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Nest thermostat. Central AC fan turns on when connected to G terminal on base

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Was installing a Nest thermostat and came across an issue with the AC. As soon as I plug the fan wire into the G terminal on the base the AC fan will run continuously. The fan goes on without the actual thermostat installed, only the base. Very strange. Anyone ever have an issue like this?

Comments

  • Ironman
    Ironman Member Posts: 7,379
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    More more info is needed.

    What type of system: gas, oil, electric, heat pump, forced air or hydronic? Split sub base with two transformers or one?

    Some pics?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • super_snop
    super_snop Member Posts: 48
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    Gas forced air furnace Using RH, W1,and C. Furnace has its own control panel WITH transformer.  Heat works fine with the thermostat. The AC Has its own 24V transformer. Was using Rc, Y1, and G. I will have to try to get some pics. Its at my friends house. 
  • HVACNUT
    HVACNUT Member Posts: 5,863
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    So there's a gas furnace with a 40 va transformer, and a separate A/C system with a 40 va transformer?
    Which fan is coming on, the furnace or air handler?
    I've never come across that specific scenario, but I've always powered the thermostat with the A/C transformer, not the heating system's. 
    Is this a new Nest? I've had numerous shorted sub bases with them. Condenser running in winter, heat running constantly, etc.
  • super_snop
    super_snop Member Posts: 48
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    Yes two separate transformers. The fan for the air handler comes on immediately when the fan wire is plugged into the G terminal. Someone had tried to install this thermostat prior to me.  Maybe its possible the base is defective or the wrong connections were made the first time someone tried to install it and fried the base. I just found it very strange the fan kicks on and runs continuously as soon as its plugged into the base. 
  • pecmsg
    pecmsg Member Posts: 4,883
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    I've found a lot of strange things with a NEST.

    Anyone's t-stat is better.

  • Ironman
    Ironman Member Posts: 7,379
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    The “G” terminal can only power one blower relay and it should be the one in the AHU. “G” connects to “RC” when the stat calls for a/c or when the “fan” is turned to “ON”.

    Make sure that you’ve got your two red wires on the correct “RH” and “RC” terminals and that the stat is set up as a split sub base, two transformer system.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • ratio
    ratio Member Posts: 3,649
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    It sounds like a shorted sub base to me. If the thermostat wasn't brand new out of the box, I'd replace it without any more diagnostics.
    super_snop
  • super_snop
    super_snop Member Posts: 48
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    I double checked all the wires. Everything is correct. I’m going to give nest a call. Maybe they will send me a new base to try out. I popped the thermostat on and went through initial settings but the AHU fan just continuously runs. I even made sure fan was not set to on.