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

  1. #7
    HVAC Thermostat Specialist

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    Remember the voltage is while there is no load on the transformer. Put a load on it and the voltage will drop. 28v is very common for unloaded transformer, anything under 30 is good IMHO.


  2. #8
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    I am also having this problem. My thermostat voltage is reading 28.

  3. #9
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    Quote Originally Posted by bsmith_tsu View Post
    I am also having this problem. My thermostat voltage is reading 28.
    It's not a problem. 28v is perfectly normal with no load.

  4. #10
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    Quote Originally Posted by cflury View Post
    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 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
    Quote Originally Posted by bsmith_tsu View Post
    I am also having this problem. My thermostat voltage is reading 28.
    Quote Originally Posted by Servicetech View Post
    It's not a problem. 28v is perfectly normal with no load.
    Although 28v without load may be normal, I think the reason bsmith and cflury brought it up is because Nest thermostat isn't working (as cflury mentioned in his original post last August), so for them at least it does seem to be a problem. It may be a problem with the Nest design if 28v is typical and expected, but still a problem if they can't get their thermostat to work.

  5. #11
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    If the 28v is in fact the reason the nest isn't working properly, yes they have some design issues. 27-28v is the most common no load voltage for a tramsformer. When I see one that reads 24v with no load it's typically very old or the customer has a lower than usual line voltage in thier house. Most transformers are designed to output at 24V under full load, which they are rarely run at. 40VA transformers are typical for most newer furances and loads are normally 1/2 that.

  6. #12
    HVAC Thermostat Specialist

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    its not a hvac equipment problem.... IT IS A NEST PROBLEM... obviously none of the engineers designing this crap have ever done any hvac field work.

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