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Consider a simple two wire thermostat for a heating system, as shown in this diagram from electrical.online.com. There are two wires going into the thermostat, one, Rh, from the 24vac secondary of the transformer, and one, W, from the heating system. When the thermostat calls for heat, Rh connects to W and the 24vac is applied to the heat system.

My question has to do with how to identify the two wires going to the thermostat (as it's not safe to assume any particular wire color scheme was used). I assume the Rh wire has 24vac but with respect to what, as there is no ground in the thermostat? If it were 24vdc, I could put a voltmeter between Rh and W and then switch the voltmeter connections and I assume I'd get +24v one way and -24v the other way. But with AC, I assume I'd get 24vac either way. So how to identify the wires?


Two wire thermostat

  • Rh wire has 24 Vac with respect to W wire .... disconnect wires from W and Rh and measure voltage ... if no voltage, connect the two wires together and measure the resistance at the thermostat – jsotola Dec 29 '19 at 21:16
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Most HVAC systems connect the 24 Vac Common to Earth ground. You can try measuring the AC voltage with one lead of your AC voltmeter connected to the Earth terminal of the closest AC electrical outlet. Then touch the other lead of the voltmeter to each of the thermostat connections.

The is not guaranteed to work but it is a test that is simple to perform and takes only a minute or two.

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  • if that doesn't work (it will almost always work) find the C terminal in the HVAC system with the thermostat off Rh will measure 24VAC to to the C terminal. – Jasen Dec 29 '19 at 20:17
  • NEC 250.20 does not require secondary grounding if primary does not exceed 150V. Furthermore, it specifically prohibits (NEC 411.5) low voltage grounding in some cases, like outdoor or pool lighting. Also, some HVAC systems use half-wave rectifiers for their own power, others use full-wave. If they both are grounded it makes a short through one of the bridge diodes. This creates a lot of confusion for electricians. Some go to extreme pains to find an eliminate grounds in low voltage systems, others just ground everything. – Maple Dec 29 '19 at 22:09
  • Note, that if secondary is floating you most likely would still see some voltage between all thermostat wires and mains earth induced by wiring proximity – Maple Dec 29 '19 at 22:12
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    @Maple: Regardless of whether the HVAC system uses half-wave or full-wave bridge rectifiers internally, every system that I have seen or worked on always uses one side of the 24 Vac as the "Common" terminal. This allows the use of external 24 Vac devices (valves, etc) without the installer worrying about those details. Obviously, the other side of the 24 Vac winding is the "R" terminal. – Dwayne Reid Dec 29 '19 at 23:46
  • @DwayneReid Of course one side of 24VAC will be "common". But "common" does not necessarily means "grounded". it is the mix of the two types of rectifiers I was talking about. See fig 4 here for explanation. – Maple Dec 30 '19 at 9:23
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it's not safe to assume any particular wire color scheme was used

Yes and no. While it is possible that an electrician could have used thermostat wiring in some unconventional way, it is more likely to have your house wired properly. The reason for this is that most (if not all) equipment has primary terminals labeled according to the color of wire that is supposed to be attached to it:

  • R, Rh, Rc (power) - Red wire;
  • W (heat) - White wire;
  • Y (cool) - Yellow wire;
  • G (fan) - Green wire.

As a result, it simply does not make any sense for an electrician to invent alternative wiring only to risk accidental short circuits in the process.

how to identify the wires?

The most reliable way, of course, is to look at the terminal block of your heat unit and make sure it is wired according to color scheme above. Pay special attention to the wires connected to "C" terminal - is any of thermostat conductors connected to it? (If yes, then someone has wired "common" to be used at thermostat in unconventional way.)

You don't need to disconnect anything or even use the multimeter on it, just look. But if you don't have an access to the terminal block there is still another way.

First thing to understand, is that the control signals going to heat, AC and fan do not power them. They usually connected to the relays inside said devices. And the other end of the relay coils is connected to common wire coming from the 24V transformer.

So, unless you have some modern devices with SSRs on the inputs, you should see around 24VAC between Red wire and each of the control wires connected to corresponding appliances.

The procedure is simple then: measure voltage between all wire pairs. One wire should give you ~24VAC to each of the other wires, and all those other wires should have 0V between themselves. That one "special" wire would be your Red, and then it is just a matter of figuring out which of the other wires controls the heater.

Note, that if you do not have AC and fan in your house then only one pair of wires will have voltage across, in which case simply use color to connect these two to corresponding terminals. (I suspect thermostat will work properly even if these are reversed by the unscrupulous electrician somewhere).

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  • The thermostats that I replaced had the white wire going to the Rh terminal and the red wire going to the W terminal, not following the wire color convention. I wired the new ones the same way and they're working (as did the old ones). Hence my comment about not relying on the wire colors and my question about how to distinguish them. I assume it wouldn't work if I followed the color convention, or would it? – Not_Einstein Dec 30 '19 at 0:19
  • See the last paragraph in the answer. With only two wires and AC it should not really matter. Thermostat simply shorts the two when temperature drops. It is when you connect the fan and/or air conditioner wires the correct wiring becomes necessary. – Maple Dec 30 '19 at 9:04

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