Cracked Heat Exchanger???

FDSjr

I recently had my oil furnace tuned up (Carrier 58HU085). The Tech told me that I have a cracked heat exchanger. As far as I know, he did not do a visual inspection. At least he did not say that he did. He showed me a measurement that he called "excess air" going up the exhaust flue when the furnace blower came on. This measurement remained stable from burner startup until the blower came on. Then it increased and continued to increase until it eventually stabilized again, but at a much higher value. He said that this is a symptom of a cracked heat exchanger and that I need a new furnace. I am a little skeptical since the furnace seems to operate fine, but in all fairness, it is 20 years old. Does this sound legitimate and what would be a reasonable value for "excess air"?

Jamie Hall

And what were the excess air values? For an oil burner, 30% is about max (somebody can correct me!). But having it increase when the furnace blower comes on does sound as though the heat exchanger may be cracked. They do... and 20 years is a fairly good life for a scorched air system. It shouldn't increase.

I might add that if the heat exchanger is cracked, it's a big big safety hazard -- I wouldn't want to run the furnace until I had verified that it either is or isn't.

STEVEusaPA

Make sure all the cleanouts are replaced properly, new gaskets, etc. I'd like to see the rest of the combustion numbers.
30% excess air isn't always doable on older equipment. It's more important to have proper draft, all air parts (burner blower wheel, air gate, end cone clean), proper nozzle, zero smoke, clean heat exchanger and flue pipe. Then the excess air and other numbers aren't really adjustable--except to add a margin of error which usually decreases CO2 and increases excess air. Higher excess air numbers when blower comes on, lower CO2, change in CO are all indicators of either improper sealed cleanouts, or a cracked heat exchanger.
If the cleanouts are right & tight, and everything else is clean and correct, then it's probably the heat exchanger. He could pull the burner and look for a crack. If not, you can rip the unit apart post mortem and find it.

FDSjr

I don't remember the exact values of excess air. I recall that the initial value (burner startup until the blower kicked in) was stable somewhere in the mid-50s. The value began to increase immediately after the blower came on. It continued to increase until it stabilized somewhere in the high 70s.

FDSjr

Since the Tech had performed all of the maintenance on the furnace before he checked the excess air, I am assuming that everything was properly adjusted, etc. In addition to the excess air measurement, other results were Efficiency 82%, CO2 10%, Stack Temp 650, Draft/Fire -.01, Draft/Breach -.03, Smoke 0.