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I have a 1988 Carrier Limited Edition furnace. The blower is constantly running at low-speed, even after cycling power. The heat cycle doesn't start (usually begins with the combustion blower starting up).

The unit won't respond to the thermost at all. No matter what position the thermostat is in, the low-speed blower continues to run. I tried removing the thermostat and it still runs. I also tried manually providing a heat signal (R-W), still runs. I also tried replacing the main blower relay control board, same result.

The secondary voltage coming off the transformer is 26vAC, so the transformer seems fine. Everywhere else the hot control voltage is around 1vAC.

I am thinking it may be the circuit board for the combustion blower, but I have no idea how to diagnose to make sure, and don't want to continue replacing parts unnecessarily.

Does anyone have any ideas to help a guy get heat in the middle of winter???

Brad
 
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Very close. It turned out to be one of the thermal cutoff fuses had failed. The fuse is a Microtemp product with an old part number that doesn't match their current numbers. Also the fuse didn't have a temp rating on it, although it was red on one end. I assumed maybe they are color coded according to temp rating, and replaced it with one of Radio Shack's red ones. Is this a safe assumption? I have emailed Microtemp with the part number to see if they can track it down, just in case.

This is a Carrier in a horizontal position in a crawl space, with 3 thermal cutoff fuses in different locations around the heat exchanger. Before I put the panels back on, I ran the furnace to watch the burners. Everything was looking fine, but as soon as the main blower turned on, a little of the flame started spilling out of the topmost element, very near the location of the fuse that failed.

I don't know if:

1) this was only happening because the panels were off and the blower was pulling air from the side of the furnace instead of from the return plenum
2) this is normal but after 20 years, the fuse eventually fails
3) this is a problem and I will be in there shortly replacing another fuse
4) this is a problem, and the replacement fuse has too high of a temp rating and I am going to be toast

Any advice would be greatly appreciated.
 

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You have at least one cell in the heat exchanger cracked. Replacing the fuseable link won't help. That is what saved your house from burning to the ground. Gosh I hope you didn't "bypass" it temporarily or you might not be around to read this message. Shut er down man its time for a new furnace. Also figure out why yours is running too hot. Don't ever close off vents or use an electrostatic "allergy free" air filter. Also if you have a cooling coil make sure it not dirty blocking air flow. Good luck and Go - Amama!
 
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