[DustinBH]

Trane (Model # N/A, built 2/1987) HVAC System with natural gas with pilot light furnace inside and air conditioning unit outside.

Hello all, I tried a search but not quite the correct answers in any of them for this problem. I had to replace my blower motor today and I believe this problem was existing before but as I became more intimately familiar with the furnace over the last couple of days I really noticed it today. My blower turns on for approximately 20-30 seconds and off again for 20-30 seconds in auto. It will stay on in on of course but I checked the temp limit switch on both settings and it continually drops in both so in auto it kicks on and off and in on mode it blows out cooler air into the house never quite heating well. The ignition system has 3 rows of flames that light up when heat is selected, I noticed when the blower kicks on the middle row is drastically reduced due to the blower fan air hitting it (bad enough to put out the pilot light when it wasn't igniting).

I have read that this is because the heat exchanger is no longer working correctly, is this the case? I am thinking that maybe when he replaced the blower after the motor change that there is a gap but I am pretty sure this was happening before today. Please help, I will try to answer questions as best I can but I cannot find the model number as the section it was printed on has worn away. Thank you.

[DustinBH]

One thing after some research I found that my Fan Limit Switch setting were: Off-140; On-150; Limit-~200, so I adjusted to the most common ranges I have found: Off-80; On-140; Limit-~200

[mygirlsdad77]

This is a clear case of a bad heat exchanger. Do not use furnace at all until you have a pro into check system. Using furnace in current state can cuase carbon monoxide(which can be deadly) to enter the home.

[KC13]

Indeed, classic symptom of a failed heat exchanger.