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 Marken • Home Power thermal editor