Furnace fan restarting

[oldguy]

Hi......my furnace runs until the thermostat reaches the set point and comes to a complete stop. Then just the fan restarts and runs for less than a minute. It only does this once in a while and yes the filter is clean. What might be the problem?

[Admin]

What kind of furnace do you have?  It sounds like you may have an old combination fan/limit control that may be wearing out.  You can adjust the fan off a little sooner or later and that may stop the fan cycling issue.

Here are some pics so you know what to look for.

It could also be a more serious issue like a chimney venting issue or cracked heat exchanger, where heat is migrating and fooling the fan control.

[oldguy]

We bought this house as a resale but it's quite a new house that we took possession of in the spring so the furnace is not old. The booklet that was left says the furnace is a Lennox model number EL296UHV. It doesn't have a chimney.

[Admin]

That model has a default fan off of 90 seconds.  So after the burners shut off when the thermostat is satisfied, the fan should run for 90 seconds then stop.

Could it be the thermostat or HRV that is causing the fan to restart?  I would remove the front door and on the bottom door there will be a small hole.  See if there are any error codes flashing when that happens.