Furnace wiring. Where is "C" connection?

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Old 01-07-10, 10:58 AM
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Furnace wiring. Where is "C" connection?

Hi, I have a forced air gas furnace. GMP 075-3 rev B (I think this is a goodman??) I'm hooking up a powered whole house humidifier (Honeywell Truesteam).

My question is very basic. So far as I know, the thermostat wires are fairly universal. R, G, W, Y, C. It's my understanding that "C" is used to power thermostats that don't already have battery power. My thermostat doesn't use "C", it uses batteries, so I can't trace the wire back to see where it hooks into the furnace.

My whole-house humidifier requires that I connect the humidifier to the "C" terminal on the furnace. Looking at the furnace, the labels for R,G,W, are clearly marked and I can see Y going out the A/C unit.

I feel like I'm missing something obvious here. How do I identify the "C" terminal? I can see references to C1 on the circuit panel, but no place to hook up.

Furnace came with home and the previous owner did not leave me the manual. So far I haven't been able to find an electronic manual online. Can anyone tell me were to get a manual for my GMP 075-3? Can anyone confirm this a "Goodman" furnace? Can't find a name on it either.

Help much appreciated!
 
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Old 01-07-10, 01:00 PM
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I think I found the manual for my furnace, but I still don't know where "C" is...
http://site.famousparts.com/goodman/PDF/IO-137CE-A.pdf
 
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Old 01-07-10, 03:03 PM
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If you look on the first page of that manual, it describes the countries where use of this furnace is suitable ----mainly European countries which use 220 volt power routinely.

If you look at the ignition control module in that manual, it's designed for 220 volt operation.

If you are in Canada or the United States, you probably have a furnace designed for 120 VAC operation, so that would not be the correct manual for your furnace.


But if you look on your furnace ignition module, you will probably see one contact there listed as C for common.


Also, these furnaces are grounded, so a C wire for your humidifier can be any good electrical connection to the sheet metal that forms the furnace chassis.
 
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Old 01-08-10, 09:41 AM
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After working with it last night, I forgot to turn the furnace kill switch back on before bed. It was a cold night! Ha ha.

Thanks for the help. I know I'm missing something pretty obvious here.

Originally Posted by SeattlePioneer View Post
If you look on the first page of that manual, it describes the countries where use of this furnace is suitable ----mainly European countries which use 220 volt power routinely.

If you look at the ignition control module in that manual, it's designed for 220 volt operation.

If you are in Canada or the United States, you probably have a furnace designed for 120 VAC operation, so that would not be the correct manual for your furnace.
Good observation on the manual. Still looking then. (I'm in US and it's 120V for sure)

But if you look on your furnace ignition module, you will probably see one contact there listed as C for common.
I checked but I'm not 100% sure what I'm looking at. I'll try to get some images up and post back soon. Looking at what I think is the module, I can see terminals for 120V, 24V, COM, and an unmarked one that grounds to the chassis. Nothing marked "C". I'll do some homework on this, but I want to post reply first.

Also, these furnaces are grounded, so a C wire for your humidifier can be any good electrical connection to the sheet metal that forms the furnace chassis.
I'm confused by this because my themostat manual suggests that hooking up the "C" wire connection will make batteries unnecessary so that implies power, right?

The other concern has to do with how the humidifier is supposed to operate. Conceptually, the humidifier isn't supposed to put moisture (steam) into the duct unless the fan is running. So it's not just a matter of powering the humidifier. I think somehow it's power to run is dependent on the power being turned "on" for the furnace fan. I may be describing the process wrong, but you get the idea. So I'm worried about alternative wiring.

True Steam Humidifier Install Doc
http://docs.electronicaircleaners.co...stallation.pdf
This is just a reference, so don't waste your time on it. (Wiring on pages 14-15 using TrueIAQ option). I'm getting some conflicting information between the printed manual and the online doc, but if I can find "C" then I think I'll be able to go forward on my own.

