If your furnace blower keeps running after the burners shut off, that does not always mean something is wrong. In many systems, the blower is designed to continue running for a short time after the heating cycle ends. This helps move the remaining warm air out of the furnace and through the ductwork. The concern is when the blower runs much longer than normal, never shuts off, or keeps blowing cool air for an extended period.
A short blower delay is normal
After the burners shut off, the furnace is still hot. The blower may continue running briefly to move that remaining heat into the home and help cool the furnace. This delay is normal and may last anywhere from several seconds to a few minutes, depending on the furnace and its controls. If the blower shuts off shortly afterward, there may be nothing wrong.
The thermostat fan may be set to ON
One of the easiest things to check is the thermostat.
Most thermostats have a fan setting with options such as AUTO and ON. When the fan is set to AUTO, the blower usually runs only during heating or cooling cycles. When the fan is set to ON, the blower may run continuously, even when the burners are off. If your blower seems to never stop, check the thermostat fan setting first.
The furnace may be overheating
If the furnace gets too hot, a safety control may shut the burners off while leaving the blower running. The blower then helps cool the furnace down. Common causes of overheating include:
A dirty furnace filter
Closed or blocked registers
Blocked return-air vents
A dirty blower wheel
A weak blower motor
Restricted ductwork
An overly restrictive filter
Other airflow problems
If the furnace repeatedly overheats, the blower may seem to run for unusually long periods.
A dirty filter can make the blower run longer
A clogged filter reduces airflow through the furnace. When airflow drops, heat can build up inside the system. The furnace may shut the burners off on the high-limit switch, then keep the blower running to cool the unit. Check the furnace filter and replace it if it is visibly dirty. If the blower still runs excessively afterward, the system should be inspected.
The high-limit switch may be staying open
The high-limit switch protects the furnace from excessive temperature.
If the furnace gets too hot, the switch shuts off the burners. The blower may continue running until the furnace cools enough for the switch to reset. If the high-limit switch keeps opening, the real problem is usually overheating or restricted airflow. In some cases, the switch itself can also fail and cause the blower to run longer than it should.
The thermostat could be malfunctioning
A thermostat can sometimes continue calling for the blower even after the heating cycle should have ended.
Possible causes include:
A faulty thermostat
Loose wiring
A stuck relay
Incorrect thermostat configuration
If changing the fan setting from ON to AUTO does not stop the blower, the thermostat or furnace controls may need to be tested.
A control board problem can keep the blower running
The furnace control board determines when the blower starts and stops. If a relay on the control board sticks or the board develops another fault, the blower may continue running after the heating cycle is over. This can also happen intermittently, which can make the problem difficult to diagnose without testing. A technician can determine whether the control board is actually responsible or whether another safety condition is telling the blower to stay on.
The blower may be responding to a safety condition
Sometimes the blower is running because the furnace is protecting itself.
The system may have detected:
Excessive temperature
An open limit switch
A failed ignition cycle
Another safety fault
In these situations, the blower may continue running even though the burners are off. The blower itself may be operating correctly. The question is what caused the furnace to keep it running.
Why is the furnace blowing cold air after the heat shuts off?
Once the burners stop, the air coming from the registers will gradually become cooler. A brief period of cooler air is normal while the blower removes the remaining heat from the furnace. If the blower keeps running long after the furnace has cooled, though, the air may begin to feel completely cold.
That can happen because of:
A thermostat set to ON
A furnace that is overheating
A stuck control relay
A limit-switch problem
Another control fault
How long should the blower run after the burners shut off?
There is no exact time that applies to every furnace.
The delay depends on the furnace design, control settings and how much heat remains in the system. What matters is whether the blower behavior has changed. If it used to shut off after a short delay and now runs for a very long time or continuously, something may need attention.
When should you call for furnace repair?
You should have the furnace inspected if:
The blower never shuts off
The blower runs for a long time after every heating cycle
The furnace is overheating
The burners keep shutting off early
The blower runs continuously even with the thermostat fan set to AUTO
The furnace is blowing cold air for extended periods
The same problem keeps returning after a reset
Replacing the filter does not solve the problem
A technician can determine whether the cause is airflow, the thermostat, high-limit switch, control board or another furnace component.
Need help with a furnace blower that will not shut off?
If your furnace blower keeps running after the heat shuts off, the cause may be a normal blower delay, a thermostat setting, overheating, a limit-switch problem or a control-board fault.
Roberts Brothers Comfort technicians can inspect the system, determine why the blower keeps running and make the necessary repairs. We provide expert furnace repair service throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
