I have a nest thermostat that need a C cable. I have a Vaillant boiler serial number GA92-120 SP that doesn't have a C connector.

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The boiler has only two wires connected from the thermostat to the boiler. Black wire connected to "T" connector and white wire connected to "TV" connector. In the middle between "T" and "TV" has a double connector with the letters "W" at the top and "Z" on the bottom but no wire connected.

There is a third wire color red that is not connected to anything. Can I use the red wire as a "C" wire? If yes, where in the boiler can I connect this wire?

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Another option is to use a "Google Nest Power Connector." Since there is no C wire connector in the boiler, it doesn't give instructions to install it. If I use the Google Nest Power Connector," how can I connect it myself?

Thank you for your help!

  • Nest can not control that without some modifications. Nest needs 24 vac transformer coming from that control board. So you need to install 24vac transformer which will also give you the C wirer.
    – Traveler
    Commented Oct 5, 2022 at 7:52
  • We have 10000 "no C wire" questions here. Have none of them provided an answer for you?
    – FreeMan
    Commented Oct 5, 2022 at 11:29
  • What make/model is that controller? (I'm suspecting it's a Honeywell aquastat from the looks of things, but can't verify the model number from here!) Commented Oct 5, 2022 at 11:47
  • @FreeMan No, I didn't find the answer on all the "no C wire" questions here. I did looked before posting my question.
    – Lunna J
    Commented Oct 13, 2022 at 1:32
  • 1
    @Ruskes Yes, I decided to use the option of installing a 24 vac transformer directly to the thermostat and that worked. I connected one of the wires from the 24 vac transformer to C and the second wire from the transformer to R. I have two wires connected to R now. The first one from the boiler and the second from the 24 vac transformer.
    – Lunna J
    Commented Oct 13, 2022 at 1:42

1 Answer 1


The L8148E can provide a C wire

Your "controller" is a Honeywell L8148E Aquastat Relay. While the L8148's installation instructions don't explicitly discuss the C-wire situation, it's clear from the schematics that the Z terminal can be used to land a C wire, provided that your thermostat doesn't draw more than 1.2A of 24V power, which is generally not an issue for most electronic thermostats.

  • Can the DVer explain please? only reason I DV'd the other answer was because it was incorrect... Commented Oct 15, 2022 at 3:11

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