gas furnace, shutting down & restarting

Old 01-02-13, 05:47 PM
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gas furnace, shutting down & restarting

Hi folks. I'll try to describe the trouble as well as I can. Thanks for bearing with me.

I have a 1993 Lennox GCS16-511-125-4P HVAC unit in my side yard. The situation is more interesting because the access panel, unfortunately, is almost up against the neighbor's fence. (I hope to replace this unit soon!)

When I turn the thermostat on, what I get is something like this:

The unit's exhaust blower starts up and keeps going.

The gas unit ignites, burns for maybe 6-10 seconds, and the flame shuts down.

A little while later, it re-lights -- again, for just a brief time. This continues.

Meanwhile, the forced air cycles on for short periods of time (maybe 10 seconds to a minute) -- so I am getting some distribution of heat gradually into the house.

I thought the trouble might be the flame sensor, but (aside from the fact that I can't spot it in this unit) my understanding is that the flame sensor would shut down the ignition attempts after a few cycles. It does not shut down the ignition cycles. The cycles will continue to repeat until I turn the thermostat down.

My two return filters are pretty clean (I removed one entirely to see if that helped, and it didn't). Floor registers are open.

If y'all have any other ideas that a layman can check out, I'd appreciate it.

The next step is to call for service, which entails lifting out a section of the neighbor's fence to properly access this thing.

Also, how bad is it if I just let it run this way?


Old 01-02-13, 06:13 PM
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Do you have a Johnson gas valve or a Johnson ignition module on this furnace?
Old 01-03-13, 07:30 AM
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If you don't see a flame sensor then it's likely your HSI is also acting to prove the flame. As a HSI ages it can go out of spec and start causing this problem. Replacing it would be your cheapest and easiest first step.

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