The Capacity of a Boiler Heat Exchanger in Gallons

Boiler Systems

• Boiler systems -- also known as hydronic heating systems -- supply heat throughout a home or building using water as a heat-containing medium. A boiler typically works as part of a radiant heating system made up of individual heating zones. Each zone uses a baseboard or radiator unit to circulate heat within an area. Boiler systems burn fuel, such as oil, coal, electricity, gas or propane to produce the heat required to heat the water. Water temperatures can go as high as 200 F, though normal temperatures range around 120 to 130 degrees.

Boiler Heat Exchangers

• Heat exchangers operate as the heat-producing component within a boiler system. A heat exchanger sits overtop the fuel burner and underneath the boiler tank. This design allows heat to rise from the fuel burner into the heat exchanger. The heat exchanger then transfers this heat into the water contained inside the boiler. The heated water moves through coils that run to different areas, or zones within a home. Since boiler heaters work within a closed system, baseboard or radiator coils send the cooled water back to the boiler tank for reuse.

Water Recovery Rates

• Water recovery rates in boiler systems determine how quickly a heat exchanger can heat the water in the boiler tank. A boiler’s heat exchanger transfers heat into the boiler tank at a rate that matches the size or capacity of the tank. A boiler’s recovery rate is determined by the number of gallons of water the heat exchanger can heat to 90 degrees in an hour’s time. To keep up with boiler tank capacities, heat exchangers must heat at least 1.5 times the amount of water contained in the tank within an hour’s time. For example, a boiler system with a 40 gallon tank would have to heat 60 gallons of water to 90 degrees within an hour’s time.

Recovery Rate Effects

• Since a boiler functions inside a closed system, a heat exchanger’s water recovery rate determines the system’s heating efficiency. As a closed system, water temperatures drops occur when water returns from zone baseboard units. For some systems, this means anywhere from 20 to 30 gallons of cool water goes back into the boiler tank. When this happens, tank water temperatures drop. Since normal operating water temperatures range around 125 F, the heat exchanger has to work quickly to get water temperatures back to normal. In effect, slow water recovery rates limit the amount of heat a system can generate.