water heater recovery rates Guide to Using an Electric Hot Water Heater For Hydronic or Home Heating

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Use a water heater for heating a home:

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

We describe using a water heater for small heating loads, and we explain the concerns for life expectancy of a water heater and on its warranty when the heater is used for other purposes. We also warn that using a water heater as the primary heat source for building space heating where heating loads are significant is likely to give a reduced water heater life and may also void the water heater warranty.

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

Sketch explaining hot water recovery rate (C) Carson Dunlop Associates

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

[Click to enlarge any image]

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. While electric or even some other water heaters are on occasion used for home heating, they are generally not applied where there is a high heating load and a long duty cycle.

Watch out: Also, take a look at your water heater's warranty and you may see as we have on review of a number of manufacturers, that using the water heater to heat the home either voids the heater warranty or reduces the warranty period.


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

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.

For your heating area where there are significant heating system loads, 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

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).

  1. Electric water heaters (though individual heating elements may fail and need to be replaced before the tank fails)
  2. Gas-fired water heaters, High Efficiency Models (LP gas or natural gas)
  3. Gas-fired water heaters (LP gas or natural gas)
  4. Oil-fired water heaters
  5. 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:

  1. 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.
  2. The chemistry of the water being heated (aggressive or corrosive water = shorter life)
  3. 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)
  4. 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.
  5. 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:

  1. Range boilers
  2. Electric water heaters
  3. Gas-fired water heaters
  4. Gas-fired water heaters, High Efficiency Models
  5. Solar water heating systems (depends on weather, season, and sun exposure)
  6. Tankless coils (depends on heating boiler size, construction [steel/cast iron], thermal mass, and fuel type [oil/gas])
  7. Indirect-fired or tankless water heaters (depends on heating boiler size, construction [steel/cast iron], thermal mass, and fuel type [oil/gas])
  8. Oil-fired water heaters
  9. 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.

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.

Reader Question: hot water is too hot when using the water heater to heat my home

I have a hot water tank which is connected into my central heating system. In the summer
I use it as an immersion heater using electricity. The problem is that the water is too
hot when I use the immersion heater. How can I adjust the water temperature. - John Newing 6/16/11


For a water heater that is used for both domestic hot water and heating, I'd suggest adding an automatic mixing valve right at the water heater hot water outlet line. The valve can be set to the desired domestic water temperature for washing and bathing; the valve works by mixing cold in with the outgoing hot.

You can also permanently lower the electric water heater temperature setting by opening the access cover to the control and setting the temperature dial to a lower number.

Also take a look at ANTI SCALD VALVES


Support for Use of Electric Water Heaters for Space Heating Applications

Reader Comment: Water heaters used for radiant floor heating may make sense in areas with milder heating load

3 Feb 2015 Steve said:

This article, while containing factual information, is not entirely accurate by ommission. While most of Canada experiences cold winters, not all areas do.

Many Canadians live on the BC coast in a very mild climate where it only rarely drops below freezing in winter. It is not clear if the application is in-floor or radiator hydronic heating. There is very large difference, the primary one being the required fluid temperature. In-floor radiant heat works best with a low temperature, while radiators require high temps.

In fact an electric hot water tank can be used to heat a house with in-floor radiant and they are thousands of dollars cheaper than boilers, and if they fail cost very little to replace; also there is very little maintenance, which can be done by the homeowner. The saving are extraordinary. The heater should be dedicated solely to heating due to slow recovery and desire to avoid mixing heating with domestic hot water. Otherwise heating water must be isolated by an approved heat exchanger.


Thank you for your interesting comments, Steve. Indeed it’s a challenge to write accurate technical data covering much of the world in one article. Indeed when heating loads are small, small heating systems, even tankless water heaters, can make economic sense, as Steve Bliss points out at HEATING SMALL LOADS

We need to be careful when claiming that water heaters are enormously less expensive than heating boilers. It's not categorically so that water heaters are "thousands of dollars cheaper than boilers" - or ... it depends.

