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Heating & Air Conditioning

Nest, external transformer. Nest only has R terminal, no Rh or Rc

last year

Hi, HVAC wizards! I have a Nest (not Learning, not E) thermostat and my house has no c-wire (details below *). I want to power it using a transformer plugged in near the thermostat. I see diagrams to do this (attached) but they all show thermostats with a separate Rc and Rh terminal. My Nest only has one R terminal.

How do I hook this up? TIA...

* It's a heat-only boiler system, 12 years old, two-wire 24V, three zones and three thermostats, but I am only installing one Nest; the other two are dumb thermostats.

Comment (1)

mike_home
last year
last modified: last year
Given the Nest thermostat you have only has a R terminal you won't be able to use an external 24 transformer without guaranteeing you don't fry the thermostat, furnace control board, or both transformers. If you want to use the external transformer then you are going to need to exchange this thermostat with Nest Learning or Nest E model.

The other option is to install a Nest Power Connector. This device is made specifically for Nest Thermostats in cases where a C wire is not available. Here is the video explaining the installation:

How to Install a Nest Power Connector for Your Nest Thermostat

If you have three thermostats, then I will assume they are wired to a zone controller. The Power Connector would be installed at the zone controller. I have never installed this device so I can't so say how well it works. There are about 2,000 mostly positive reviews on Amazon.

Nest Thermostat C Wire Adapter

It is a good deal for $24.99 if it works reliably. Let's us know if you try this and the results.
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