Central air fan will not shut off after heater stops.?

Question

After the ambient temperature in the house reaches the setting point, the gas heater turns off itself, but the fan still stays on indefinitely. I tried to leave the fan switch at ON and AUTO; but still the same. This central air heater is perhaps 24 years old. What could be the root cause ? Bad thermostat, relay switch, or control board. Please, help with your advice. Thank you.

Answer 1

Get "the man" out to investigate and fix it.

Answer 2

For a system of this age, two possibilities come to mind:
First, and most likely, is the relay that supplies power to the fan has burned contacts and they are sticking together. Replace the relay.

Second possibility - The Thermal Switch (mounted on or in the plenum) is sticking, similarly to the relay. Replace the switch.

To determine which is the cause, carefully (WARNING / DANGER - DANGEROUS VOLTAGES PRESENT AND INJURY/DEATH CAN RESULT FROM ELECTRIC SHOCK) check the voltage at the coil of the relay. If the control voltage to the relay drops, but the relay does not, bingo, bad relay. If the control voltage never drops, it's the thermal switch.

Other possibilities exist, but these are the two most likely.
Good luck, and please be careful.

Answer 3

Heaters are designed to have the fan stay on after the funace/ heating elements have shut off. They serve the purpose of cooling those elements off after they are finished as well as getting that air you paid to have warmed circulated. This usually only last for a minute or two after the heating source has switched off. Could depend on the age of the unit.

Answer 4

ARE U SWITCH IT TO FAN IF NOT CALL OVER a friend it not too expensive

Answer 5

change relay switch.