View Full Version : New Thermostat with 'c wire' hookup

01-04-2011, 07:13 AM
I have a Goodman GMPN100-4 REV B, and am attempting to hookup a new 3m Filtrete Thermostat with WIFI from Home Depot. It takes batteries but it also requires a 'c wire' hookup for the WIFI to operate. I have removed the stock Honeywell 4 wire thermostat with R, Y, W, G hookups. There are unused wires that run from the thermostat to the furnace that I can use for the 'c wire' connection. It is my understanding that the 'c wire' hookup is for 24v power.

My question is: Where do I hook up for the 'c wire' connection on the circuit board ? Within the furnace I have an open place labeled '24V HUM', which is unused. There is also some sort of transfomer with multiple wires connected labeled 24 on the lower and 120 on the upper with multiple red, blue, and white wires leading out. Can I attach to either one or am I totally wrong?

Any help would be appreciated and I will respond to any questions, if any.


01-04-2011, 10:58 AM
The red wire is usually the 24vac hot lead. The thermostat connects that to one of the other wires to turn on the desired function. Keep in mind that any circuit requires two leads for a complete circuit, but it might be referenced to ground rather than being floating. The transformer takes the line voltage (120vac) in, and reduces it to the 24vac required to run the various control circuits. The HUM lead may only be on when the fan is called for, and not continuously, so may not be a good choice. You'd want to verify anything with a multimeter prior to connecting it to ensure you don't fry something.

01-04-2011, 05:04 PM
24V hum would indicate a connection for 24 volts to a humidifier. It is a source of 24 volts you could use. C could mean a lot of things. You have to find out on the wifi gizmo if it is for a common, or a 24V hot.

01-04-2011, 05:35 PM
On some, they switch the humidifier so it is only powered while the fan is on so it can't do anything without the air flowing...ensure whatever you choose is powered all the time for proper operation to the new device.

01-04-2011, 06:38 PM
the 'C' is for common. Does that mean I can use the 24V HUM slot?

coming out of the transformer is:
upper left labeled COM is a white wire going to the circuit board labeled PRI TRNS
upper right labeled 120V is a black wire going to the circuit board labeled PRI TRNS
lower right labeled 24V is a red wire going to the circuit board (plugged in with a group of wires/not really labeled)
lower left labeled 24V is three blue wires-one going to what appears to be ground, one going to the circuit board (plugged in with a group of wires/not really labeled), and one going out of the furnace with a massive gob of wires/not sure where leading

Should I connect to the blue wire or the furnace chasis?

01-04-2011, 10:08 PM
Measure between the red and the blue wires. If you get 24vac, the blue is the common. Essentially, it sounds like you want 24vac available at the thermostat to power it (rather than just switching it to one of the control lines). Normally, they don't need that there, and only switch one side through the thermostat to one of the other wires back through a relay (or other electronic device) to turn something on...the common is connected on the other side of the relay so there is a complete path. The safest thing is to call 3M to verify this is what they intended. A transformer has (at least) two windings: the primary (in this case 120vac - stay off that side!) and the secondary (24vac). To complete a circuit, you need to supply both sides of the secondary to a device if it needs 24vac power, rather than just switching something.

01-05-2011, 06:21 PM
Thanks guys for all your help. The 'C' is Common and I connected it to one of the Blue wires coming out of the 24vac side of the transformer which also goes to the furnace chasis.

Just started using this thermostat and I LOVE IT. I can control the thermostat from my iphone anywhere.

Thanks again.