Secondary heat exchanger failures

zepfan

We have had several Bryant and Carrier furnaces (mostly the carrier 58mcb models) that have had the secondary heat exchangers fail. Most of the furnaces were made between 2004 and 2011. In all cases we try to get the customer to take the manufacturer‘s credit, and replace the furnace. In some cases the customer chose to replace the secondary heat exchanger.

Most of the time that fixes the issue, and the furnace with the new exchanger operates safely. However in two cases that I know of the furnaces still produced excessive levels of CO in the stack (over 800 ppm) I went on a call yesterday with a Carrier 58MCB furnace that was manufactured in 2011. Sometime between 2011and 2019 the secondary heat exchanger was replaced. The furnace yesterday was producing 30,000 PPM in the stack. The homeowner called when they noticed the odor outside when the furnace was on. Obviously I shut the furnace off, and we are working with the homeowner on replacing it. We did not install the furnace or the replacement exchanger.

My question is in both cases if the secondary heat exchangers are for the most part new, the furnace is firing at the rated BTU input, manifold pressure in range, flue and combustion air pipes are the proper size, and un obstructed ,why then would these furnaces be producing an incredibly high level of CO? Has anyone else had this experience?

Carrier has not been a lot of help on this issue. It also has been my experience that the Carrier/ Bryant brand even when new produce the highest levels of co, as compared to other brands. Not a dangerous level when new, but higher thenthe rest. Thanks to all.

Jamie Hall

The only way you can get CO levels that high is with insanely poor combustion — in fact, it's kind of a wonder that the combustion is even remotely stable. I'm not familiar with the geometry of the fire side of those furnaces, but for some reason you are managing to burn way rich or, possibly, too cool where secondary combustion air is introduced, if that's a feature for complete combustion to take place.