Fixing A Furnace

It’s the eve of Christmas Eve, and when I returned from running a couple of errands this afternoon my mother noted that while the thermostat was set to 68 the temperature was 60. Um, hmm. “Well, it’s 2:30, call someone right now,” I said. The burners in the furnace would light but no main blower activity.

A skinny guy named Red rolled up in his van and took a look at it. He mucked with it a bit and it started working fine. “I don’t know why I’m even here,” he said. “It’s all working fine.” I explained what I’d done before he got there, that I could see the burners come on but the fan wouldn’t go.

“That makes no sense,” he said. He explained how the automatic fan control works, and if the burner was going it would turn the fan on.

“Could the automatic fan control have been stuck or something, and you’re moving it unstuck it?” I asked.

“Maybe… I don’t know. I’ll probably get half way home and it’ll fail again, since we can’t figure this out for sure, but it’s working fine now.”

So he leaves, and I turn the furnace back on because I want to see it work. Through the little port you can see the igniter glow, the burners light… and then they shut off. WTF. I call him back, explain what I see. “It’s your flame sensor. I’ll be right back out,” he announces, and hangs up. Twenty minutes later and some sandpaper on the flame sensor (to remove soot) and it works again. Yea!

Some observations:

A) Furnace manuals have approximately zero troubleshooting information in them. There isn’t even a mention of the flame sensor in this one. I find that odd, since the flame sensor is a big reason furnaces will stop working. A simple table of failure modes and potential solutions would be nice.

B) When you’re troubleshooting appliances it isn’t enough to see something running. You need to watch it for a little while to see if it stays working. In this case, before the tech got there I’d seen the igniter light the burners, but I didn’t watch to see if they stayed running. Of course, they worked fine while the tech was here…

C) Why do product designers place screws behind things like the gas pipe to the burners? I think every engineer that designs a product requiring maintenance should be required to take it apart at least three times, with tools normal people will have, before the design is finalized.

Hey, at least I know how furnaces work now.