jaredharley.com | Blog

I mentioned last week that our furnace was acting up. After crawling up there and restarting it, everything worked fine, so we thought the problem was a single occurrence. It turns out that we were wrong. Since that post, every time the furnace should have heated the house and didn't, we would flip the power switch off and back on, restarting the furnace. It would start up again, and we would hope it work. This led to several very chilly mornings.

So, I turned to the interwebs in search of information. It turns out a few other people have posted on home repair forums about the same model furnace, a Lennox G40UH(X) having problem heating. The combustion air inducer would turn on (the initial motor sound we could hear), and then the burners would light. They would burn for 4 seconds while the furnace checked for a flame, and when the check failed, the furnace would turn off the gas, reheat the ignitor, and try again. According to the maintenance manual, the furnace tries 5 times to light the burners, and if it fails, it enters a WatchGuard mode (the error code I saw blinking on the LEDs), and waits one hour to try again, preventing leaking gas or overheating.

The answer I found online said that the flame sensor sometimes needs to be cleaned, especially if there's buildup on the sensor. So today, I climbed into the attic with a piece of steel wool, found and removed the sensor, cleaned the buildup on it, and then put it all back together.

So far, the furnace has worked each time it should have - we'll have to see if we wake up to a house that's 60 degrees again tomorrow morning to see if my fix will be permanent.

For anyone who needs it (ie, Google searchers):

• Lennox G40UH(X) User's Information Manual [pdf]
  
• Lennox G40UH(X) Service Manual [pdf]