Cost comparisons between heating boilers and water heaters vs. replacement life are interesting. Boilers matching radiant heat specs well range from about $1,200 USD (Argo Electric AT2-series) to over $3,600 (Peerless PureFire condensing gas boiler).

Water heater prices also will range widely by type & fuel. At the low end are electric water heaters in the $225-400 USD range (simple electric, e.g. Kenmore) with prices easily zooming up to the same range as some pricey heating boilers (Rheem-Ruud 200 BTUH Gas unit) at $2,800. USD.

Making water heater vs. heating boiler cost comparisons needs to consider some additional expenses

The life expectancy of a water heater is not calculated based on the duty cycle it will experience as a space heater but rather as a water heater. On the other hand, an added difference in water heater performance when it’s used for heating is that in areas where water chemistry is aggressive (corrosive) a water heater used to make domestic hot water will have a more challenging time than the same unit used to heat the same volume of unchanging (and perhaps treated) water for space heating.
Things that affect water heater life are discussed

My own views in the question of using a water heater for radiant heating were shaped by reading what the various manufacturers recommend regarding use of their equipment.

But I agree that for light heating loads a water heater, properly set up for that application, may perform just fine.

And it's worth noting that building codes do not prohibit using water heaters in these other heating applications.

Also see REFERENCES where we cite independent research on the use of domestic water heaters for different heating applications including radiant flooring heating and ancillary heat.

Reader comment: water heater longevity improved in closed-systems

28 Feb 2015 Gavin said:

I'd like to add to Steve's comment regarding the use of an electric hot water tank in a 'closed' system. First the longevity of a hot water tank in this system is greatly increased due to the fact that the oxygen in the water is virtually non existent after a short time of use (assuming you've installed an air bleeder). Secondly in this situation the water heater is only operating intensively for four month out of the year, and would be off for six months. This setup will outlive a domestic only or open system setup. The elements are typically around $30 each to replace.

There is also the some what controversial, but highly effective method of rewriring the water heater to operate both elements at the same time. Most master plumbers know about and have done this before. In this situation a 60 gallon typical hot water tank with 4500 watt elements will produce approximately 28,000 BTUs which is a substantial amount of heating potential from one tank. Also some manufacturers recognize this wiring setup as a viable option where there is a higher demand for hot water. (You need heavier gauge wire and larger breaker, or second wire and breaker and wiring scheme for thermostats,


Thanks Gavin, I agree about the oxygen, am nervous about the wiring changes, and remain a bit conservative on this general topic in part because I figure the manufacturers have a reason for voiding warranty if their water heater is used as a space heater.

Domestic Hot Water Tanks as Space Heating Appliances in Canada

5 March 2015 Gavin added:

Check out the last page of the Giant water heater brochure, it states the following,

"All models are suitable for combination potable water and space heating applications. Toxic chemicals, such as those used for boiler treatment, must NEVER be introduced into this system. These units must NEVER be connected to any existing heating system or component(s) previously used with a non-potable water heating appliance."

It appears the exception to the rule is actually Canada. Virtually all hot water tanks are certified for space heating here. Look at Canadian Mortgage and Housing Corporation's article [below]. CMHC made a study back in the 80's using domestic hot water heaters in place of boilers up in northern Canada. The findings are interesting and led to the approval of DHW heaters for space heating.

The approval we don't have is for the wiring for simultaneous energizing of the heating elements in electric water heaters. However, many of the commercial grade tanks do allow for either simultaneous or non.

Excerpting From & Comments on CMHC's Report on Pros & Cons of Using Electric Water Heaters as Space Heaters

A CMHC study performed for Yellowknife, included considerations such as transport cost as well as heating loads that will be different in different climates.

