Thermostat issue or Contactor


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Old 07-20-13, 05:12 PM
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Thermostat issue or Contactor

I have a Grand Air A/C unit (no heat pump) that is 11 months old and I usually run it at 78 year round. Well I was hot last night and turned it down to 74 and forgot to turn it back up today. I went out for the afternoon and now the outside compressor continually runs and the inside air handler frosts over. I turned the thermostat to the off position but the outside unit continued to run. So I pulled the outside breaker and left everything off and let it sit for over an hour. The inside unit defrosted, so I tried to turn it all back on. When I turn the thermostat to off the outside unit still runs but the inside fan stops but the inside unit frosts over again.

It is a 5ton unit and plenty big enough for our 2000 sq footage if that mattered.

I've looked through responses and had some say thermostat, some same contactor stuck.

Just want to see what answer I can get here or could it really be either?

Also it is Saturday, so I won't be able to get anyone out here to help till this week. If I run it with the fan on instead of auto would it still freeze up or would it hurt the unit? It's is going to be really hot tommorrow without air.
 
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Old 07-21-13, 04:33 AM
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My guess is that the contactor is stuck in the "on" position. Turn the power off, and remove the access panel so that you can see the contactor. Turn the power back on, and if the outside unit (compressor/condensor) turns on, take a voltmeter and measure the voltage on the low voltage (coil) side of the contactor. If the thermostat is not calling for cooling, you should not have any voltage across the coil. If there is no voltage, the contactor is bad. If there is voltage (~24VAC) across the coil, then there is something wrong with the wiring or thermostat. Try removing the thermostat and see if anything changes.

The evaporator coil is likely freezing up because the compressor is running but the air handler (blower) isn't moving air over the evaporator to remove "cold", thus it's freezing up. As long as the blower is running while the compressor is on, everything should be OK, so you should be able to run it like this until you can get it fixed. In fact, on these really hot, humid days, I set my fan to ON so that it keeps circulating the air within the house. I think it makes the house feel more comfortable when the compressor isn't running.
 
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Old 07-21-13, 09:23 AM
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at the outdoor unit, remove one of the control wires from the contactor.... does it shut off?
 
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Old 07-21-13, 09:35 AM
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Bob you said "take a voltmeter and measure the voltage on the low voltage (coil) side of the contactor. If the thermostat is not calling for cooling, you should not have any voltage across the coil. If there is no voltage, the contactor is bad. If there is voltage (~24VAC) across the coil, then there is something wrong with the wiring or thermostat."

Question - Am I understanding correctly -
If I have the thermostat set down to cool to turn on, there should be voltage across the coil, if there is no voltage -contactor bad

OR if thermostat not set to cool there should be no voltage, if there is voltage then it is a wiring or thermostat issue.
 
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Old 07-21-13, 10:27 AM
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Of the two statements/questions you just posted, the first one is incorrect, the second is correct.

The contactor is actually a relay. When 24VAC is put across the coil terminals, the relay turns on and the contacts close, thus turning on the compressor & condensor fan. When the thermostat calls for cooling it closes a circuit sending 24VAC to the contactor coil, thus turning on the outside unit. When the thermostat is not calling for cooling, there should not be any voltage on the contactor coil.

hvactechfw had a good, simple idea (no voltmeter required). Merely remove one of the low voltage wires that connect to the contactor coil (power off when you do it). This removes all possibility of a bad thermostat or shorted wiring and leaves just the contactor. When you turn the power back on, if the outside unit turns on, the contactor is bad (contacts welded together).
 
 

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