Nest Thermostat power issue

RyanNYC1 · January 28

I have the following setup:
-Weil McLain EG-65 steam boiler
-McDonald & Miller Unimatch Water Feeder
-Safgard 400 Series Low Water Cut-Off
-Nest Thermostat (wiring: white wire into the W1 slot, red wire into the RH slot).

Problem:
When the low-water indicator flashes on the Safgard Low Water Cut-Off, the Nest thermostat shows “no power.” After the automatic feeder adds water, the Nest turns back on after a few minutes.

Is there a way to prevent this issue?

EBEBRATT-Ed · January 28

To be reliable the Nest needs constant 24 volt power. Power comes in on the R wire but you need a "C" or common wire for them to work

The issue is that the Nest has a large battery that needs charging. They are supposed to recharge on the off cycle. Some boilers do not have a large enough transformer to charge the nest even with the C wire.

The best way in my opinion is to add a separate

RIB RELAY & TRANSFORMER FOR NEST.pdf

Uploaded Jan 28, 202591.65 KB

transformer and relay. Others may comment with other ideas. This is just my opinion.

See the attached

HVACNUT · January 29

You can power the thermostat using R and C on the transformer to Rc and C on the thermostat. Then Rh and W on the thermostat to G and Y on the transformer thermostat wires.

EdTheHeaterMan · February 3

What @HVACNUT suggestion may or may not work. Read this comment for how to use an isolation relay

EdTheHeaterMan

Feb 3, 2025

This comment tells you why you need the relay
https://forum.heatinghelp.com/discussion/comment/1833986#Comment_1833986
Here is the diagram from the comment:
https://us.v-cdn.net/5021738/uploads/RCB49I69B2BQ/screenshot-2025-01-11-at-8-35-59-e2-80-afpm.png
And to add the relay would look like this based on your equipment's diagram
The relay on the top that I added may look like the transformer that is powering the burner, but it is not, The top Transformer.Relay Combination is the additional isolation relay that is needed to properly set up your Nest Thermostat:
https://us.v-cdn.net/5021738/uploads/9PZU9O2F7A5K/screenshot-2025-02-03-at-12-03-57-e2-80-afpm.png
But your equipment may not be wired according to the original factory diagram, since I see some components that are not original to the equipment that have been added.

EdTheHeaterMan · February 3

Here is the diagram from the comment:

And to add the relay would look like this based on your equipment's diagram

The relay on the top Combination Relay/Transformer that I added may look like the transformer that is powering the burner, but it is not, The top is the additional isolation relay that is needed to properly set up your Nest Thermostat on this steam boiler.

GW · February 4

Been there, you need to power the nest with full-time 24 V, which would likely require the use of a relay.

Or, run a 4 wire, two of the wires are the power and common, two of the wires are your call for heat.( r and w) . No relay needed in this circumstance.

Nest Thermostat power issue

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