My Tappan air handler won't turn off even when the thermostat is in the off position. The unti is 11 years old

Hi Sounds initially like a fan relay, however if you have put in a new thermostat make sure it is programed for the type of system gas or electric heat. Hope this helps have a great day.

Make sure the fan switch on the thermostat is set to "auto". The "heat-off-cool" switch on the stat should have no effect on the fan except to possibly change fan speed when changed from heat to cool.