you might however

Check with your electrical company to see if they can give you preferential rates for using electric heat
Be sure your home is draft free and super-insulated. See these articles that can help improve the energy efficiency of your building:
Set back your thermostats as much as you can - see THERMOSTATS

Here is a rough guess at the relative life expectancy of several types of water heaters, provided all other water heater life factors (discussed at AGE of WATER HEATERS) are the same (comments are invited Contact Us) (shortest life first).

Electric water heaters (though individual heating elements may fail and need to be replaced before the tank fails)
Gas-fired water heaters, High Efficiency Models (LP gas or natural gas)
Gas-fired water heaters (LP gas or natural gas)
Oil-fired water heaters
Solar water heaters have a relatively high purchase cost but probably lower maintenance cost than oil and gas fired equipment

Conditions Affecting Water Heater Life

Keep in mind that conditions besides the type of water heater can dominate its life expectancy. Some of these water heater life expectancy factors include:

The duty cycle or level of usage of the water heater (more use = shorter life). Using a water heater to heat a building in a cold climate is going to work that device harder than if it were just making domestic hot water for washing and bathing.
The chemistry of the water being heated (aggressive or corrosive water = shorter life)
The construction of the water tank, methods and materials used (look at the water heater warranty period; longer warranty probably means a more durable water heater tank)
Proper installation of the water heater; improper fuel or water piping connections or inadequate chimney venting on fossil-fuel fired water heaters can lead to safety problems or leaks and early heater failure. Improper location or improper position of the water heater (such as mounting some water heaters horizontally) can lead to early failure of the heater as well as voiding the manufacturer's warranty.
Proper maintenance; almost no one we've encountered drains and services their water heater on the schedule recommended by the manufacturer

For complete information about water heater life and things that affect how long a hot water heater will last, see our full article at AGE of WATER HEATERS.

As we illustrate with Carson Dunlop's sketches shown here, in order of speed of re-heating or hot water recovery time, listing slowest-recovery time to fastest recovery time we'd list water heater types as follows:

Range boilers
Electric water heaters
Gas-fired water heaters
Gas-fired water heaters, High Efficiency Models
Solar water heating systems (depends on weather, season, and sun exposure)
Tankless coils (depends on heating boiler size, construction [steel/cast iron], thermal mass, and fuel type [oil/gas])
Indirect-fired or tankless water heaters (depends on heating boiler size, construction [steel/cast iron], thermal mass, and fuel type [oil/gas])
Oil-fired water heaters
Instantaneous water heaters

Don't confuse water heater recovery rates (how fast we can heat water) with water heater operating costs, which we discuss at Water Heater Operating Cost Comparisons. Recovery rate is measured in gallons per hour or gph. Water heater operating costs are compared using a standard measure of energy cost in therms.

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Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Ben V. (anonymous) for email suggestion of this topic 12/03/2009
A.O. Smith water heater safety sticker: 182734-000 Rev.00 observed on the gas fired water heater shown at High Efficiency Gas Water Heaters

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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

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Advice on Using an Electric Water Heater for Home Heating by Baseboards or Radiators?

Question: How Can I Improve the Efficiency and Reduce the Operating Cost of my Electric Water Heater Used to Heat my Entire Home?

Answer: Electric Water Heaters Designed for Producing Domestic Hot Water

Conditions Affecting Water Heater Life

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair