ASK THE EXPERTS: Combination Water Heating


I have a combined bath and utility room of about 180 square feet that sits inside a larger, unheated barn space. The floor slab has not been poured yet and I would like to use radiant floor heating for those two rooms. My plumber suggested using a conventional tank-style water heater to provide both domestic hot water and heat for the radiant floor pipes. He said he could rig up a heat exchanger on a conventional water heater but I thought it would be better to go with a product that is designed for that purpose. The problem is that virtually every combination water heater I can find is gas-fired. I would prefer using electricity because I already have a 4.5 kW PV system that can meet some of that load. Do you have any suggestions? In the future, I might consider adding a solar thermal system to preheat water.

Dennis Favello • Union Dale, Pennsylvania

Many people use conventional water heaters with radiant floor heat delivery systems. Numerous solar companies offer tanks with an integrated heat exchanger and electric element backups (see “Solar Hot Water Storage: Residential Tanks with Integrated Heat Exchangers” in HP131).

Consider the amount of heat that’s delivered for each type of fuel. Gas and propane heaters usually have 35,000 to 50,000 Btu burners and can normally heat a reasonably well-insulated area of 1,200 square feet, perhaps more. An element for an electric water heater is usually 4,500 watts, although 5,500-watt elements are available. There are about 3,412 Btu in 1 kWh, so a 4,500-watt element will produce a little more than 15,000 Btu per hour.

Even when a tank has two elements, they are never on at the same time, so the total heat output is limited to one element. For your application, this is plenty of heat—if the room is insulated. The electric water heater is a better environmental option if you have any excess winter production from your PV system or a significant part of your electricity is generated with renewables.

An external heat exchanger option suggested by your plumber is a good idea with either gas or electric heaters, but will require two circulator pumps. External plate-type stainless steel heat exchangers are very efficient, though, and, in some cases, more effective than integrated heat exchangers. The electric heater is also a better option for a backup for solar collectors, since electric heaters have fewer standby losses because they don’t need a flue pipe, which is normally needed to vent gas and propane water heaters.

Chuck MarkenHome Power thermal editor

Comments (1)

Fred Golden's picture

Can you buy and sell your electric with a on peak or off peak meter? I was offered a chance to sign up for off peak billing in Portland OR. The cost is basically about 10 cents per KW for a normal service, or you can sign up for off peak at 4 cents per KW, yet mid peak is most of the hours per week at about 8 cents per KW, while on peak is 12 cents per KW. If you can schedule to warm the room and heat the water tank during the off peak hours, and only run it at night, you can save a lot of money by heating at night.

For the size of room, it might be cost effective to run a 3 KW electric heater, if you will only be heating the room for a limited amount of time each day. Basically turn on the heater while inside, then adjust to say 45F to prevent the room from freezing while you are not there. Much less capital expense too. A 3 KW electric heater should cost no more than $150 and can be portable. Or buy 2 each $25 Wal Mart special 120 volt 1500 watt heaters, and be done with it.

Once you change to solar heated water, then consider your options. You can have a hot water coil, say 50' of 3/4" copper tubing and a small pump to move solar heated water through it. By keeping it all in drinking water tubing, I see no need for a heat exchanger. This tubing can be located adjacent to the walls, and heat the room without a fan to the desired temperature, depending on the solar hot water available. The pump will come on with a thermostat. I might also suggest a two position thermostat, with the second higher temperature for when the solar heated water is above 140F, to warm the room to a higher temp at that time, using more of the hot water.

If the room to be heated is larger than 180 square feet, then the heat load might be significant, and a 12,000 Btu ductless heat pump could be used. However that would be most effective only if the room is in the 250 to 400 square foot size range. The efficiency of a ductless heat pump ranges from 3 HPSF to 5 HPSF. So a electric heater can produce about 3,410 Btu's per KW consumed. A 3.0 HPSF heat pump will basically transfer 3 times as much heat (about 10,000 Btu's in this case) per KW of energy used by the compressor from the outside air into the inside air. To justify the $1,500 cost, it would need to be a much larger area. You can heat a 180 square foot room with a 1,500 watt $25 heater a number of years before consuming $2,000 in electricity.

If the room is insulated with 3.5" of fiberglass and outside is insulated with 2" foam board, then the total insulation will be about R20? This will slow the heat transfer, and also because it is indoors no air drafts will cause the air to flow outside freely. It would not take to much heat to warm such a small area.

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