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Questions tagged [common-wire]

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External (C Wire) Power Supply Use for Cooling Only unit

Question regarding using an external 24V power supply to provide power to a smart/WiFi thermostat. I’m not clear on why both Honeywell and Emerson/Sensi docs state that the use of an external power ...
Paul's user avatar
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2 answers
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Another C-wire for Nest thermostat question

Some background first. 30-ish year old furnace/boiler with a zone system and six zones. This was my parents full-time residence in rural Maine, but after my father passed away in 2022, and my mother ...
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Connect c wire at old furnace

I don’t have an air conditioner and have the same issue. Blue wire for c at thermostat but blue wire not connected to anything at old furnace. The blue wire is with a red wire and a beige wire near on ...
Renee B's user avatar
1 vote
2 answers
148 views

How to make c-wire to a new boiler using Fast-Stat common maker?

I'm seeking assistance regarding connecting a C-wire (for Ecobee) to a new boiler using the Fast-Stat common maker. The new boiler currently has two thermostat wires available for connection. However, ...
Kamsu's user avatar
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2 votes
2 answers
207 views

Ecobee Thermostat / Common Maker / Gas Valve

I have a two-wire thermostat and an old forced air gas furnace in my house. When you open up the cabinet, inside you find a Dayton 2E228 Gas Valve and it is wired to an external 24VAC Transformer ...
vicatcu's user avatar
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1 answer
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Is it ok to supply power from one device a use Common on another?

I have a valve relay with R and W going to a stat, and there’s a spare wire I want to connect to a C wire. The relay doesn’t have a Common and I need the common for a wifi stat. I have a separate zone ...
user158117's user avatar
3 votes
1 answer
287 views

Can neutrals be landed in an old panel which acts as a junction box?

I have a metered panel which had been stripped down and wires jumped over to new panel. The feed is still here but also jumped over. Can I land my neutrals from the new panel inside of the old main ...
Jo_mls's user avatar
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2 votes
2 answers
244 views

Which Circuit Does this Switchbox Common Wire Belong To?

I'm replacing a single-pole switch with a Lutron Caseta, in a switch box that houses two switches, each on a different circuit. The old switch that I've removed was connected to two wires: line (red ...
Maaaaaarc's user avatar
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1 answer
744 views

C terminal voltage on AC control panel

My smart thermostat shut itself down reportedly (as it ran some time on internal chargeable battery and I browsed through the settings) because the C terminal stopped receiving 24V power supply. That ...
amphibient's user avatar
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1 answer
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Can I connect my thermostat's R and C wires directly to the transformer?

One day my parent's furnace and thermostat stopped working. The thermostat is a smart thermostat that depends on 24volts to the R and C wire. We measured voltage between the R and C wire and was ...
Vanny Sok's user avatar
1 vote
1 answer
11k views

Where do I connect the C wire in my furnace?

I am upgrading my thermostat to a Nest, and would like to add a common wire. My current furnace controller doesn't have a C terminal, but the current thermostat wire has 2 unused wires (black and blue)...
Nathan's user avatar
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