It's the kind where there is an inducer (exhaust) blower. The inducer function gets proved via a pressure switch, sends power to the pilot valve and a thermostat switch heated by the pilot which when cold sends it a spark generator and when heated sufficiently, to the main gas valve.
I was trying to run down a problem that I think stems with that pilot t- switch (or maybe with the pilot flame itself) as the main gas frequently gets cut off while heat is still be called for. It keeps shutting off then quickly restarting. I don't know if that issue is related to what I discovered.
The burners are like four blow torches feeding into the heat exhanger.
Around the exchanger inlet by the third burner, I noticed some cracking and burned metal.
I thought uh oh. This could be bad.
So I ran the blower alone to check it out. As this is a downflow, the blower is blowing down through the exchanger and with the A/C coil under acting as a resistance, the air through the exhanger will be somewhat pressurized. With the blower running I put a lit candle near each entry into the heat exchanger. No sign of wind. No fluttering. Just a nice gentle pull on the flame *into* the HE probably because it was still hot and creating a slight chimney effect.
Whew, I thought.
Then I went back to my issue. While watching the burner operate I noticed that the flame on the third burner which started out looking like the others started to change a bit. I'd seen this difference before and thought maybe the gas flow was slightly impeded so I made sure the orifice was clear. Well, I started staring into the heat exhanger opening that it was feeding into and started seeing weird motions of the gas flame. I got to thinking that the HE might have a crack that is opening under heat.
So I immediately killed the heat, kept the blower going and got the candle and sure enough the flame blew around a little bit when held by the third opening. I think there is little doubt that there is a heat exchanger crack.
So I need a new furnace. Right away.
I know this cannot be ignored. The question is whether I need them out ASAP in the morning or I have a couple days to weigh proposals.
Before you answer:
Because this is a downflow and because the A/C coil means the HE is pressurized on the air side, it's not going to suck in fumes through the crack. In theory it could blow fumes out of the furnace into the room but I think I would see that when the flame is no longer being drawn into the HE. The crack is small enough not to overcome the inducer and general upflow of the hot gases.
There is a CO detector just outside the furnace closet.
Is this a busy time for HVAC because it's cold or a slow time because people are more concerned with Christmas shopping?