How Long Should the Fan Run After the Furnace Shuts Off?

Does your furnace blower fan keep turning on and off, or run when your furnace isn’t heating? This is a common HVAC problem. Various underlying reasons can leave you asking, “How long should the fan run after the furnace shuts off?” 

The fan will normally kick in 1½ to 3 minutes after the thermostat signals the heater to start. It should then keep the blower running during the heating cycle. When the cycle ends, the blower fan should turn off a few minutes later. 

But if this isn’t the case, here are some possible scenarios:

The Blower Fan Turns On/Off While the Furnace Is Heating

The fan should not turn on and off while the furnace is in a heating cycle. To troubleshoot the problem, you can check the air filter. If it’s dirty, then airflow over the heat exchanger will be limited, causing the furnace to overheat and shut off, while the blower will turn on to help cool things down. A dirty blower fan in the air handler can cause a similar problem; wiping the blades clean can resolve it.

When these steps don’t resolve the issue or the filter/fan isn’t dirty, you’ll need to call a professional. Leaky or poorly designed ductwork, an oversized furnace, or abnormally low or high heat exchanger temperatures can result in the blower fan constantly turning on and off.

The Blower Turns On/Off Without the Furnace Running

A faulty fan limit switch is the most likely reason. It controls when the furnace blower turns on and off in response to feedback from a temperature probe. This allows the blower to operate when the temperature of the heat exchanger increases. But the heat exchanger’s temperature should be relatively constant when the furnace isn’t running. Fortunately, a professional can quickly diagnose the problem and repair or replace the switch.

It’s also possible the limit switch is in manual override mode. To check the switch, look under the furnace hood. The fan limit switch is a small white button or knob that, when pushed in, will allow the fan to run constantly. To reset the switch, pull it out (so it’s in “auto” mode).

The Thermostat Is Not Set Properly

If the thermostat is set to “on”, the fan will run constantly, including between heating cycles. Simply switch it to “auto” if this is the case. Then the fan will run only in conjunction with the furnace. 

Once you’ve reset the thermostat, lower the temperature to below that of the room. The system will trigger the furnace to turn off as it won’t be called to provide additional heat. Now, the fan should turn off in 3 to 4 minutes.

However, the furnace fan may not turn off. That can mean the furnace itself is running constantly, which might indicate something is wrong but can be normal especially if it’s a cold day. In abnormally cold weather, the furnace will work harder and longer to keep your home warm. Also, if you’ve set the temperature very high, the furnace and fan will run more to meet the demand.

Call Sky Heating & Air Conditioning

Our trained heating experts can look at your furnace if you’ve been asking, “How long should the fan run after the furnace shuts off?” First, look at some of the potential reasons above. But if you can’t determine the cause of the issue and resolve it, the friendly technicians at Sky Heating & Air Conditioning can help. They can correct any problem and are available 24/7 in an emergency. 

Maintaining your heating system can avoid many issues. Learn about our Signature Protection Plan, which includes a furnace and HVAC maintenance strategy. 

For assistance, schedule a visit online or call (503) 673-9083 today.