I have checked other posts regarding this and don't see a solution yet.

I have purchased a new thermostat that requires a c wire. My old thermostat's c wire didn't work so I went ahead and added a brand new 18-5 thermostat wire and wired everything up. Unfortunately, I am getting no power. Here is my old set up:

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I stupidly did not get a before of the furnace set up, but there were two wires that were connected to it that actually don't run to the thermostat but come from somewhere else. They are red and white:

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Here is my new set up using my new wire:

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And at the furnace:

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So anyways I get no power at the thermostat and am wondering if those extra wires are the culprit, any help is appreciated.

  • R and C look right to me. Surely the thermostat, at least, must be powering up? Furnace may not work but one thing at a time. Mar 31, 2018 at 19:57
  • No I flipped the power back on and nothing. Thermostat won't even turn on
    – Mr Man
    Mar 31, 2018 at 20:32
  • @Harper I went up and tested those extra wires with my little volt tool and they aren't hot (at least hot enough to trip the little tool)
    – Mr Man
    Mar 31, 2018 at 20:33
  • This is a standard forced-air furnace with A/C right? Mar 31, 2018 at 20:38
  • @ThreePhaseEel I am no expert but I am pretty sure that is the case. It is gas heat with A/C
    – Mr Man
    Mar 31, 2018 at 20:39

1 Answer 1


You said your old thermostats C-Wire did not work so you wired a new one. You said your meter only measures 50V or higher (I never saw a tool like that before) - so how do you know the c wire was not working?

Your old thermostat was not working ?

Your old thermostat picture shows something like this:

C or G?? , [Y?orX? - most likely Y], W , [RH RC]

It would have been a better photo if the letter codes were visible; these codes are standardized and they do have meaning as to what circuit they operate.

Common (24VAC) - should be a black wire - blue is for other things, G is for Ground green wire , if Y Cooling On Signal [Most likely Y for Yellow] , W Heating on Signal - white wire , [ Rh 24VAC Heating , Rc 24VAC Cooling - red wires ].

The additional wires that you show removed - I am thinking out loud here - they go to your outside Air conditioner Condenser? They turn it on. You can check the wiring diagram for the condenser to find out for sure - and ohm the leads ..by opening the condenser panel - disconnect the red and white wire that goes into the unit - tie them together - then go back inside and measure continuity - if continuity - then you know which wires go to the AC unit - put the wires in the condenser back to the way they were before and connect the wires inside to the proper terminals. Red probably ties in to your Yellow Wire that is wrapped, and the white wire to the common - (Black tape the white wire so you know it is really black).

Now if they do not go there - then they must turn something else on - that will need to be verified where they really go to by continuity testing .. but I think the AC Condenser is where they go.

I can't see your Air Handler control board terminal labeling very well - so under the white wire - the label is ? and next to it is that a Y ? for Yellow like if AC is on don't allow heater to run ?

Your yellow line is not hooked up so it goes somewhere - perhaps to that Y terminal on the AH control board and something like the red wire ?

The answer here has a layout of what wire is what and the proper color scheme - although there are some missing exceptions like X which could be a heat circuit if I am not mistaken ..

Oh and by the way if all else fails - and you have not done so already - Please check your circuit breakers and check for any fuses that you might have in the system - just in case (sometimes that old adage of making sure we really have power gets skipped - especially when we have pulled things apart and perhaps mislabeled something or did not take note - figuring that out makes people forget that simple stuff - even the pros - we are all human. I remember having a 20 year old washer and dryer set - I replaced the timer in the washer and my dryer went out a month later - I had no heat but it spun - with no time to spend checking anything I quickly ordered a timer as I assumed what my problem was. Upon pulling the dryer out and getting behind the unit - I did that yes let me check the power before I go digging. One leg of my circuit breaker in the service panel was blown. I had that Homer Simpson moment Doh!

  • Thanks for the comment. I ended up connecting the yellow from air handler to yellow at thermostat and leaving everything else the same. When that didn't work I gave up and called an electrician. He said everything was wired exactly the way it should be, but at some point I blew a fuse. So 5 minutes and $100 later everything worked. This question/answer doesn't have a lot of relevance anymore, but I would gladly mark you as the answer if you add an edit saying to check fuses or something
    – Mr Man
    Apr 1, 2018 at 14:24
  • @Dan - Added - done.. I should have put that in my answer to begin with - I think everyone has done that at least once in their lifetime check everything except...
    – Ken
    Apr 2, 2018 at 8:20

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