Home heating question: testing for common wire at thermostat

Is there a way to test for the existence of a common wire at the thermostat using a multimeter, or does it have be tested at the furnace itself?

The system is pretty old (heat only); the current thermostat uses only 2 wires. I found a third wire in the wall that wasn’t connected to anything and was wondering if I am lucky and this is a common. Is there a way to tell just by using a multimeter, without going down to the furnace and mucking around in there? (it’s a cramped space, not fun to work in)

Simple thermostats don’t have a common wire. They’re like a simple light switch.

Trying to install a “smart” thermostat? Some do need a C or common wire (usually blue) for power, and some don’t. The Nest can work with just red and white (R & W, or occasionally labeled as R1 & W1). On the other hand, I’ve got a Wi-Fi enabled Honeywell that does require a C wire.

If that third wire isn’t connected to anything at either end, you’ll need to find out where to connect it at the furnace to provide power.

Yes. I actually tried the Nest because of their claim that a c-wire is not needed, but it didn’t work for me. Only later after I returned it, did I find this other wire buried in the wall.

So you’re saying there’s no way to test this wire with a multimeter? I don’t know what it’s connected to at the other end, if anything, and I’m trying to avoid tracing it to find out, because I’m lazy.

It’s easier to test at the furnace, using a tool called a ‘flashlight’. Look for a wire of the same color at the furnace and see where it’s connected (if it is).

I would doubt that the third wire was connected to anything. The only test that you can do is test for voltage between the wires. 1-*2, 1-3, 2-3. any line with 0 volts test for continuity.

Except don’t go testing wires for continuity unless you kill the power first (unless you have a multimeter that’s designed to deal with that).

I’m not sure what you’re testing in the above quote, but the easiest thing to do is go open up the furnace and see what, if anything, the last wire is connected to. It’s a $250 purchase, might as well go check the wiring instead of guessing.

If you do not want to walk downstairs to check at furnace then test. If you have 0 volts between two wires you can test for continuity.

If you get 0 volts between two wires, they might both be hot and generally you don’t want to read continuity between two hot wires. Some DMMs can handle that, some will blow an internal fuse (and sometimes toss a big spark out at you in the process).

In the three smart-thermostat installations I have done, each time there was no C wire at the thermostat, even in newer furnace installations.

In two cases there were spare wires in the cable that were coiled up behind the mounting plate. In the third case there was only a 2-wire cable behind the thermostat so we had to run a new wire from the basement (in our church), with multiple conductors.

Each time the C wire hookup was dead simple: I took the panel off the front of the furnace and found where the thermostat wires all went in, and it was very easy to spot the terminal where a C wire would go if there were one (probably a Googleable topic). I then used a screwdriver to connect the spare wire of my choice (e.g. blue/white) to that terminal and then did the hookup upstairs with the same wire.

How old is this furnace?
Is it gas or other?

Old furnaces used termocouples for the thermostat - there was a pilot light, which heated the thermocouple, causing a trickle voltage - enough to operate the gas valve.

If you have one of those, the only current is only one side. A meter may not even try to work.

Time to look at the wiring inside the box - if there is a pilot light, it may have two wires leading out - those are the thermocouple.

I installed a Honeywell WiFi thermostat. My house has a modern, heat-only furnace with just the two wire switch-type thermostat wiring (and the old ‘round’ style simple thermostat). I’m a tech guy so the WiFi part was relatively easy, but I’m not an HVAC guy at all so I was very apprehensive with trying to find the C wire (or common power) source at the furnace control box to send power to the WiFi thermostat.

So I just bought & used a separate power adapter with the right voltage from Radio Shack and stripped the wires and hooked it up to the thermostat directly.

If you are reading 0 volts and both are hot leads then with no voltage there will be no spark. If the wires are connected together at the source you will get continuity, if only one lead is connected at the other end no continuity. No damage to your meter.

I’m not totally following this conversation - I understand in principle what a continuity test is, but are there situations when performing one isn’t safe?

And, can it actually help me determine if a wire is the c-wire or can it only tell me it’s connected to “something”? (I suspect the latter.)

I guess as others have said, best is to just go down to the furnace and see if I can find the other end of that wire.

Wait, what? Are you saying you just set up an alternate 24V source and wired that directly into the thermostat’s common connection? Can you describe this in a little more detail? I had always wondered if something like that would work, and have found these other devices claiming to help you add a common wire when you don’t have one, but the installation still seems a little complicated and barely any easier than getting a real C run from the furnace.

Going to the furnace is the place where I would start. Easiest.

The time it is unsafe is if your are not use to working with electricity. And if the circuit for the stat is 120 vac then the danger increases.

Red hot…White neutral.

Trying to remember the colors again on 7 wire stat. :confused:

Red -heat strip
Blue -
Green - fan
White -
Orange - compressor
Yellow -
Black -common

…i cant remember.

I’m a Hvac tech and It sounds like you have a heating only system with no A/C correct? If that is in fact the case and you have a third wire not hooked up at the Tstat you need to go to the furnace and verify where the other end is connected (it most likely isn’t connected to anything) and wire it to the terminal labeled c …Traditional wiring of a is R=24v hot ,W = heating ,G = Fan ,Y =Air conditioning ,C=24v ground or common. Save yourself trouble and turn the power off to heating and cooling equipment when wiring a Tstat.Not so much for your safety but to protect the transformer from getting shorted to ground.

If in doubt, take a few pics with your cell phone of both the stuff behind the thermostat and the stuff behind the door of the furnace (not blurry and with enough light) and it looks like you got the right folks in this thread to immediately identify what is what.

Yes, that’s exactly what I did and it worked perfectly. AC power is power, it makes absolutely no difference if it originates from the furnace, the WiFi thermostat simply needs a power source for its LCD display and WiFi functionality.

Here’s a video describing how to do it. Note that I have a log home without sheet-rocked walls so the wire hanging down from the thermostat wasn’t a cosmetic issue (I didn’t have to fish it thru the wall). I have oil-fired hot water without a heat pump or fan, a simple two-wire thermostat circuit. Also note that although the guy in that video says that the polarity of the AC adapter wires doesn’t matter, I hooked the positive to the ‘C’ terminal and the negative to the ground (he calls it the ‘RH’ terminal but mine was labeled ‘R/RC’, two terminals with a jumper connecting them). I did it this way because, well, it just made sense to me. Maybe it doesn’t matter (AC polarity is weird).