Gas Vent Damper

Don10000

RE: Field Controls Gas Vent damper Model WM 4-8
My understanding is that the vent should open when the Gas boiler is calling for heat and close when not calling for heat. I've noticed the damper opens and closes every few minutes even when the boiler is idle and not calling for heat. Is this normal or is something wrong?

HVACNUT

What thermostat(s) do you have?

Don10000

I'm using a nest thermostat

HVACNUT

Don10000 said:

I'm using a nest thermostat

I knew that. Just wanted to verify. 2 wires or 3 at the Nest?

Don10000

You hit the nail on the head! I have 2 wires, I only have a red and white wire coming out of the wall. I guess I need the Common wire? Do I have to try and run another wire from the thermostat to the C of the boiler x former or is there a way around doing that.
I checked the voltage across the thermostat at the boiler (Y&G) and noticed only 20 volts. After I read your message, I put my old thermostat back in and it worked fine.

EdTheHeaterMan

A common wire may help mitigate the problem. The Nest on 2 wires will steel power from the system 24 volt transformer to charge the onboard battery when there is no call for heat. With a C wire the battery is always charging. That draw on the system sometimes is enough to close the circuit for enough time to make the damper rotate open as if there is a call for heat. Once the load is measured by the Nest thermostat, the R to W opens instantly to allow the damper to go to the closed position. By that time the damper is already rotating.

That is a very touchy thermostat and does no play well with anything that is not a furnace with or without air conditioning. Boiler wiring often has stuff that Nest does not get along with like Zone Valves, Low Water Cut Offs and vent dampers. Run that extra wire and see it that solves the problem.

HVACNUT

Can you post a pic of the boiler wiring diagram? 

Don10000

Wiring diagram:

EdTheHeaterMan

@Don10000 said Mr. Ed, I added the common wire from the thermostat to the C terminal of the Xformer and it solved the problem. The damper isn't randomly opening and closing. I can't thank you enough!

Thanks for the update. We always like to hear the success stories.