I checked. Quite clean really. I don't know how many hot air furnace designs there are but this one looks very robust against buildup. There are really no small passages to clog. I am sure if the burner was adjusted poorly soot would cover all of the heat exchanger area but again, no small passages that would be easy to clog.kevm14 wrote:I also need to pull the flue stack off the furnace and check for soot buildup. I believe my furnace runs quite cleanly but I haven't checked this area yet.
I have the LB168, biggest "low boy" in this series. Fun fact: divide output BTU by input BTU to get rated efficiency. Mine is 80% which is quite standard for oil hot air. So I pulled the big 8" flue pipe off. I am not sure the guy who serviced this 3 years ago even did this. If not that it's been something over 3 years since this was cleaned. It's not bad at all. That big hanging plate I think serves to slow down the heat and transfer to the heat exchanger (and air) before rushing up out of the fire box. Junk sitting on top of it may even increase the efficiency of the unit. I vacuumed it out anyway. 10 minutes later, clean. Looking at these again, this stuff mostly didn't even look like soot. Almost seemed like the inside of the heat exchanger was painted or coated and this was the coating having peeled off. If that is the case, it has probably been a LOT longer than 3 years since the upper part of the heat exchanger was cleaned. I'm going to say the heat exchanger actually looks quite good.
Now, for the SECOND half of this....of which I have no pics, see that long tube that goes away from the camera in each shot? Best I can figure that is some kind of a soot trap. That pipe is a dead end but is part of the fire box area in that it probably gets quite warm when the burner is on. Right below that pipe is the return area with the filters, blower and so on. So some heat actually pre-warms the return air before getting blown up around the sides of the fire box area. I removed the access cover and vacuumed it out. It wasn't terrible but there was still probably 1/8-1/4" of crap at the bottom of it. Worse though, the very end of the pipe is actually rusting, providing a direct suction path from the flame area into the return air that eventually comes out of the vents. This is likely why I smell diesel byproducts sometimes.
I tried a few different things like cutting out a patch from an old, unused 6" flue pipe. I ended up losing one of my self tapping screws and gave up on the patch. I just shoved some pink fiberglass insulation in there to try stopping the majority of the air flow. I think what I'll do is patch it from the other side next time I change my filters.