How I handled a cracked heat exchanger on my old furnace - what I'd do differently

My situation was very similar a few years back when I owned a Lennox unit. After a technician confirmed excessive gas pressure causing damage, the replacement quote was around $3,800. My furnace was about ten years old, wich made the decision really tough. I was worried about the longevity of the old unit even after fixing the immediate problem. I decided to go with replacement because the unit was starting to fail in other areas as well. It was making weird squeaking noises and the main board had alredy shown signs of strain. I opted for a slightly cheaper, but highly-rated, unit from a competitor instead of sticking to the same brand, which made the overall replacement cost significantly lower. The new unit was only $300 more expensive than the repair, which made the difference easy. I would suggest looking closely at the condition of other components, like the blower motor and heat exchangers, before committing to the repair. Sometimes if you have to replace two things simultaneously, you might as well just put in the whole thing.

One thing I'd add is to ask if they'll do a combustion test after any repair. We had marks on our exchanger a while back and the second company found the real issue was a cracked vent pipe letting the burner run wierd. Fixed that for a couple hundred bucks instead of the exchanger story I was hearing. Might be worth getting someone else to confirm the diagnosis before you spend anything big.

Been fiddling with gas furnaces since the days of pilot lights and standing pilots, and one thing I learned the hard way: verify the input yourself. Old trick is clocking the gas meter, watch the dial for a minute with everything else off and compare it to the rating plate. Takes ten minutes and tells you if the 4.5 figure is even real. I had a tech tell me my old clunker was overfired once, turned out his gauge was off and the furnace was running right on the money. Get the numbers from the meter before you sign anything.

One thing I'd want pinned down before spending anything is what exactly those marks are, because discoloration or flaking on the exchanger gets called damage a lot faster than it gets called a crack. I had a tech tell me mine was finished years ago, and it turned out to be surface oxidation you could wipe with a finger. Ask them to show you the actual breach, ideally with the camera photo they surely took, and get the diagnosis in writing on the quote. If they can't point to a specific split or hole, the 4.5 figure becomes a much bigger deal than some staining that may not matter at all.