Bryant furnace cycling on limit

zepfan

Having an issue with a Bryant furnace model# 313AAV060110ABJA. made and installed in 2011. The furnace was locked out on limit. Filter was clean, reset the furnace and it ran for about 20 minutes with no issue, while a combustion test was being done on the furnace. All readings were normal with both the stack and undiluted CO to be well under 100 ppm. Around 20, and 35 ppm, temperature rise through the furnace was about 58 degrees, manifold pressure was right at 3.4" wc. Then the furnace cycled off on limit again. The furnace has two dime type limit switches in the middle of the furnace about six inches apart. The one dead in the middle with a temperature probe on the back of it was reading 110 degrees, the one to the right of it was reading 330 degrees. I took the limits out, and the did not see any cracks in the cells, and with an inspection camera I could not see any either. The only thing that I can think of is one of the cells on the right has a fairly large crack that I just can not see. I would think my co readings would be a lot higher if that were the case, but that is the only thing that I can come up with and was posting to see if anyone else has has a similar issue with this same model furnace. The furnace is run of natural gas, installed in a basement, and is an up flow application. Thanks to all.

EBEBRATT-Ed

So If I got this right you were reading 330 degrees on the air side of the heat exchanger (not the combustion side) while the furnace was running?

Doesn't sound good. Is it possible your probe was hitting the heat exchanger?

Probably time to strip the furnace a little to take a better look. Maybe make a phone call and find out if a heat exchanger is available before you do that.

Did you check the Co at the supply registers

mattmia2

Does the flame change when the blower turns on?

HVACNUT

How did you take the temperature reading, infared? 
If the readings are accurate, then it could be a blockage on that side but I think you'd see it with the flame pattern or test results. 
Pics?

zepfan

Thanks to all that posted. I measured the co at the registers and their was 0 PPM. The home also has multiple co detectors. The readings were taken with a Fluke thermocouple and was placed directly behind the dime type limit, at maximum 1" behind into the furnace jacket. The flame does not change when the blower turns on and the flame pattern appears normal.

captainco

Why us there a 110 degree limit in between the heat exchanger/ Was it replaced with the wrong one? The temperature in that location if centered should be 140 to 160 degrees, To the right or left it is going to be 330 degrees and up to 500 degrees right next to the heat exchanger,

CO gets diluted 1000 tines when it enters the air stream. If you read 1 ppm of CO at that point the furnace would be making over 1000 ppm.

When CO is coming out of the registers it is because it is being sucked into the return from the space and therefore should have been measured in the space.

CO alarms that don't do anything until the CO is over 70 ppm for at least an hour aren't much of a good indicator until CO gets deadly.

Tim McElwain

Just as a note of modern day procedure it is not feasible any more to change or rep[air heat exchangers.

JUGHNE

Are you sure both thermodiscs are limits?
If so they would be connected in series.
Could the lower one be a fan switching function?

Wiring diagram would show.