furnacethermostat-c-wire
I have an older unit Goodman furnace with a c-wire that isn't connected to the unit. In order to install a wifi thermostat to supply continuous power I need to connect the c wire. I see spots for the W, G, and R wire connections, but no c wire. An alternative would be to do a power adapter at the thermostat, but I'm hoping for some guidance on where I could connect the c-wire for power. Thank you!
The thermostat you posted has no common terminal (C). The blue wire that should be wired to common at the thermostat is cut off and wrapped around the harness.
You show red, white, yellow, and green wired to R,W,Y,G terminals on the thermostat, but there is no common terminal on that thermostat and the blue wire that should be wired to common is unused at the thermostat yet wired to common at the air handler.
Is there 24 volts at the air handler low voltage terminals (R and C)? You should, if not check for the door panel switch being closed. It must be closed or tape the switch closed if trying to work with the door open.
The circuit board in the air handler's circuit board should have an automotive type 3 amp fuse: it may be bad. If not and the breakers are on and proper voltage is to the furnace, then the transformer is bad.
If you switch the fan on does the fan work? If so this proves you have both high voltage and 24 volt low voltage control power available. The thermostat you show has no provision for the common side of power and looks to be battery powered. Perhaps a Maple Chase brand thermostat?
After more carefully reading your question:
Neither G nor W1 are suitable to use as a common (C). The C connection is there in the unit as you said, but not present on the rear terminal strip.
Based on the manufacturer schematic, it does appear that "W2" on the rear terminal strip of the single stage unit is truly not connected to anything, so you should be able to extend your "C" terminal from the interior circuit board to W2 on the rear terminal strip without any adverse consequences as you mention. It would probably be a good idea to relabel "W2" on the rear to "C" as well.
Best Answer
There is a 24 volt transformer in the top-left corner of the photo. It has a red wire from one side of the secondary (24 volt side) going into the multi-position plug onto the circuit board. The other secondary wires are blue; it looks like there are three and one of them is a jumper down to the mounting foot of the transformer. Based on that I'll say C ("common") in this unit is wired to the chassis.
To verify use a volt meter in AC mode. Measure voltage from R to chassis (should be about 24-30 volts) and W to chassis (should be about 0 volts). Note that because this is all in the blower compartment there is a safety switch you'll have to override in order to turn on the power for this test.
Since there's apparently no terminal designated for the thermostat C wire, in your case you could attach the C wire to one of those screws where a blue wire attaches to the sheet metal.