The relay between 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.
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 through before additional 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 absorb more heat, according 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.
The fan limit switch on these gas and oil furnaces energizes the fan when the heat exchanger reaches a certain temperature. For most thermostats, the white wire runs to the fan limit switch. It acts as a safety device, because if the burner gets too hot, the white wire will shut it off and keep the fan running to dissipate heat.
The heating relay is separate from the fan relay. The control board has several safety switches before it goes to the furnace to keep it from overheating. Electrical heaters should have a thermal safety fuse installed by a Denver furnace repair company that will physically melt and break the electrical connection if the furnace gets too hot.
Because furnaces are so dangerous, they have numerous fail safe controls, some of which may be manual reset. This means it will stop your house from burning down, but you may have to call a Denver HVAC technician to come reset the fuses. If the switch was tripped for no apparent reason, ask the technician which switch was tripped so that you will know where the problem is if it happens again. Most fuse and breaker switches can be replaced at a minimum cost.