Case of the Furnace With No Flame

Recently the gas furnace in my rental home in Winnipeg stopped, and it was freezing out. I knew the previous renters replaced the furnace motor at my suggestion for a rent reduction. It had an electronic spark ignition for the pilot light. I had a wire beside it to sense the flame temperature. The sensor wire was free to rotate in the bracket. I surmised that the wire moved out of place. If the wire were out of place, it would stop the furnace from going from pilot to operating mode.

My first move was to use adhesive, which could secure the rotating sensor wire in place even at high temps. Then I bent the wire so it sensed the outer cone of the blue flame, rather than the center. Voila, the sensor heated up quickly, and the main flame started a few seconds later. The furnace appeared to be fixed, and the wire shouldn't drift with vacuum service.

It was wise that I had used insulated-handle pliers to adjust the flame sensor wire as it tends to arc from the igniter to flame sensor when metal fills the gap and the pilot arc starts. Otherwise you have to hit the furnace kill switch, adjust it, then turn it on and wait 45 seconds or so until the arc starts on the pilot gas.

So it was fine except that half the time the furnace motor wouldn't cut out after the flame stopped. Yet recycling the power on the slow-speed switch ON reset the motor to the off state. Hmm. Perhaps excess dust was insulating the hot air sensor now, and the reading tool was offset on the high side from stiction with hysteresis on the threshold. Recycling the power seemed to fix it.

On checking the spring thermostat on the furnace hot air duct, I found it just needed to be raised on one of three settings, namely the one for flame-off low-temp fan cutout. I adjusted it from 90F to 100F, and that fixed the problem. Obviously the spring had gone out of calibration somehow.

Then, two days later, the thermostat stuck on the ON position even when disabled. A low battery indicator was on the digital thermometer. Normally this would not leave the furnace stuck ON, so I bought three new AA batteries and hoped that would fix that issue. Removing the panel from the wall stopped the furnace even when set to the OFF state. I lucked out. I didn't really want to buy a new $5,000 furnace now after spending $5,000 on materials and tools to rent this house.

This entry was submitted by Anthony Stewart and edited by Rob Spiegel.

Anthony Stewart started life as an EE design engineer in 1975. He has worked in telemetry design, aerospace design, and nuclear robotic inspection design. He has also designed T1 test equipment.

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