to the refrigerant coil in a Heat Pump system or a Central AC. Copper is often specified in supply and return manifolds and in tube coils. Heated water is pumped through the heat exchanger then back to the boiler to be reheated Sequence of operation Thermostat calls for heat Source of ignition is provided at the boiler Circulator initiates water flow to the hydronic coil (heat exchanger) Once the heat exchanger warms up, the main blower is activated When call for heat ceases, the boiler and circulator turn off Blower shuts off after period of time (depending on the particular equipment involved this may be a fixed or programmable amount of time) See also Forced-air gas Copper in heat exchangers References Heating, ventilation, and air conditioning