First time posting, and I need some help.

For the last three weeks my wife had noticed that sometimes the heater would not kick on. By the time I came home to check, the heater was working.

The house got cold about 2 this morning, and the heater would not click on. I moved the tab and heard it click, but nothing else.

I switch it back and forth from heater and air conditioning, thinking it might have stuck. That didn't work. I noticed that when I pushed the switch for the fan, the fan came on.

The unit is about twenty years old. What would I check next? Could it be the thermostat going bad? My email is mitch9518@netzero.com

Thanks