Boiler Size Calculator

How It Works

Boiler sizing must account for both space heating and domestic hot water (if using an indirect water heater). The space heating load depends on your home size, insulation, and climate. The domestic hot water load adds roughly 30,000 to 50,000 BTU for an indirect tank that heats water on demand. Modern condensing boilers operate at 95 percent efficiency or higher and can modulate their output down to 20 to 30 percent of capacity, which makes slight oversizing less wasteful than with older non-modulating boilers. Baseboard heating requires higher water temperatures (160 to 180 degrees F) than radiant floor (90 to 130 degrees F), which affects boiler efficiency since condensing boilers work best with lower return water temperatures.