Gas and Oil Home Heating Furnaces - Reznor F100 pilot will not stay lit

A long story as short as possible. I have a Reznor F100 ser# ANK31k5n65696 manufactured Nov. 1988. Since then it may have run 90 days. The pilot light will not stay lit. The thermocouple has been replaced 2 or 3 times, the thermostat wiring has been replaced, I have jumpered the thermostat wires to test the thermostat, I have replaced the gas valve and the pilot assembly. The pilot will not stay lit after holding the red button in for a slow count of 60. Now what!

I presume the pilot burner lights as long as you are holding down on the control button.

I further assume that the pilot flame engulfs the end of the thermocouple.

Check to see if there are any wires or leads coming from the thermocouple or the fitting where it fits into the gas valve. Such leads could lead to a resettable limit switch that checks for a plugged venting system.


I'd verify that the thermocouple is producing 15-20 millivolts when it is heated. Unthread the thermocouple from the gas valve and measure the millivolts from the copper tubing to the center of the thermocouple fitting that screws into the gas valve.

Next I'd measure the resistance of the magnet circuit in the gas valve itself. Turn off the power to the furnace, and set your multimeter to measure oh-- in the range of 20 ohms or so.

Connect one probe to the furnnace chassis and insert the other probe into the gas valve thermocouple fitting, right in the center where the insulated end of the thermocouple makes contact. If that is an open circuit, the gas valve is defective and needs to be replaced.

If it's a short circuit (zero ohms) the gas valve also need to be replaced.

There are other things to test, but that's a start.

I presume the pilot burner lights as long as you are holding down on the control button. Correct

I further assume that the pilot flame engulfs the end of the thermocouple. Correct

Check to see if there are any wires or leads coming from the thermocouple or the fitting where it fits into the gas valve. Such leads could lead to a resettable limit switch that checks for a plugged venting system. There are 2 wires that plug on to spades of a small plastic part, the part is then inserted into the valve. The other end of the plastic part has a round tab that the thermocouple contacts when screwed in. These wires run into the side of the furnace, there is no resettable switch visible. I still need to pull the outer skin off that side of the furnace to see where those wires, and the 2 red wires that plug on to spades near the pilot gas supply tube go to.

I'd verify that the thermocouple is producing 15-20 millivolts when it is heated. Unthread the thermocouple from the gas valve and measure the millivolts from the copper tubing to the center of the thermocouple fitting that screws into the gas valve. Do I need to heat the thermocouple or is this it’s state at rest?

Next I'd measure the resistance of the magnet circuit in the gas valve itself. Turn off the power to the furnace, and set your multimeter to measure oh-- in the range of 20 ohms or so.

Connect one probe to the furnnace chassis and insert the other probe into the gas valve thermocouple fitting, right in the center where the insulated end of the thermocouple makes contact. If that is an open circuit, the gas valve is defective and needs to be replaced. This is going to take three hands, or some creativity. I understand it happens, but it is a new valve and thermocouple.
I still have to clean off the 5-6 inches of snow we got today, so I may not get this done tonight.

If it's a short circuit (zero ohms) the gas valve also need to be replaced.

There are other things to test, but that's a start. If you want to list any more tests you would like to see done don't hesitate to point me in there direction.

Thanks for the input

I'd concentrate on where those wires coming from the thermocouple are going. You can test those with an ohmmeter and more than likely you are going to find an open circuit, which is why the pilot wont stay lit. You can verify that by connecting the two wires going to the thermocouple together and you will very likely find that the pilot light will then stay lit.

But those wires are leading to a safety switch of some kind with has opened, so it's not safe to operate the equipment until it's been checked out and repaired or verified that the equipment is safe to operate.

Most likely the switch is a draft limit switch that opens when the furnace isn't venting properly and the combustion gasses are coming into the dwelling space. The limit switch you are looking for is probably in the draft diverter, a sheet metal opening in the furnace above the heat exchanger but below the vent pipe.

OK SeattlePioneer I'll start there. Talk to you later in the day.
Thanks