The relay in the middle of the air handler control unit and the blower motor is called a fan or heat relay. If the thermostat fan setting is "on" then the relays do nothing but keep the fan running. Otherwise, when the air handler controller asks for heating or cooling the relay switches the fan on.
Fan and Heat Relays in the Air Handler
The relay allows the fan a short delay to spool up before it starts heating or cooling. This increases efficiency by allowing th blower to push already conditioned air in the ducts straight through before further heating or cooling takes place. Instead of that air being wasted in the handler it is used for cooling your house.
Cool evaporator coils work best, so when the cooling cycle starts you will be operating at maximum efficiency. The cool coils can suck up more heat, agreeing to Denver air conditioning services. Some controls use a fan limit switch to set when the unit turns on or off. The fan limit switch is used on furnaces where the fan control circuit is mounted on the heat exchanger.
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