Many residential hydronic baseboard systems are designed to operate at water supply temperatures of 180–200 degrees F whenever there’s a demand for heat. Their control method is simple; when a thermostat calls for heat, the circulator is turned on, and within seconds boiler water is streaming through the distribution piping and baseboards. If this water is already hot, (say from previously supplying a domestic hot water load), and the distribution piping and baseboard are relatively cool, the familiar snapping and popping sounds of the expanding piping indicates the heat is on.
While such sounds may be a familiar — even reassuring — sound in some homes, they are very unwelcome in others, especially when loud enough to wake occupants in the middle of the night.