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Join Date: Jan 2012
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01-23-12, 03:08 PM   #16 (permalink)  
Also, it seems that I may not need that SPST relay after all, as long as the Nest circuitry is properly designed, which can be checked with a multimeter. Here is the explanation from a user named JackFlash at Ask Metafilter (I posted this question there as well):

It is possible that what you are trying will work, depending on how the thermostat is designed.

The reason is that there are two independent circuits. Remember that current has to flow in a complete circle. Drawing it out on paper may help.

The first circuit connects one terminal of the transformer to RH, through the thermostat, out C and back to the other terminal of the transformer. That provides 24VAC between the RH and C terminals and powers the computer circuitry of the thermostat.

The other circuit is furnace thermopile to RH, through the thermostat heat switch, out the W terminal to the gas valve which is grounded to the other side of the thermopile. This circuit sees only the 750 mV of the thermopile. Since there is no connection between the gas valve/thermopile and both terminals of the transformer, this circuit does not see the 24VAC.

This means that making the connections as you propose should work. Just make sure that the C wire connects only to the thermostat and to the transformer and is not grounded to anything else in the system. Since the 24V is AC and not grounded, it doesn't matter which terminals you connect to RH and C.

As a side note, on a more typical 24VAC system, one terminal of the transformer connects to RH and the other terminal of the transformer connects both to the C terminal and the gas valve through the W terminal. So in this case, 24VAC goes through both circuits. So how does the thermostat computer get power if there is no C wire on a 4-wire system? In this case the power for the computer is generated by the connection between RH and W. This current is the same that would turn on the gas valve, but when the heat switch is open, only a tiny amount of current flows to provide power to the computer, not enough current to activate the gas valve.

Since your 24VAC transformer is only providing a tiny amount of power to the thermostat computer, your transformer is way overkill. Instead of 50 watts you probably only need 1 watt since the transformer is not powering the relays in the furnace, only the computer chip. So you could use a much smaller transformer but the one you selected is okay.
...

He made a second post on how to check to make sure the circuits are in fact isolated:

By the way, one way to test this is to first connect the transformer to RH and C. This should provide power to the thermostat computer. Also connect RH to R on the furnace. This provides the thermopile voltage to the heat switch. Do not connect the white wire. This will prevent any possible damage to the gas valve. Then adjust the thermostat to turn on heat which closes the switch on W. You probably will hear a click as the switch closes. Measure the AC voltage between W terminal and the white wire. It should be 0. Then measure the DC voltage between the W terminal and the white wire. It should be about 750 mV DC (assuming the pilot light is on). Next adjust the thermostat to turn off heat. Both AC and DC voltage between W and the white wire should be 0. If so, hook up the white wire and you are good to go.

The SPST relay method will work, but shouldn't be necessary if the thermostat is designed as expected with the W and C terminals isolated.