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The inductor motor starts only when R and W are shorted on the control board. The wires to the motor do not have power otherwise. The fuse seems good though I'll be replacing it tomorrow just to make sure. When the thermostat is plugged in and tries to initiate heating, VRW=28 V. Shorting R and W with alligator clips does start the motor so it doesn't seem to be a control board problem, at least on the high-voltage side.

I do not know exactly what to expect when checking the wires behind the battery-powered thermostat but shorting RH and W there does not start the furnace. RC is left open for some reason, though the manual says that it should be shorted with RH. We have no A/C and nothing outside so there shouldn't be anything else between the thermostat and the terminals on the board.

The furnace was working normally last week but we're moving slowly so no idea when it may have stopped. The only "unusual" event since is that we switched the breaker off and on last week.

Where is the problem likely to be?

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If connecting R and W at the furnace turns on the heat and connecting R and W at the thermostat does not then there's a problem with the wire from the furnace to the thermostat.

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  • I considered this but I'm not familiar enough to know: does this suggest that only W is disconnected somewhere in that section since it should be comparable to R and R is ~28 V?
    – gormadoc
    Commented Apr 19, 2021 at 5:16
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    if there is a good strong 28V between R and C at the thermostat location, a break or disconnection in the W wire is the most-likely problem.
    – Jasen
    Commented Apr 19, 2021 at 5:22
  • I'll see about having it fixed and then accept. Thanks!
    – gormadoc
    Commented Apr 19, 2021 at 5:24
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    @Jasen or where the last shelf or picture was fitted :)
    – Solar Mike
    Commented Apr 19, 2021 at 10:27
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    Seems like a finishing nail was driven too far into the baseboard and severed W there. Running new wire fixed it.
    – gormadoc
    Commented Apr 20, 2021 at 3:08

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