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?
Best Answer
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.