I have the wiring configuration for a RHEEM thermostat Pictured Below. On it, there is a "V" wiring connection which according to the manufacturer is for "Pulsed width modulation" type heating. On the other end, the furnace, it connects to a "V/W2" connection which I have verified is pulsed width modulating.

Initially, Nest showed that I was incompatible due to the "V" wiring but learned that it was because they thought it was "milivolt" designation. However, after talking with them and researching, I learned that this was not the case and should be able to connect the V/W2 into the Nest "W2".

Will this accomplish the same thing? I learned that W2 is for 2nd stage heating which doesn't seem to be exactly the same thing as Pulsed Width Modulating heating.

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Edit - Found in manual: From Manual

To set the furnace for operation with two-stage thermostats, set switches 1 & 2 of SW2 to the “ON” position (See Figure 56). Note that these switches should be in the “OFF” position from the factory. With both switches in the “ON” position, the furnace can still recognize a “V” signal present and will still operate with a modulating thermostat. However, with both switches of SW2 in the “ON” position, the furnace is set to operate with a two-stage thermostat as well. With a two-stage thermostat (installed as shown in Figure 50) and switch settings configured as described above, during a call for heat, the furnace will operate as follows:

So it is currently operating in "modulating" thermostat mode but is able to operate with Nest in a "2-stage" mode by simply flipping a dip switch. Then the "V" wire will act as "W2" wire.

  • W2 is typically used for a "second stage" heat or something like emergency heat on a heat pump system. Unless you can find specific information saying it works with your equipment (PWM or "V wire"), then I would assume it won't work.
    – JPhi1618
    Dec 13 '19 at 20:21
  • One reason I think it might is that the furnace has a "V/W2" designation Dec 13 '19 at 20:22
  • 1
    PWM is essentially an analog signal instead of just on/off. If a heater could run at a percentage (most can't) instead on/off, PWM would be a way for the thermostat to assume more responsibility in system's control. For example, it could run at 100% when you're away, but only 75% when home, to make it quiet. Or, it could see that "gee, i'm only 1deg cold", so it runs at 50% to catch-up, whereas it ramping up 5deg would use 100%.
    – dandavis
    Dec 13 '19 at 20:46
  • Thanks @dandavis. Do you know why they aren't that common. It seems like that would be a useful technology but I don't see a lot of information. Does it not work very well? Dec 13 '19 at 20:51
  • I don't know much about them, but i'm sure they work fine. It's likely a matter of compatibility and momentum. Mercury thermostats are just on/off, and that's basically guaranteed to work, so that's what most digital thermostats do and most furnaces support. It's trivial to run under-floor heat at a percent by switching it on and off several times a second, but it's complicated to do that for gas-based forced air; many solenoid valves are on/off, what do do about the fan speed, etc...
    – dandavis
    Dec 13 '19 at 21:11

It looks like this type of heat is called "modulating heat" or a "modulating furnace". I can't find any information saying that Nest supports that.

The labels on the thermostat and furnace are meant to be a convenience for the installer, and you really have to refer to the equipment documentation to see what it does. The common labels of R, G, Y, W, C are fairly standard, but once you go beyond that, there are conventions, but you have to research to know the exact use of R, W2, B, etc.

That's a long way of saying that just because the furnace happens to have a W2 label and the Nest also has a W2 label doesn't mean very much. You have to dig into the documentation (as you have done).

  • Thank you for your help. It looks like my furnace can fairly easily be switched to "2 stage mode" and will work with the Nest W2 Dec 13 '19 at 20:40
  • @jwillis0720, that's interesting. I did see some discussion about "how efficient" the modulating furnaces were and some people were recommending against disabling that feature. I haven't used one of these heaters. Don't accept an answer any time soon - someone else that knows more might have a good answer.
    – JPhi1618
    Dec 13 '19 at 20:43
  • It doesn't look like that they are that common, so I think maybe they came and went. I just switched it to two stage mode even though I have a modulating thermostat forcing it to operate in 2 stage mode, and will know very soon if I can detect a difference. Dec 13 '19 at 20:47

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