I'm trying to upgrade to a new line voltage thermostat. The new thermostat has many more connections, including three fan speeds. My current system has a manual fan switch on the wall so all I need the thermostat to do is allow power to flow to the fan coil unit.

This diagram tries to show how the system is wired and a proposed new wiring. I propose this because I've tried attaching the heat and cool and neutral from the new thermostat to the R, and the black to the black. The white wire is capped at the thermostat and junction box ends, but I'm proposing to use that as the neutral connection at the thermostat and add it to the W to W current connection in the junction box. Any advice on whether this would work?

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(source: blogspot.com)

  • The whites in the junction box should be pigtailed together regardless of what else you do. – ratchet freak Feb 27 '15 at 13:37
  • What is the make and model of the new thermostat? Why are you changing the wiring in the junction box? – Tester101 Feb 27 '15 at 13:40

Not sure what model equipment you're using, but it seems like the new thermostat requires a neutral connection. If this is the case, you'll want to make the connections as follows:

At the Thermostat

  • Connect the black wire to the B terminal in your diagram.
  • Connect the white wire to the N or neutral terminal.
  • Connect the red wire to the Heat (and Cool if that's how it was on the old thermostat) terminal(s).

At the Junction Box

  • Connect the white wire from the cable going to the thermostat, together with the other white wires in the box.

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