Heat only thermostat wires....white, white, green?

Old 09-19-14, 12:37 PM
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Heat only thermostat wires....white, white, green?

I'm finishing up replaceing all the thermostats in our 5 zone, heat-only, oil-boiler system. The first four that I replaced each had two wires- 1 red & 1 white. I am on the last themostat now and the wires coming out of the wall are 2 white and one green. There is a red wire that goes back it to the wall, as pictured below.

The old zone 5 thermostat was a mercury one with the 2 white wires attached to one terminal and the green one to the other. I didn't take a picture of the old setup but there is a picture of the old thermostat below.

It is ZONE 5 in my diagram below. It's connected in someway to the wiring of ZONE 4. Zones 4&5 are an addition. The other 3 zones are wired independently.

I would like to install a Honeywell T8775A1009 round digital that I have(There is a Honeywell T8775A1009 in zone 4 as of yesterday) if I can figure out the wiring and attach the 4074FAB resistor onto the taco ZV correctly(resistor is installed in the picture on ZV5 but I removed it until I figure out the correct wiring.


more pictures
Thermostats Photos by snowday133 | Photobucket

Zone 4 left, Zone 5 right

Old 09-19-14, 05:06 PM
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Looking at the manual for your zone valve I'd say that the 2 white wires are the R wires and green is the W wire.

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I have not had good experiences with power stealing thermostats.

A Honeywell thermostat that uses batteries would be a better choice.
The RTH5100B would be a better option for a non programmable stat and would not require adding a resister.
They run ~ $35 at Home Depot.
Old 09-22-14, 09:23 PM
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Thank you-
I never would have figured out that two whites are now red & green is now white. I went ahead and hooked up a battery-powered Honeywell there a few days ago and it has been running perfectly now. Confirmed.


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