I have an old furnace-20-30 years old which the fan doesn't turn off. We have an old Honeywell thermostat with a the mercury tube inside. I was thinking I could just replace it with a newer programmable one. Will this solve the problem? Is there anything else I can try before replacing it? I have no experience with thermostats but am somewhat handy-I've repaired a dryer, have tools and know a little about electronics. Thanks for any help!
Sam

Your furnace probably contains a fan and limit control. That is probably the cause of the problem and not the thermostat.

Your furnace probably contains a fan and limit control. That is probably the cause of the problem and not the thermostat.

How do I find that and reset it? It's an old Williamson Temp-o-Matic furnace. Thanks for the help!

There is usually no reset for a fan and limit control. Usually it is a
Rectangular box with wires going to it.
.
Sometimes if you tap on it it might stop blowing but it will need to be replaced.
It is probably as old as the furnace and has seen better days.