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Thread: Common 'C' wire reading 29V rather than 24V, any ideas?

  1. #1
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    Exclamation Common 'C' wire reading 29V rather than 24V, any ideas?

    Hello All,
    I just had a new Carrier heat pump and fan coil installed in my house and the voltage between the R and C wires is reading 29V. The Nest thermostat will not work if the voltage is above 27.6V. My HVAC installer tried swapping out the transformer from 230 to 240 but was only able to reduce the voltage on the C wire to ~28.75V. Does anyone have any ideas of how I can lower the voltage so that the Nest will work?
    Thanks,
    Chris


  2. #2
    HVAC Thermostat Specialist

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    When measured with a voltage meter what is the line voltage to the transformer.

  3. #3
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    The voltage going into the house from the box is high @ 247 instead of 240. Not sure why that is the case, would that effect the voltage coming out? I would think a transformer would be designed to handle that sort of variance and output the correct voltage. Perhaps I can use a higher guage wire for the 'C' wire?

  4. #4
    HVAC Thermostat Specialist

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    On a transformer a higher than rated incoming voltage will produce a higher than rated secondary voltage. You should talk to your electrical company about your high voltage. A typical HVAC transformer secondary voltage will range from 26vac to 27vac when the fan and compressor are off. Changing the C wire gauge will not help the voltage.

  5. #5
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    Is the voltage on the common AC or DC?

  6. #6
    HVAC Thermostat Specialist

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    The transformer line side (240vac side) is the primary side. The secondary side (24vac side) is volts AC. The secondary side will have two wires. If one of the secondary wires is grounded to the air handler chassis the wire that is ungrounded will have 24 volts AC. The wire that is grounded will have 0 volts AC. The grounded secondary wire will be your common. wire

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