I have a 20year old Heil gas updraft with electric start and it has stopped starting. I checked out all electrical connections and the pressure switch and all wiring seems to be OK. I cleaned the contacts anyway and checked all wires connected by wire nuts. I also had a heater man come out and he said the unit checks out fine. Of course it ran most of the time while he was here.

I can tap the heater or more particularly the manifold and it will start and run anywhere from 20 minutes to 5 hours and then stops again. Flame is good when it is running.

Bad ground!

Thank you. I re-tightened the ground but still nothing... Bob

You have a intermittent problem and they are almost impossible to find as you have found out. I would start by hooking a volt meter up to the gas valve and when the furnace does not run see if you have 24 volts to the gas valve. If you do not work your way back up the control system until you find the defective item. The reason I mentioned the gas valve first is because it is mounted onto the manifold. Good luck.
BTW if you do have voltage at the valve and it does not open you have found the problem.

Thanks, I will try that at first chance. This is a big help. Will let you know how I make out.

Bob

Go to the sticky at the top of the heating forum and find the sequence of operations for a gas furnace. Follow your furnace and see where the sequence stops. Then check that component.