One burner not efficient, heat exchanger clogged with soot.

[diamond dogs]

Hi, new to the forum, first post. I have an older (92) York 32000 btu 80% furnace in my addition (just 2 burners). Thankfully I have a carbon monoxide alarm in there, because it went off today.

After shutting off the furnace and airing it out, I observed that one of the burners had some soot marks around it and was burning without much force / yellow flame. checked the vent and it was coated with soot, but otherwise not clogged. so i guessed the heat exchanger might be clogged. I was able to look in the top after tilting the condenser up, and everything looks pretty clean in there, didn't notice any obvious leaks. took the exhaust blower / plate off and it was full of soot, so was the heat exchanger for that burner, just absolutely caked inside. the included pic is after i vacuumed it out a bit. so im assuming that the middle section that i cant see or get to is also very clogged with soot.

so a couple questions, what might cause one burner to not work properly while the other is fine? a clogged tip (nozzle?) is the only thing i can think of. second, if i am able to get that working, I'm worried about the soot in the exchanger still blocking flow, how might i clean it? i thought of spraying a bunch of brake cleaner through there followed by compressed air? i believe that the carbon monoxide was coming out of the manifold because there wasn't enough flow to draw it into the exhaust.

im also wondering, is this worth fixing or is it time for a new furnace? I know its getting to be that age when i should start thinking about that anyway. unfortunately. i was going to save up and get some high efficiency furnaces for both this and the main house next summer. I could get a used furnace in the meantime, but the small size is harder to find (also its propane) and i cant really put a larger furnace in there because its in a fairly small room.

also the main house furnace has a strange problem as well. seems to work fine, but when i turn the heat down for the night, it goes past the temp i have set it to and doesn't come on. i have observed the exhaust blower going and the ignition element glowing with no gas. it has a new exhaust blower by the way.

any help would be appreciated, as usual this comes at a not so great time.

 

[Dana]

A hot air furnace that's 26 years old isn't usually worth spending any serious money on. If the heat exchanger is leaking carbon monoxide between the fire side and the room air side it's probably not worth fixing.

Before just replacing like for like, run a heat load calculation on the zone it's serving. In many cases a ducted or ductless cold-climate mini-split heat pump (yes, even in MN) will be more appropriately sized for the design heating load, and would provide high efficiency air conditioning in summer as well. There are also small hydronic air handlers designed to run off the water heater that can fill the bill for small to medium sized loads.

If using an online tool such as loadcalc or coolcalc for estimating the heating requirements, be very aggressive in your assumptions on R-values and air tightness or it will oversize the equipment by 50% or more. Ideally with 1 or 2 stage hot air furnaces the output would be no more than 1.4x the load at the 99% outside design temp, which is enough to cover Polar Vortex cold snaps, yet still deliver long, comfortable cycles during normal cold weather rather than the hot blast followed by the chill.

Some model numbers would be useful for making educated guesses as to what might be a suitable replacement for the defective York, or the diagnosing the symptoms of furnace that seems to be misbehaving.

 

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[WorthFlorida]

Good move on the CO detector. Every home should have one.
Your heat exchanger could be leaking from corrosion (rust holes). At the burners there is a primary air mixture adjustment. Check that it is clear and not covered by soot and spider webs. Soot could indicate an incorrect air mixture. Then I would get a furnace tech to check it out. A good one should have a CO test tool.

 

[WorthFlorida]

As far as your main house furnace it maybe the control board that signals to what turn on and off.

That picture, is the bottom that shows up black, it it steel or a big rusted out hole?

 

[diamond dogs]

thanks so much for the informative replies, sorry i let this thread die but i got swamped with holiday stuff. made some progress!

the small furnace in the addition, i ended up cleaning out the heat exchanger tube that was all sooted up. ran a bunch of brake cleaner through it as well as compressed air and a vaccum on the other end. i also took the orifice out and blew that out, although it looked perfect. when it was all back together it worked fine, flame seems identical on both burners now and no carbon monoxide. i checked the heat exchanger pretty good and cant find any holes or anything, other than the rust on the front part there which did not go through (the dark area on the bottm was just soot) i looked in the top when spraying the brake cleaner and figured any leak would show up with dripping fluid. so i think for now that furnace is ok, i will keep an eye on it..

the main house furnace has repeated the problem a few times, its verry intermitant. last time it did it, i saw a verry small flame by the igniter. again the induction blower works, ignighter glows, but no gas. im suspecting the gas valve, but i didnt think to test if its getting power the last time i had the issue. i will test that next time.

its an armstrong guj100d14-1a btw. I looked up the gas valve and its about $250, not really something i want to spend on a furnace that old. is it possible to repair the gas valve, if thats the problem?

thanks again.