AC not controlled by the thermostat

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Old 06-30-20, 02:47 AM
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AC not controlled by the thermostat

Hello!
I have an old central AC unit that cools the house but has to be switched on and off from the breaker. As soon as the breaker is switched on, regardless of whether the thermostat is set to heat, cool or off, the condenser comes on. The only way to turn it off is by switching it off again at the breaker. It appears to be wired to the furnace and those wires are then connected to the thermostat, but it just seems to not be "speaking" to the condenser. I've included pictures of the furnace control board and the thermostat wiring. The furnace is a Lennox Elite Series. Thanks for any help you can provide.

Lennox Control Board

Noma thermostat

Lennox wiring diagram: one thing I do notice here is there appears to be a common wire that runs from the control board to the thermostat that my system does not have. Not sure if that's part of the problem.
 
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Old 06-30-20, 04:37 AM
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I’d start by checking if the contactor has welded closed.
 
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Old 06-30-20, 05:45 AM
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Hi, did you replace the Tstat?
Geo
 
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Old 06-30-20, 06:43 AM
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Thanks for the reply Geo.

Actually, yes I did replace it with a Nest. I forgot to mention that in the original post. I copied the wiring from the Noma thermostat but even with the nest it still doesn't seem to control the condenser. Here's a pic of the nest wiring:

Nest wiring
Thanks for the reply roughneck77. Not sure what you mean by welded. Do you mean he might have welded something at the condenser?
 
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Old 06-30-20, 07:45 AM
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Were you having this problem before you changed the Stat? as for the welded contactor open the cover in the control area on the condenser, the contactor will have some large wires on it, be sure the power is turned off to both air handler and condenser and see if that part of the contractor moves freely in and out.
post a pic of what you have there and we can point you in the right direction.
another thing you can try is to remove the 2 wires from the Y terminal on that control board and see what happens then.
Geo
 
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Old 06-30-20, 08:07 AM
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Yes this behavior was happening before. We bought the house about a year ago and the owner wasn't willing to fix it. I'll try your suggestions and post back with pics in a few hours. Thanks again!
 
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Old 06-30-20, 08:22 AM
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Hi, this link will give you idea of what the contactor looks like, it may or may not have a cover on it, if it does remove the 2 small screws on the front, also one other thing to try is remove one of the small on the side of it and see if that makes a difference.
https://www.homedepot.com/p/Noark-40...20G7/300813144



 
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Old 06-30-20, 08:35 AM
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Contactor is in the condenser. they can weld closed and cause symptoms such as what you have.
That nest will need a common wire. Not having one can cause an already unreliable thermostat to perform more poorly then it already does. I’d suggest removing it and instead using a product from Honeywell or Ecobee.
 
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