I have a Carrier Day & Night furnace. Two years ago, the fan would not turn off. A repairman came out and replaced the circuit board. That fixed the problem (I had a $35 copay on my home warranty, so it was no big deal).

The same problem has happened again. Putting the thermostat in the “OFF” position does not help. The only way to get the fan to turn off is to unplug the power cord on the furnace. Coming back several days later and plugging it in causes the fan to turn back on. With the fan running, I can adjust the thermostat and get the heater to ignite (in heat mode) or the A/C to turn on (in cool mode).

I had a repairman come over to provide an estimate on a new system since this one is so old and is probably getting near the end of its life. In the process, he took a look at the furnace and told me the transformer is putting out 120V instead of 28V, and that probably fried the circuit board and caused the fan to turn on full time as a safety measure.

Since this is now the second time this has happened, and I no longer have the home warranty, I’m hesitant to shell out the estimated $500 and would prefer to troubleshoot and repair the problem myself. I’ve got a volt-ohm meter and am very comfortable with electronics. I just need guidance on how to troubleshoot this problem. Also, I checked out the secondary on the transformer, and it’s putting out 28VAC, not 120VAC. So I don’t have much faith in that repairman.

From what I’ve read on the internet and on this forum, the problem is probably either a bad relay on the circuit board (Carrier part number HH84AA020) or the limit switch (the part number on the one in the plenum is HH12ZA174A). With all power turned off and everything at room temperature (the furnace has been unplugged for several days now), the limit switch is an OPEN circuit. Is that what it should be for this model of limit switch?

Even though the thermostat is in the OFF position, can it still be causing a problem? If so, would disconnecting all of the wires from the thermostat be one way to eliminate it, or would that normally force the fan to turn on anyway (I think the repairman had done that, and the fan kept going)?

Is there anything else I can do to isolate the problem (short across the limit switch, check continuity across the relays, etc)?

If the problem is the limit switch or the relay on the board, disconnecting the thermostat wires wouldn't make any difference. I think the limit switch should be open when cool. You could try jumping it and see if the fan stays off. If it still runs, then the problem is the relay. Removing and replacing a relay on a circuit board would be a pain, but not impossible.

Labman,
Thanks for the quick reply. If I do as you suggest with the limit switch, and the fan continues to run, then is the circuit board the only other thing that it can be? If so, I will purchase another one rather than trying to replace the relay. But I want to make sure that's the problem first. Is there any way to confirm the circuit board is the problem (maybe by reading the resistance across the relay terminals to see if they're stuck open or closed - if I knew which way they're suppose to be)?

Once you had the board out, and you identified the contacts, if the line and load are continuos, you have pinpointed the problem.

I hate trying to answer questions like this. I know what needs to be done, but it is hard to do it, and I don't want to say spend big bucks on a circuit board and be wrong.