Gas and Oil Home Heating Furnaces - Bad heat exchanger

Is it possible for a faulty heat exchanger to cause the high limit switch to trip? Our HVAC tech said the heat exchanger was cracked and causing the flame from the burners to back draft and trip the switch. The furnace is 13 years old but everything inside looks like new.

He observed this?: That perhaps the flame was being pulled down by the inducer motor perhaps? One crack? Several? Chunk(s) missing from it? ( A few years back I observed bizarre flame action in a newer high efficiency furnace I got called to investigate, and actually found a chunk missing out of it! Landlord had a new heat exchanger installed by a tech).

I am not a licensed furnace man, and what I am about to say is speculation:

I can't think how this would actually make the furnace inside get hotter though. I think on the furnace I had the chunk out of, was just upsetting the flame sense. But I cannot say for certain. It is possible that you have something going on that caused the exchanger to crack in the first place, where say there was too much heat retention isnside. Perhaps filter not being changed frequently enough, or insufficient cold air return or blockage, or too many heat duct dampers or registers closed/blocked, or an A-coil above the unit for a/c is all plugged up (goes hand-in-hand with not changing the filter), or from poor combustion and yellow flame it is all carboned up inside, or too high of gas pressure as some guesses. But -I- can't see, directly-related, how even an inverted flame would cause this temp rise in the unit.

Welcome others to weigh in on this.

He is probably talking about the flame rollout switch, which would possibly trip with a cracked heat exchanger. Either way, he should have done a visual inspection and notified you of the location of the crack at the very least. These switches are there for a reason, safety. If there in fact is a crack and the switch were to fail, then it could easily result in death. Get a second opinion.

If it is cracked, a decent service person can find it & show you the crack.

I have seen this happen before (flame rollout effecting the limit switch). In one case the cause turned out to be an extremely dirty heat exchanger. The second time it was because a squirrel went down the chimney and died right inside the homeowner's vent pipe close to the furnace.

If the heat exchanger looks like brand new inside, then I would check also to make sure the unit is venting properly. Is there enough combustion air? (It sounds like maybe there is not). Is the furnace in a sealed room or is there an exhaust fan that is robbing the furnace of the air it needs?

Your furnace may or may not have a cracked heat exchanger, but if your tech says there is one, then ask him to take some carbon monoxide readings and let him show you the readings. That will tell you for sure if there is a crack.

Oh yes, I saw this happening a third time. In that case it was a 35 year-old suspended unit heater with a rusted-out heat exchanger. It even had two visible holes in it.

When the fan came on it would blow air right through those holes and would partially blow the flame out of the heat exchanger. Of course, as soon as the main burner shut off the fan would also blow out the pilot light too.

I got called to that one because the guy was wondering how come his pilot light kept going out. :)