# 24VAC Common for New Thermostat



## Plumber101 (Feb 25, 2009)

I have a Goodman GMT115-5B furnace and outside condenser:










The cable to our thermostat has 5 wires with the Red, Yellow, Green, and White being used by the thermostat. The 5th blue wire is unused at both the thermostat and at the furnace.

My new thermostat (Honeywell TH8320ZW100 Z-Wave) requires a 5th wire for 24VAC Common:










Here are some pictures of the board in the furnace:



















Here is what I found on Goodman thermostat wiring:










If you look in the wiring diagram you see the Chasis ground this will be your C connection. Goodman is a cheap furnace in my opinion and they most of the time use the furnace case as ground instead of providing a terminal connection

This is how I'm currently wired:










I have an unused 5th (blue) wire in the cable to my thermostat location. I just need to know where to hook it up at the furnace. Can anyone point me in the right direction?

Thanks!

*Look above for red type*


----------



## breezin (Mar 3, 2012)

So I would attach the unused blue wire at this point:










Thanks again for the advice!


----------



## hvactech126 (Nov 11, 2010)

yes, attach the wire at the point


----------



## biggles (Jan 1, 2008)

see that "powder blue" right off the TR is the common side of the 24V up to the stat that is a constant as long as the furnace has power....you want to read R to C on the stat subbase.reason they call for a C the stat is digital electronic w/LEDS the old feedback trick won't do it.if you hook it up without the TRUE 24Vs it will only light up,but have no action to the furnace or condenser.anyone of those lite blue are C your spare just happens to be BLUE could be any color,but r,g,y,w for the service guy walking up to it wax on wax off thing there!:wink:


----------

