Linked Questions

3 votes
3 answers
12k views

Where's the C terminal on my boiler control?

The new thermostat I purchased doesn't run on batteries, and needs 24 volt power. I have only a red and a white wire connected between the boiler and the thermostat, but there is also a third, unused ...
Mike N's user avatar
  • 31
0 votes
1 answer
23k views

How to connect thermostat C wire to Weil-McLain CGa boiler? [duplicate]

I have Weil-McLain gas boiler for hot water heat and going to install WiFi thermostat that requires common 24V 'C' wire. I do have a free wire in the cable but cannot figure out where should I connect ...
Dmitriy's user avatar
2 votes
1 answer
17k views

How do we replace an old 3-wire thermostat with a 4-wire one?

How do we replace an old 3 wire thermostat with a 4 wire one? We have an AC/heating unit, but the wiring for the thermostat does not have a yellow wire. What can we do?
Irene's user avatar
  • 21
2 votes
2 answers
10k views

Can I use a WiFi thermostat without a C wire?

I have a very old Honeywell programmable thermostat. I am going to replace it with a White-Rodgers UP500W Emerson Sensi WiFi Thermostat. I just want to know if the WiFi unit will work without a C ...
qinking126's user avatar
0 votes
3 answers
13k views

How to move from RH/RC thermostat to C thermostat in AC only system?

I see this is a pretty common question, but the answer seems like it might be dependent on your thermostat and air handler, so I'm posting with my specific questions. I'm upgrading from a White ...
Fuzz Evans's user avatar
0 votes
1 answer
10k views

Why isn't the "C" or Common wire working on my Nest thermostat?

I Have connected the Blue wire to the C terminal on a 3rd Generation Nest and to the Blue wires on the Transformer....... But the Nest Thermostat does not show the Blue wire connected on the Equipment ...
Ed J Panco's user avatar
2 votes
2 answers
5k views

How do RH/RC/R and Y/W relate for HVAC?

I have the following notes for HVAC (that I've compiled from reading through here): Thermostat wiring: C low voltage power wire that brings power from heating/cooling equipment to thermostat ...
user avatar
3 votes
1 answer
7k views

How can I add a "C" common wire to this system? [duplicate]

I would like to add a "C" / Common wire hookup to my hvac system and get a fancy new thermostat. I've read How can I add a "C" wire to my thermostat? and still have some questions. First of ...
Raolin's user avatar
  • 133
3 votes
2 answers
3k views

Can I use an unused wire as a "C" wire?

I just bought a new thermostat that is Wi-Fi enabled and it requires connection to a "C" wire. The old thermostat does not have one running to it, but there are other unused wires. The thermostat ...
user14503's user avatar
1 vote
0 answers
5k views

How can I add a common wire to the existing furnace wiring?

My current furnace has Rh and W1 to the thermostat. I'd like to add a C for my new networked thermostat, but I'm not sure where to connect it. From my research it sounds like I should be able to just ...
fracai's user avatar
  • 111
3 votes
2 answers
2k views

Can I use the unused blue wire as a C wire?

I'm installing a wifi-capable thermostat in my house which for obvious reasons requires a C wire. The old school mercury thermostat I'm replacing didnt have a C wire connected but I've checked the ...
J. Smith's user avatar
1 vote
2 answers
3k views

Can I use the Y wire in place of the C wire for my new thermostat?

I'm looking to wire a Honeywell Lyric T5 to my furnace (I do not have an air conditioner, I do have a humidistat). My current wiring at the thermostat is G,W,Y,R. At the furnace it's G,R,C, and 2 at W,...
RookieMistake's user avatar
0 votes
1 answer
2k views

Can I directly add a Common wire to this point on a 24VAC transformer, or will load be too high? (70mA)

Referencing This excellent answer I have a two wire furnace controller, 24VAC, no common. I measured the current draw of the relay (when the thermostat wires are shorted), it draws 0.2A Wish to add an ...
Mtl Dev's user avatar
  • 133
0 votes
1 answer
3k views

Add a c-wire to 25+ year old Rheem furnace

I've looked at just about every post on adding a C-wire to power a WiFi thermostat but remain uncertain on how to proceed. The current installation on a Rheem RGAC-080A is two wire as here: XFMR - ...
geoB's user avatar
  • 211
1 vote
1 answer
3k views

How do I connect a C-wire on a Triangle Tube Prestige Solo 110? [duplicate]

I purchased a Honeywell RTH6580WF to use with my recently installed Triangle Tube Prestige Solo 110 propane boiler. I only have two wires at my existing heat-only thermostat. I understand that the c-...
Jon's user avatar
  • 11

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