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Honeywell WIFI Themostat and the dread C wire - Does it apply here?

Honeywell WIFI Themostat and the dread C wire - Does it apply here?

Old 12-26-16, 03:43 PM
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Honeywell WIFI Themostat and the dread C wire - Does it apply here?

I recently purchased a Honeywell rth8580wf and after reviewing the install methods I came to the "c" wire problem and I called Honeywell up and they explained that the "c" wire provides 24volts to power the unit constantly, ok I get that.

But here is my question:

When I moved into my house in 99' (the house was built in 74') it has an oil furnace with hot water baseboard, in 99' I wanted to update the thermostat but found then that it was 120v line voltage and purchased a "Adapt-A-Stat" from Home depot (it fits neatly in the wall gang box the thermostat is mounted to (here is a url to it)


It is a 120v to 24v transformer, been using it for 20 years simply connected the existing Black & White to the Transformer (the existing ground wire was left unused) and the "Adapt-A-Stat" output (2) dark blue wires that connect to the "W" & "RH" and a Jumper wire from "RH" to "RG"

Putting a Volt meter after the transformer confirms it is out-putting 24v combined across the W - RH terminals on my thermostat.

Correct me if I'm wrong If my existing thermostats is using 24v and the new Smart Thermos require 24v shouldn't I be able to use a transformer that had 3 wire (simply adding a single 24v dediated line and the existing 24v split like the "Adapt-A-Stat". After all I'm already stepping down from 120v to 24v. Does this exist?

Old 12-26-16, 10:14 PM
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Here is the modern version of what you are asking for but does require three wires to feed it: Aube Technologies RC840T-120 Electromechanical Relay with Built In Relay

Note that one person found it did not put out enough power to drive the Ecobee wifi thermostat so don't know about the Honeywell (has worked with Nest). There is the heavy duty solution: Emerson 90-113 Fan Control Center

Last edited by Astuff; 12-26-16 at 10:30 PM.
Old 12-26-16, 10:50 PM
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Welcome to the forums.

A similar thread.....

I was unable to find what that Honeywell stat draws so I can't tell if the 840 will run it.
Old 12-27-16, 10:02 AM
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I take it that you can't fish a new low voltage cable up to the Tstat location,sure would be easier and a better install.

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