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An HVAC blower has two operating modes: intermittent and continuous. The fan setting on your thermostat allows you to select blower operation. For most people, the desired thermostat fan setting is “Auto.” The Auto mode means the blower turns on when the furnace or air conditioner cycles on and then turns off when the furnace or air conditioner cycles off. Conversely, if the thermostat fan setting is “On,” you'll get nonstop, 24/7 HVAC blower operation.
Why Not Continuous Air Circulation?
To some people, constant air circulating in the house — even when the AC or furnace is off — may be desirable. However, it’s worth noting the definite downsides to this option. The electric motor that drives the HVAC blower fan consumes a substantial amount of electricity, so nonstop blower operation will raise monthly bills considerably. Also, running the blower fan all day and night imposes substantial wear and tear on the blower motor, potentially leading to early (and expensive) replacement.
Why Does the Blower Run All the Time?
If the thermostat fan setting is “Auto,” the system blower should be turning on and off in sync with furnace or AC cycles. If the blower fan continues to run even in the “Auto” mode, fan limit switch problems are another possible reason.
The fan limit switch in the furnace air handler is the component that actually turns the blower on and off with HVAC operation. It also safeguards against dangerous overheating by shutting the furnace off automatically if the temperature in the unit rises too high. In certain circumstances, a malfunctioning fan limit switch may keep the system blower running continuously. This can be due to:
The override button on the limit switch is set to keep the blower fan running continuously.
The limit switch requires replacement.
Issues with thermostat wiring such as shorts or disconnected wires.
Because the fan limit switch also prevents dangerous furnace overheating, any problems with the furnace relating to this component requires diagnosis and repair or replacement by a certified HVAC technician only.
For more information about issues such as nonstop HVAC blower operation, consult the professionals at Air Assurance.