In a typical radiant heating system, hot water circulating through pipes installed in the subfloor or under it warms the floor, and the heat rises by convection to warm the room. Boilers that provided the hot water in older systems tended to be oversized; as smaller ones became available, people started substituting hot water heaters, because they are more economical. Not all water heaters are efficient enough to economically power a radiant heating system, though.
Types of Radiant Heating Systems
Early versions of radiant heat systems featured a dedicated boiler, and many modern systems that employ hot water heaters are similar. A second option, however, is to supply water for both the heating system and other household needs from the same heater -- which the Department of Energy estimates can produce a 27 percent energy savings for the average household. But in particularly cold climates, water in the under-floor pipes may freeze if it isn't constantly circulating, so a third option is to circulate glycol through the radiant heat system and pass it through the water heater using an internal coil.
Water Heater Efficiency
The main problem with connecting a radiant floor heating system to your existing water heater is that the heater may not be able to recharge quickly enough to handle the increased heating needs for the floor system. To keep the temperature of the water hot enough to warm the rooms where you have radiant heat, you may have to turn up the water heater, so when the radiant heating system is off, the water may be too hot to use. The person who designs your radiant heat system will probably recommend replacing your exiting water heater with a more efficient one, which will add to the installation costs.
The water heater you use to supply water for a dual-purpose system must be rated to supply both heat and domestic drinking water. Some jurisdictions may require you to install a timer on the pump that circulates water through the heating system, because without it, water could stagnate and become potentially unfit for consumption. This can happen in any plumbing system that remains unused for an extended period, however, so not all building inspectors consider it a problem. In all systems, whether they include a timer or not, a thermostat usually controls the circulating pump.
The feasibility of using a water heater -- rather than a boiler -- to power a radiant heat system ultimately hinges on how much heat is required to keep the space warm. If the loss of heat through doors, windows and unsealed gaps is too great, the water heater will overwork, driving up energy costs and leading to premature heater failure. As part of the installation of an effective system, be sure to add proper insulation, seal gaps with caulk as necessary, and retrofit old doors and windows with energy-efficient ones. These modifications drive up installation costs, but produce substantial savings in the long run.
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