# amateur HVAC question. Connecting a 'C' wire. **URL:** \ **Category:** Factual Questions\ **Created:** [December 16, 2016, 4:35pm UTC](https://boards.straightdope.com/t/amateur-hvac-question-connecting-a-c-wire/774865 "2016-12-16T16:35:53Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Quintas](https://avatars.discourse-cdn.com/v4/letter/q/48db29/32.png) [@Quintas](https://boards.straightdope.com/u/Quintas)\ **Post date:** [December 16, 2016, 4:35pm UTC](https://boards.straightdope.com/t/amateur-hvac-question-connecting-a-c-wire/774865/1 "2016-12-16T16:35:53Z")
Purchased a new programmable thermostat that requires a ‘C’ wire for power. A/C unit outside and gas heat in attic. After removing the old thermostat, ive got my red,green,white and yellow wires connected to their respective terminals on the heater. I’ve also got 2 unused wires. One is gray, one is orangish. The gray one is unconnected at both ends, the orangish one is unconnected at thermostat but is connected to a terminal labeled ‘C’ on the heater. I connected the new thermostat and used the orangish one as the ‘C’ wire and thermostat does not power up. Checked it with a volt meter and nothing. The question is: should I add it to the ‘R’ terminal on the heater since thats 24VAC power? And secondly, why is there a C terminal on the heater with no current? ---
**Author:** ![DCnDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dcndc/32/2842_2.png) [@DCnDC](https://boards.straightdope.com/u/DCnDC)\ **Post date:** [December 16, 2016, 5:32pm UTC](https://boards.straightdope.com/t/amateur-hvac-question-connecting-a-c-wire/774865/2 "2016-12-16T17:32:30Z")
This article may help you: > **[Thermostat C Wire: Everything you need to know about the "common" wire](http://smartthermostatguide.com/thermostat-c-wire-explained/)** > > Back in the olden days, thermostats were simple on/off devices that didn't need their own continuous power supply.  Modern thermostats with Wi-Fi and backlit display, by contrast, need a steady supply of juice.The C wire, or "common wire"... ---
**Author:** ![Driver8](https://avatars.discourse-cdn.com/v4/letter/d/b5a626/32.png) [@Driver8](https://boards.straightdope.com/u/Driver8)\ **Post date:** [December 16, 2016, 6:06pm UTC](https://boards.straightdope.com/t/amateur-hvac-question-connecting-a-c-wire/774865/3 "2016-12-16T18:06:36Z")
It seems odd that that running thermostat C to furnace C isn’t working. I think that’s what you want. I recently got a smart thermostat and didn’t have a spare wire in the wall for this. I just did the fan wire repurposing the link above tells you not to do, since we’ve never used the fan independently ever. Maybe there are some jumpers on the furnace that have to be set correctly? ---
**Author:** ![Quintas](https://avatars.discourse-cdn.com/v4/letter/q/48db29/32.png) [@Quintas](https://boards.straightdope.com/u/Quintas)\ **Post date:** [December 16, 2016, 7:56pm UTC](https://boards.straightdope.com/t/amateur-hvac-question-connecting-a-c-wire/774865/4 "2016-12-16T19:56:09Z")
> [@Driver8](#): > > It seems odd that that running thermostat C to furnace C isn’t working. I think that’s what you want. I recently got a smart thermostat and didn’t have a spare wire in the wall for this. I just did the fan wire repurposing the link above tells you not to do, since we’ve never used the fan independently ever. > > Maybe there are some jumpers on the furnace that have to be set correctly? I’ll prolly do the same you did. You mean using G as C and jumper from Y to G with no independent fan control? ---
**Author:** ![KCB615](https://avatars.discourse-cdn.com/v4/letter/k/ea666f/32.png) [@KCB615](https://boards.straightdope.com/u/KCB615)\ **Post date:** [December 17, 2016, 12:43am UTC](https://boards.straightdope.com/t/amateur-hvac-question-connecting-a-c-wire/774865/5 "2016-12-17T00:43:19Z")
I ran into the same problem, with having a “C” on the boiler that didn’t do what I wanted it to. I ended up wiring an additional transformer at the boiler just to feed the thermostats. It’s the same style transformer as the boiler controls, zone valves, and thermostats already use. I want to say it was about $35 for the transformer, junction box, and conduit nipple to mount it. Several days of cursing solved with a 15 minute installation. The wire meant to feed the C just goes to the new transformer; it’s just a different power supply. I already had 5- or 7-conductor wire from the thermostats to the boiler, so the wire was in the walls. If you have 2 conductor wire, you’re up a particular creek sans paddle. ---
**Author:** ![Me\_Billy](https://avatars.discourse-cdn.com/v4/letter/m/e480ec/32.png) [@Me\_Billy](https://boards.straightdope.com/u/Me_Billy)\ **Post date:** [December 17, 2016, 5:44pm UTC](https://boards.straightdope.com/t/amateur-hvac-question-connecting-a-c-wire/774865/6 "2016-12-17T17:44:33Z")
Don’t assume any particular wire is connected to any particular terminal at the other end! Verify what each color is connected to. Best to have and read installation instructions for the furnace/air handler and the thermostat. These can be obtained from the manufacturer’s web site if you know the manufacturer and model number.