Weird A/C problem


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Old 05-27-16, 07:13 AM
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Weird A/C problem

My in-laws have a conventional single stage Trane central AC with a White Rodgers stat. When the system is set to "off" on the stat and the fan switch is moved from "auto" to "on," it takes a few minutes before the fan comes on. When the system is set to cool and the fan is set to auto on the stat, the condensing unit outside comes on immediately, but the indoor fan does not. But the indoor fan does not come on after a few minutes either. What could be the issue?
 
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Old 05-27-16, 09:48 AM
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My first thought is a defective thermostat.

When the stat is put into FAN ON mode the blower should start immediately.

When the stat is put into A/C mode and there is a call for cooling..... the outside condensor should delay several minutes before starting but the blower should come on immediately.
 
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Old 05-27-16, 10:17 AM
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Thanks. In my own unit, the condensing unit (and indoor fan) starts immediately upon a call for cooling. My own stat (and I thought most others as well) is set up with a five minute delay on break (upon completion of the call for cooling) rather than delay on make (upon the call for cooling). I think theirs is as well since the condensing unit starts up as soon as there is a call for cooling. But it's possible that the board in their furnace imposes a delay when fan on is selected at the stat. But that still doesn't explain why the fan doesn't come on at all when calling for cooling. Hate to replace the stat if it's the board.
 
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Old 05-27-16, 10:20 AM
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The delay on make is just for the compressor...... not the inside blower.... and keeps the compressor from short cycling on a power failure or if you change the stat setting and it was in the middle of a cooling cycle.
 
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Old 05-27-16, 11:10 AM
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You may have to bypass the T-stat and play with the wires(G, Y, R, etc..) to check it out.
 
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Old 05-27-16, 11:27 AM
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If I do that, connecting R to Y should energize the condensing unit and R to G should turn on the indoor fan, correct?

PJmax: I understand that the delay applies only to the compressor, but since I can hear the outdoor unit start immediately upon a call for cooling, I assume the stat is designed to delay on break.

Still don't know if the Trane board is designed to delay fan on when setting the stat's fan setting to "on".
 
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Old 05-27-16, 11:42 AM
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If I do that, connecting R to Y should energize the condensing unit and R to G should turn on the indoor fan, correct?
Correct.

Some furnace control boards have both delayed turn on and timed off settings for the blower.
 
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Old 05-28-16, 05:15 AM
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Tried a spare stat I had lying around and got the same behavior. Weird thing is that if system is set to "off" on the stat and I set the fan to "on," the fan will start. While the fan is running, if I set the system to cooling (while the house temp is greater than the set temp), the fan immediately turns off. I'm not familiar with this unit, but my suspicion is a board in the furnace. That's why I like my 32 year old Rheem. Both the AC (I know, it's only 9 SEER) and the furnace still run like a champ and the only electronics is the ignition control.
 
 

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