24vac transformer wiring

Discussion in 'HVAC Heating & Cooling' started by Master Brian, Apr 23, 2009.

  1. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    I am trying to wire up a new humidistat/thermostat. It is a Honeywell Vision PRO IAQ with interface Module.

    Problem is I need to wire up the "C" wire, which is listed as the 24vac Transformer Common. I have the wiring diagram for the furnace as well as for the thermostat, but nothing says which wire is actually the common.

    Is there an easy way to find this out? According to the diagram for furnace there is a Black, White, Blue, Blue to ground, and Red wire coming out of the transformer. It looks as if the Black and white wire are High Voltage wire, which really only leaves the Blue wire as the common. Would that be correct?

    I feel fairly confident about the rest of the wiring and this may even be more clear once I get a good chance to focus on it. Earlier I was somewhat rushed and my 3y/o kept pestering me. Not a good mix, but here is what I believe are the rest of the connections.

    The circuit board on the furnace shows 3 wires going to T-sat....W, G & R. I am pretty confident on those. W, being the stage 1 heat and going to W1 on the t-stat module's board. G being the fan relay wire and going to the G on the module's board. Then R should be the Red wire which comes from the 24Vac Transformer and goes to the R on the module's board. I am then left with the Yellow wire, which I realize comes from th condensor and goes to the Y (stage 1 compressor relay) on the module's board.
  2. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    I seem to have figured it out.
  3. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Depending on the price of the transformer and the price of a fuse, you may want to fuse the 'former.
  4. MaintenanceMan

    MaintenanceMan In the Trades

    Messages:
    54
    Location:
    Indiana
    Blue should be your common wire, by the way you described it. Looks like you already had yourself straight.
  5. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    Yes, thanks for the responses but it seems to be working and it does seem to have been the blue wire.
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