Firing issue with Carrier condensing furnace

zepfan

I installed four new burners in a Carrier condensing gas furnace model# 58MXA080-F-1-12, that has been converted to PROPANE . The old burner's ends were collapsed. The furnace is from 2001. After installing them, and making sure they are aligned, I am getting a backfire when the furnace lights. I have checked to make sure that the manifold pressure is good (10" wc), and double checked the alignment. The furnace uses an igniter.
The furnace at times will light off smoothly with no issue. But for every smooth light off, there are four to five mini explosions when the furnace lights off. I have worked on these furnaces before, and know that they are prone to leak in the secondary heat exchanger, but with this one I see no evidence of water like it has been leaking. It just will not light off properly. Any assistance will be much appreciated. Thanks to all

captainco

It would be a lot easier to solve a problem with some combustion analysis numbers.

Ironman

Make sure the crossover passages between the burners are clean and properly aligned. Even new burners can be oily and that will cause dust to stick and block theses narrow passages. If that's the case, take them out and wash them in hot water and blow the passages dry with compressed air.

MilanD

The age is prime for the inducer motor fan (plastic fan disc mounted inside the furnace on the shaft of the motor, not the motor itself) to break off or crack. This could explain it, maybe? I have the weathermaker version of this one, and mine at one time after both primary and secondary heat exchangers were replaced under warranty, would intermittently have issues with the vacuum switch making because of it. We were done replacing 2 inch pvc vents with 3" before we figured that one out. Depending on the cracking, it may cause quick drafting issues that would manifest as backfiring, but not show up when you measure it. Pull off the fan from the back of the secondary heat exchanher, and make sure it's solid and not cracked. Maybe.

Harvey Ramer

I guessing Ironman probably hit the nail on the head. I've had to take those assemblies apart before and wire brush them to clean out the crossover passages.

Tim McElwain

That is all correct and I have had new burners sent that the crossover was not fully open.

Steve Minnich

Spider webs will stop or slow gas big time in the crossovers.

zepfan

Thanks to everyone for the responses. The secondary heat exchanger turned out to be bad. When I went back to the job, and got the furnace to fire off, The co level in the exhaust pipe immediately jumped to well over 600 ppm. I shut off the gas, and am working with the homeowner to get the heat exchanger replaced under warranty, or replace the entire furnace. It must be restricted somewhere, and that was what was causing this backfire issue. I have never seen one do this before, but now I see where a bad heat exchanger would cause this. Thanks to all.

Tim McElwain

Usually caused by a build up of condensate and failure to flush the secondary heat exchanger annually. It eventually corrodes and blocks movement.

MilanD

Interesting indeed! Glad you caught it. When monkeying around that secondary HE, do double check the fan blade assembly on the inducer. Mine was cracked and broke off when it was being removed, without us noticing it was broken...