What is the best way to check for blocked heat exchanger?

Poconoplumber

I have a lennox LP gas furnace I was asked to work on in a vacation rental home. The exhaust fumes are bad. Everything I have researched comes back to blocked heat exchanger. I do not have model # for this post . Gas pressure is ok and flame is not effected when blower comes on. Without taking furnace apart, what is the best way to check for blocked heat exchanger (primary or secondary)?
Thanks

Tim McElwain

Did you do a combustion analysis? If so what was the draft reading first then what are the other readings? What do you mean the exhaust fumes are bad? Do you have a carbon monoxide reading?

Does this unit have a secondary heat exchanger? If so check on a possible plugged condensate line.

Tinman

There's also usually physical evidence of a plugged heat exchanger. Soot, burned wires, flame rollout, yellow tipping of the flame, tripped rollout switches.

Rich_49

Put a canary by the furnace and see if it dies .

RJ_4

Make sure that the flue is installed to code , and isn't one of your problems

icesailor

You can have a blocked secondary HX and not know it by how described.

I had a plumbing account with two gas WA units. improperly vented because they were too close to the ground and behind bushes. One showed a "sooty" exhaust. They both had higher than I felt were OK CO levels. I notified the caretaker to get it checked. The following Spring, nothing had been done. I checked them again and both had the same high (one was less than 400 PPM, the other was 90PPM. I called the caretaker, He swore I never told him. In spite of the large yellow lined legal note pad with the digital printouts stapled to it with information as to what was wrong on the kitchen counter. So much for caretakers checking houses. The building is next to a high cliff next to the ocean where the wind blows hard all winter. Salt corrosion every where. So, I called the gas provider who services the gas for the customer. They came out and looked. Said it was fine. Told them that 400 PPM was outrageous for that 90+ furnace. He checked the "Shotgun Burners". The cast iron middles were all expanded from oxidation cutting the gas flow down. A common problem for these type of burners that are close to the ocean. Such a problem that the gas guy went back to their shop where they had cases of replacement shotgun burners of the same type. He fired it off and proclaimed it resolved. I said "Wait a minute. Before you go, let me test it again." So I did. The good one was 1200 PPM. The bad one I shut off before it wrecked my instrument. The old, bad burners passed a restricted amount of gas and the HX would take it. They couldn't take the new ones. The gas guy couldn't give a rat's. I went on-line and found the manuals. The manuals ladder charts said that those symptoms meant a blocked secondary HX and couldn't be replaced. No how, no way. It had to be replaced and had a lifetime warranty.

I don't do replacements of HX's on old 90+ WA HX's.

If they are acting up, and you have serious moisture or humidity issues, check the burners. If they are bad, they can hide a problem.

IMO