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TLDR: I think I need to flip R and W wires, and to splice the blue (C) wire to the brown wire coming of the transformer. Is this correct?

Long version:

Like many others on here, I'm trying to add a C wire to my thermostat, and to my furnace so that I can use a smarter thermostat.

The thermostat has the red and white wires connected to their respective R and W terminals. The blue wire is present, but not connected.

According to my understanding of how thermostats are supposed to work, the R wire is supposed to be connected to 24v, while the W wire is energized only when the thermostat is calling for heat.

On my furnace, however, P3-9, the always-energized 24v wire, which is white, connects to T2, which is connected to the W terminal on the thermostat, and P3-7, the 'energized with heat wire' is red, but is connected to the R terminal.

Shouldn't these be switched?

Using battery power it doesn't matter, but I'm concerned that if I add a smart thermostat I'll need this to be correct.

Similarly, I believe that I can splice the Brown wire that goes from opposite the white wire on the transformer to P2-2 to the terminal block and then to the B wire going to the thermostat.

Is this correct?

I've annotated the wiring diagram with what I think are the correct fixes.

Lots of WiresWiring diagram edited

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The only things you need is a tap from Pink to connect to the B common wire and to flip the red and white wires on the Thermostat terminals: block.

Write down a big W above the T1 conn. and R below the T2 conn. label so you or whoever has to look at it later doesn't get confused.

  • That agrees with my analysis as well, but the red/white thing seems a bit non-standard. Guess nobody cared when this furnace came out. – gbronner Oct 11 at 19:12
  • Followed these directions and new thermostat is working – gbronner Oct 14 at 14:56

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