I have totaline P374-1000 thermostat connected to a lennox heating unit. The heater goes on periodically even though the thermostat is in the OFF state. Is this normal or what might be the problem?

The biggest problem is the lennox furnace,,
Disconnect the tstat wires from the furnace end,, be sure to tag or mark each wire so you can put them back where they came from,, do this on a warm day that the furnace isn`t needed,, watch the furnace to see if problem is still there
If the prob. Goes away you can be pretty sure its either in the tstat or a short in the tstat wires between the furnace and tstat

Thermostat
Bought house with Gibson furnace
I do not have a book
Need instructions for wiring thermostat
It has heating and cooling

Jimjim,, you need to start a thread of your own, if you want the guys to see it and help you with answers