On average, 15% of a commercial modulating/condensing boiler’s runtime is spent at high-fire, according to Jim Schnorr, president of Lancaster, Pa.-based Thermal Solutions Products. The unit coasts along at medium- or low-fire for the other 85% of the time. This is the “sweet spot” in which the unit is condensing, providing the high efficiency it was designed for.
Ideal condensation — which facilitates high-efficiency operation — occurs in the heat exchanger when the difference between supply and return water temperatures (Delta-T) is near or greater than 20° F.