Home air circulating fan runs constantly.


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Old 08-22-14, 01:04 PM
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Home air circulating fan runs constantly.

I have a small problem with my home AC system in that the air circulating fan will not shut off as long as the system has power. I have done some troubleshooting, but would welcome any additional ideas.

Equipment:
Trane XB series furnace and air handler (TUC80 something I believe) - installed 2007.
White Rodgers 50A55-474 control board.
Honeywell digital thermostat.
Older Trane compressor/condensor unit with evaporator on top of newer furnace.

The problem:
A week ago I noticed one evening that the compressor outside was running but the house air circulating fan was not. This may have run like that for about 10 to 15 minutes. I switched off the cooling at the thermostat and the compressor stopped. The low battery indicator was showing on the thermostat so I replaced the batteries. When I called for cooling about 20 minutes later, the compressor kicked on, but the house air circulator fan didn't, so I switched off the cooling and set the thermostat fan control from auto to on. Still no fan.

Down in the basement I checked that there was no icing at the evaporator, and then took off the access panels at the front of the furnace. After checking for obvious loose wires I pressed the panel safety shut off switch and the air circulating fan started running; it hasn't gone off since! My ac cycles perfectly, it cools the house, but the house air circulating fan runs all of the time which eventually gives me ear ache since I have to listen to constant complaints about sleeping in a draught.

I checked the thermostat output at the furnace control board with a multimeter - all switching is correct. It calls for heat+fan, calls for cooling+fan and calls for fan alone when set appropriately. I have swapped out the thermostat with a spare and no effect. The fan runs even when all thermostat setting are off.

I have given each relay on the control board (a White Rodgers 50A55-474) a good tap, several times. The board diagnostic light flashes slowly continuously, which I read indicates normal functioning.

My diagnosis is a bad control board, but before I experience the pain of buying a new one I thought I would check that there is nothing I am missing. My understanding is that there is no switching on the furnace that would malfunction and keep the circulating fan on, but when it comes to reading circuit diagrams in conjunction with a control board, I am a prize idiot.

Any ideas?

Thanks
 
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Old 08-22-14, 01:35 PM
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I agree, it's sounding like a bad control board. However, before you buy a new one, with the thermostat turned off, check the voltage on the "G" terminal on the control board. This is where the thermostat wire coming from the G terminal on the thermostat connects to the control board. If everything is working properly (with the thermostat turned off), you should read 0VAC from the G terminal to common (usually the furnace metal). If you're reading 0VAC, then it's likely the control board that's bad. However, if you're reading ~24VAC (which is what you should get if the thermostat was calling for the blower), it's worth investigating why there's voltage present. Try disconnecting the thermostat wire from the G terminal on the control board and see if anything changes.
 
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Old 08-22-14, 02:26 PM
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Thanks Bob.

I will check the voltage when I get home this evening. With the thermostat disconnected from the board, except the 24V supply (red), I get 0V when nothing is called for on the green wire and 24V when the fan is called for. Both thermostats I have tried seem to work perfectly. I've also tested them for switching by testing continuity between red and green, yellow and white while they are disconnected - all switch as expected. But I do not recall testing them while they were connected to the board. I will try.

And I should add that the fan runs when just the green wire is removed from the board.
 

Last edited by AMDKS; 08-22-14 at 02:27 PM. Reason: Added details
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Old 08-22-14, 02:38 PM
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It sounds like a bad control board. The reason I asked you to disconnect the green wire at the control board is because if the green wire was shorted to the red wire (thermostat wires in the wall), it would give you 24V at the green wire and the blower would run. By disconnecting the green wire from the control board, you've ruled out shorted thermostat wiring as a possibility.
 
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Old 08-22-14, 02:55 PM
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I agree.

I am not familiar with these systems and considered the possibility that a bad sensor on the furnace may call for continuous fan operation, for example if there was a high temperature situation in the furnace heat exchanger that called for continued circulator fan operation. I believe some older systems did have a thermostatic switch to control the circulator blower after a heating cycle, but I have no idea about these newer systems. Hence my post.

Thanks again for the input.
 
 

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