The study reported the following installation cost comparisons:

Installation Cost Comparisons: Domestic Heating Boiler vs Water Heater Used for Space Heating

Cost Item Heating Boiler DHW Tank
Capital Cost $4,334 $2,324
Installation Cost 300 160
Shipping Cost 120 45
Total Installed Cost $4,754 $2,529


Notes & comments from InspectApedia: these opinions are not quoted from the CMHC study.

Amounts are in $CDN

The study did not compare the effects of different heater fuel options: gas, oil, electricity - rates vary widely by area.

Watch out: the study did not comment on the BTUh output requirements as they vary considerably in different climates. A high BTUh output requirement in a cold climate may be beyond the output of many water heater models or may significantly reduce the service life of a DHW heater used for space heating. Certainly some water heaters are capable of a high input BTUh - 200-700 BTUh (more than a typical boiler needs to heat a home, since the water heater is shooting for a quick recovery time).

Watch out: This study omits reporting on the annualized cost of these two heating approaches to include differences in fuel cost, heater efficiency, and possibly significant differences in the typical service life of a heating boiler vs. a water heater. The authors noted that they assumed a typical 79% efficiency level for heating boilers and only a 58% efficiency level for domestic water heaters. That 21% absolute percentage points difference means that if you converted your DHW heater (used for home heating) to a mid-efficiency heating boiler using the same fuel (presumably oil?) using the boiler you would see a 38% improvement in fuel efficiency over the orignial DHW heater!

Missing from the analysis was comparison of a direct-vented high efficiency boiler vs. a direct-vented high-efficiency water heater. For example, without going to extreme (and possibly fussy or hard-to-maintain equipment), U.S. Boiler Company offers the MPO-IQ, a 3-pass oil fired hydronic heating boiler with an 87% AFUE. That boiler can be direct-vented, avoiding the cost of installing a chimney in new construction.

  • High efficiency oil-fired DHW water heaters that we found operated in the 82% efficiency range. (HTP SuperStor). estimates water heater efficiencies ranging from 60% (conventional gas storage heater) all the way up to 1.20% for solar with electric backup. But they too end up at about 82% as the upper range for conventional water heaters, noting interestingly that larger water heater tanks tend to be less efficient.
  • High efficiency AFUE heating appliances (boilers for example) operate in the 90% to 98.5% AFUE range.

Maintenance costs are also assumed to be different (probably lower for a water heater in general but possibly higher for a water heater used for space heating). The authors reported annual maintenance costs for Yellow Knife as:

  • $298./ unit furnace maintenance cost
  • $242./unit - Boiler maintenance cost
  • $90./unit - DHW maintenance cost

Watch out: it may not make sense to compare a furnace installation or maintenance cost to a water heater installation and maintenance cost: where are the heating distribution system costs for the water heater vs. the duct work cost for a furnace. Or if a water heater is to be used in a water-to-air installation (which may be a neat approach to converting a forced warm air heat source) one needs to add to the DHW heater cost the cost of the water to air heat exchanger as well as additional controls. - Ed.

As well there are very signficant price ranges for both heating boilers and water heaters. For example a cast-iron heating boiler might be expected to have a longer life than most water heaters of any design. - Ed.

Watch out: also missing from this analysis is the question of how domestic hot water is to be produced if the DHW heater is dedicated for home heating (thus claiming a longer life than otherwise). One might consider other alternatives and cost comparisons to obtain a more accurate picture of all of the cost implications of DHWs for space heaters such as:

  • The requirement of a second water heater for domestic hot water
  • Comparing two water heaters and their installation, operating cost, fuel efficiency, maintenance cost, and life with a high quality cast iron boiler with a high efficiency, attached to an indirect-fired water heater - operating as an additional zone off of the heating boiler.

The following note from the end of the article is also significant for what it says and implies (in our OPINION) - Ed.

Manufacturers of domestic hot water tanks should make design improvements in the area of efficiency and life span of the tanks. Side wall venting should also be incorporated into future designs.


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