fan works with heat but not with ac


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Old 05-06-21, 03:07 PM
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fan works with heat but not with ac

I finally turned my ac on after winter but got no air coming through the vent. The outside fan comes on fine but the fan inside the furnace does not come on. When turning the heat on, the fan will run fine and there is power going to the fan motor. But on AC mode, I don't get any power going to the fan motor at all. No voltage from any of the wires but on heat, all wires have voltage. I replaced the capacitor and it did not fix the problem. I checked the voltage in my thermostat and they all are fine and reading 24v. Why am I getting voltage to the fan on heat and the fan runs on heat but I get no voltage on ac and the fan does not run? Is it a bad control board? I attached pictures. Thanks.

 
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Old 05-06-21, 03:34 PM
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Welcome to the forums.

Please post the part number off that blower/timer board.
 
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Old 05-07-21, 01:22 AM
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lennox furnace blower control circuit board kit 48k98 bcc3-2 65k29. I bought a new control board and will get it tomorrow so I will replace it and see if it fixes it. But I'm still confused as to why the fan comes on when in heat but it won't come on when in ac. They both share the same fan but no voltage goes to the fan in ac but it does when in heat.


 
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Old 05-07-21, 11:22 AM
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You need to remember that a blower runs at multiple speeds.
Typically high speed for A/C and medium speed for the furnace.

Blower Speed Taps:
Blower speed tap changes are made on the BCC blower control board at the upper right corner. Unused speed taps must be secured to the dummy "D" terminals on the BCC. The active heating tap is connected to the "H" terminal and the active cooling tap is connected to the "A" terminal.


You could try swapping wires on A and H as a test.
If that doesn't change anything.... try a jumper wire from R to G.
Does the blower run now ?
 
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Old 05-07-21, 12:23 PM
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I put in a brand new control board and still the same issue. If I jump R to G, then the blower runs. Nothing happens when I swap A to H. I have a voltage meter and when I run heat, both A and H gets voltage but when I run AC, neither of them gets voltage. So the control board is telling the fan to run on heat but it won't tell it to run on AC. But I'm not sure why both A and H is getting voltage on heat. I assume that H would get voltage on heat and A would get on AC.
 
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Old 05-07-21, 05:33 PM
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Both aren't getting power. You're measuring thru multiple speed windings on the motor.

Your thermostat connects R to G when the A/C is called for.
Remove thermostat and connect R (red) to G (green). See if blower runs.
If it does.... there's something wrong with the thermostat.
Possibly low batteries if it's a battery powered model.
 
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Old 05-07-21, 09:17 PM
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When connecting R to G, blower does run. It's a Nest Thermostat and the battery level is showing 3.86V, which is normal but I did just charge it for 2 hours and still same problem. I will charge it overnight just to make sure. The thermostat does show that G is connected and it recognizes a fan. It allows me to run fan alone but the blower does not turn on when fan is on. The only time the blower turns on is in heat. If you think the thermostat is defected. I don't mind going over to Best Buy to buy a new one to see if that is the issue.

 
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Old 05-07-21, 10:47 PM
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It is very possible the nest sub base is defective. The sub base contains the switching components.
This is the weak link in the nest thermostat.
 
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Old 05-07-21, 11:16 PM
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I measured the voltage for G by the control board with thermostat set to fan and got 0. With thermostat set to AC, Y showed 26v but again G is 0. But when I removed the thermostat and connect G and R directly, the blower goes on and the volt for G by the control board is 26v. So it does appear the thermostat is not working correctly. Will get a new one over the weekend to see if this fixes the problems. Thanks for all your help!!! Much appreciated!!!
 
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Old 05-08-21, 06:21 PM
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HOLY ****. It turned out to be the thermostat. U know your stuff!!!! Got a new one, put in the base and still same problem but once I put the new thermostat on, it worked like a charm. Turns out the base is okay but the actual thermostat is the problem. Do you have any email? Would like to send you a $50 starbuck gift card for helping me! THANKS THANKS!!!!!
 
 

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