Many people use a water heater for radiant heat instead of a boiler because it COSTS LESS. Of course, cost is important, but there are many other important factors as well. A high quality water heater is more energy efficient and friendlier to the environment. Quality water heaters will be made of stainless steel for many years of trouble free service. It is important to note that the same cannot be said for cheap, low end water heaters.
Why a water heater and not a boiler?
Radiant heat runs on warm water. Boilers are designed to make very hot water for radiators. Many cannot run well at lowered temperatures. They require additional components so they can provide lower temperature water without damage. Many boilers are subject to “cold shocking” damage when a lot of room temperature water comes back upon start up unless more expensive controls are added.
A water heater should not be used as a cheap shortcut for radiant heat. We only recommend water heaters if they will be high efficiency units. These units must be made with long lasting materials and be powerful enough for the job. But water heaters are designed from the beginning to operate at lower temps. Lower temps are good for efficiency. Modern water heaters can condense the flue gasses right down to water in a simple non mechanical way because of their low operating temperature. Boilers are made to boil and they do not condense as well as a unit that is designed to operate at low temperatures.
The Polaris and Noritz water heaters are two examples.
At Radiantec, we look for units that have a large submerged flue and units that vent with plastic pipe. In that case, almost all of the heat goes into the heating system and little heat is lost up the chimney. The exhaust is so cool that it cannot melt plastic.
An additional benefit is that no chimney is needed at all with plastic vents. Direct venting is possible with high efficiency units, and this saves money.