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I have a wifi programmable Honeywell thermostat. Worked fine for 3 years, this morning I turned on my gas furnace for the first time this year and it would not go off, even when I set the "Heat" / "Cold" / "Off" to "Off."

Called Honeywell, and they told me to check the voltage "between the R and G wires" on the thermostat to be sure I was getting around 24 v. Not sure what they meant by testing "between the R & G wires." I've tested the R & G wires independently and they vary from 12v to 16v. I connect the COM end to a screw in the thermostat's case and the other end to the R or G wire screw. Are they suggesting that I touch the red cable to the R or G wire screw and them COM into the other wire screw?

Sorry, but I'm confused about the "between the R & G wires" comment. (Between the R & G wires is the Y wire, but I don't think that's what the tech was asking me to do. Difficult to communicate, because tech was on phone and spoke with a very strong, almost unintelligible foreign accent.

Also, why does my voltmeter sometimes read 13.0 (for instance) and at other times read -13.0? (with a minus sign in front of it).

  • Hello, and welcome to Stack Exchange. At least some of these wire carry A/C current; is your voltmeter set on A/C? – Daniel Griscom Sep 29 '18 at 15:49
  • place one probe on R and the other probe on G .... the display shows the voltage between R & G – jsotola Sep 29 '18 at 19:52
  • why meter shows negative: meter has two inputs positive(usually red) and common(usually black) ...... if the red probe is negative when compared to the black probe, then you will see "minus" ...... example: measure voltage on a battery, then reverse the probes and measure again – jsotola Sep 29 '18 at 19:58
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Are they suggesting that I touch the red cable to the R or G wire screw and them COM into the other wire screw?

Yes.

Voltages are potential difference, they are relative not absolute. You can often measure with respect to protective ground but it is usually more appropriate to measure between two current-conductors (e.g. between circuit ground/common/neutral/negative and circuit supply/line/live/positive)

In extra low voltage DC systems (<50V), screws in the case may not be electrically connected to the circuit under test and so may not be useful in measuring voltages.

why does my voltmeter sometimes read 13.0 (for instance) and at other times read -13.0?

Because sometimes you connect the voltmeter leads the other way round.

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You have the meter set to DC voltage. We know the meter is in DC mode because it is giving a "-" indication sometimes. It will never do that in AC mode.

Since you are in the wrong mode, the results you are getting are junk.

Thermostats use AC voltage. Switch the meter to AC for a meaningful reading. The voltmeter can't do this automatically, you must do it.

In AC mode, it doesn't matter which probe goes on which wire, switch them and you will get the same reading. AC doesn't have a polarity like that. (Mains AC is polarized but in a different way.)


Connecting a probe to a screw in the thermostat's case is absolutely meaningless and stop. I get it, you're used to low voltage gadgets and expect the usual "low voltage DC, negative common" like everything else. Thermostats are weird because of their pre-diode heritage as mercury switches.

You should be connecting the two voltmeter probes onto the two places they specify.

When they say "between X and Y" they mean put one meter probe on X and put the meter probe on Y and don't put any meter probes anywhere else. Not on chassis. Not on other meter screws that happen to lay between the specified screws, that's not what they mean by "between".

  • When I talked to Honeywell they said the thermostat i have runs on DC not AC. Thanks for info on the peculiarity of 'stats and explaining about "between X and Y." I thought the Common probe ALWAYS was supposed to be for a ground (like the black side on car jumper cables). I bought the V/M after talking with Honeywell tech who said if the voltage wasn't right (around 24+ or - 5), a new thermostat wouldn't work. She was difficult to understand because of her accent, but i DID get that much! (It was the guy at Ace Hdwe who said the COM side of the V/M was for grounding.) – L. Hendrick Oct 1 '18 at 23:32

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