I’m trying to install a smart thermostat and my furnace does not have a C terminal. The thermostat has an unused blue wire, and I connected it to what I thought was a common, but the unit is not powering on. Any help would be appreciated.
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It's probably the black wire in the upper left that is labelled "COM". But if you posted a picture of the schematic diagram that would help.– jwh20Jul 26 at 21:55
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Schematic added.– bperhamJul 26 at 23:30
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3 Answers
Just screw it to the chassis
The wiring diagram for your unit says to connect the C wire (whether from the thermostat or the condenser) to chassis ground, so you can use a Garvin GSST (and perhaps a 18AWG-rated ring or spade lug) to connect the wire to the furnace chassis directly.
answered Jul 27 at 2:18
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I just noticed you wrote this comment... My current connection to the light grey wire also has a connection to the light blue wire, which goes to the chassis. Does that not achieve the same as screwing directly to the chassis? How does screwing the common to chassis send power to my thermostat?– bperhamJul 27 at 5:57
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@bperham -- you can try swapping the push-in connector out for an appropriately sized wirenut, as I suspect it's not making good contact with the blue wire from the thermostat cable Jul 27 at 11:48
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Still not powering on. I connected to the chassis and used a voltmeter to see 26 V when touching the red wire and chassis nut. The blower kicks on and doesn’t stop when I put the unit face on, but it doesn’t power up.– bperhamJul 27 at 16:46
The "C" connection is here:
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The "C" is just the ground connection. Where are you connecting the 24V wire, i.e. the "R" wire?– jwh20Jul 27 at 18:21
The wire with the "wago" connector seems to be the common. check it with a multimeter between it and the red one coming from the transformer (it should read 24V).
My idea is that t-stat wire is too thin to make good contact in wago connectors. Try a standard nut (or fit the blue in the same hole as a side of the thicker wire you've already tapped into).
Also check continuity of the blue wire in the wall, it may be broken, or have incomplete splices inside the wall.
