Restarting gas furnace


Old 09-20-03, 03:16 PM
Mark B
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Restarting gas furnace

I turned off the gas to my forced air furnace a month ago to run new gas pipes in the kitchen, then neglected to turn the gas back on to the furnace since the weather was still warm. I turned the gas back on today, using the in-line valve, and the furnace won't ignite or run. It's a Lennox, circa 1990 installation (model GH19P3/4-100). Can't find an operating manual on the internet to help me. It has electronic ignition, according to the instructions on the service door panel. I'm not sure if this means it has the hot glow ceramic ignitor or not. There is no manual pilot igniting switch or button on the unit. I've turned the pilot light switch on the Honeywell control unit to 'on' (there's no 'pilot' mode, just 'on' and 'off') and let it sit for a few minutes at a time, but no ignition (the house temp is 60 degrees,and the thermostat is set at 70). All of the electrical connections are secure, the service switch is on (never turned it off when I shut off the gas).

Any idea what could be wrong? It worked fine all last winter and I had it serviced just before last winter season began. Someone mentioned that air could have built up in the line between the valve and the control unit (about 2 ft. in distance). Thanks for any advice.

Old 09-20-03, 07:41 PM
Jay11J's Avatar
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Location: Rochester, Minnesota
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You will have spark igntion system.

When you turn the t-stat up, for call of heat. spark should start snapping, and shoudl hear it..

I bet it maybe in lock out.. Leave t-stat on "heat" and higher than room temp.. turn power off.. wait a few sec, then turn the power back on at the furnace.. You should get snapping sound till a pilot lights.

You may of had air in the line, and went into lock out.

If that don't work.. If you have "Auto-Fan-On" on the t-stat.. switch it to on and see if the blower runs.. if it runs then you have power for the t-stat..

Try those and get back to us...
Old 09-20-03, 10:23 PM
Mark B
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Jay- I've tried switching the furnace on and off numerous times, hoping to hear the ignition try to start up, but nothing...I probed around the circuitry with a meter and it seems that my problem may be in a faulty circuit somewhere. I get full voltage to the input for the 24 v. transformer, then nothing coming out. I then checked out the transformer and it is OK, but I don't have return juice on the neutral leg. So, I may have a defective circuit, which would keep the furnace from starting.

I'll be changing the service switch, which seemed loose, since it didn't completely cut off electric power to the unit in the 'off' position. Maybe that's all it is. I'll also be replacing a funky switch that is labeled 'fan operation' that virtually came apart in my hand when I tried to turn it off (old house with funky wiring!). Seems that this was a leftover from the original furnace setup that allowed you to just run the fan for air circulation, and it may just have been left in the curcuitry to complete the loop without rewiring. I'll post again when I do this and see what happens.


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