Twenty-five years ago, just about every residential and light commercial hydronic heating system used a single boiler as its heat source. Most of these boilers had a "tankless coil" for producing domestic hot water. Most served two to four zones of high temperature heat emitters, such as fin-tube convectors or cast-iron radiators. Fuel was relatively cheap, and so was skilled labor. Energy conservation was hardly a priority. Oversizing was common and not viewed as a problem in the American tradition of "Bigger is Better."
Times have changed. Today, many residential and light commercial hydronic systems serve multiple and diverse loads, such as space heating, domestic water heating, pool heating and snowmelting. At times, the instantaneous demand for heat can be very large, while at other times, a mere trickle of heat is needed by one small heating zone.