After I posted here I found a lot more "true steam" posts at DoItYourself.com Community Forums > Plumbing, Heating, Cooling, A/C, Fireplaces, Air Filtering & Water Conditioning Systems > Humidifiers and Dehumidifiers/. I posted there with a link to here, but I don't mean to cross post and I'm sorry for the mess. I'm only responding here but Jay11J did comment over there.

http://forum.doityourself.com/humidi...ml#post1675087

Again, sorry for the mess. I'll try to get some images up.
 
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Old 01-08-10, 10:06 AM
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It requires two wires to power the thermostat without a battery, and two wires to operate the humidifier.

The thermostat will use power from the R terminal, and use the C, "Com", or a connection to the furnace chassis as the second connection.

The humidifier presumably has a connection off the furnace module that turns it on when appropriate (perhaps using a humidistat) and again uses the Com terminal to complete the circuit.
 
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Old 01-08-10, 10:41 AM
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Originally Posted by SeattlePioneer View Post
It requires two wires to power the thermostat without a battery, and two wires to operate the humidifier.

The thermostat will use power from the R terminal, and use the C, "Com", or a connection to the furnace chassis as the second connection.
Ah, okay. Well, it's really looking like the "C" connection as concerns my furnace just means ground to chassis.

Let me get a little off topic for a second. For my thermostat, R, Y, W, and G are already hooked up (which is probably obvious). My thermostat does allow for hardwire power with battery backup. I assumed when I put it in that this meant I'd have to tap into a junction box somewhere. At the time this was too much hassle, so I just decided to let the batteries be the only power source. However, there is a blue wire in the thermostat wiring that's available but unused. Are you saying that if I hook the blue (C) wire to the appropriate thermostat terminal and then ground it to the furnace that I'll have power to the thermostat and then I'll be able to use the batteries just for backup? Is this correct?

The humidifier presumably has a connection off the furnace module that turns it on when appropriate (perhaps using a humidistat) and again uses the Com terminal to complete the circuit.
You know your stuff. What you describe is how I interpret the wiring diagrams provided with the humidifier. Looks like "C" must be what grounds to the Chassis. I'm still working on those images, but if I can confirm I have a goodman then that sure sounds accurate. For furnace, C = wire that grounds to chassis.
 
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Old 01-08-10, 11:07 AM
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Shows images of my furnace:
Pictures by mypicstrix - Photobucket
 
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Old 01-08-10, 11:51 AM
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Originally Posted by petethebuilder View Post
However, there is a blue wire in the thermostat wiring that's available but unused. Are you saying that if I hook the blue (C) wire to the appropriate thermostat terminal and then ground it to the furnace that I'll have power to the thermostat and then I'll be able to use the batteries just for backup? Is this correct?




Probably. That would be common, typical and expected. Consult the installation manual of the thermostat to be sure.
 
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Old 01-09-10, 06:00 AM
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Originally Posted by SeattlePioneer View Post
Probably. That would be common, typical and expected. Consult the installation manual of the thermostat to be sure.
SeattleP,

Well, I got it going last night without blowing anything up. Can't say it's had a big impact yet, but time will tell. I'm still playing with the settings, including an alternative way to wire it, but at least I know what I'm dealing with now.

Just want to say thanks for your help. I know I was being rather picky about the whole thing, but the last thing I wanted to do was short out my furnace in the middle of winter.

Anyway, thanks again SeattleP. Your help was spot on.
 
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Old 01-09-10, 07:21 AM
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You correctly identified the "C" terminal in photo #5.

For clarification, the thermostat connects the "R" terminal to "G" to run the fan, "R" to "Y" compressor, "R" to "W" for furnace. A hard wired tstat need "R" and "C" to power itself.
 
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Old 01-09-10, 07:39 AM
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Originally Posted by Jarredsdad View Post
You correctly identified the "C" terminal in photo #5.

For clarification, the thermostat connects the "R" terminal to "G" to run the fan, "R" to "Y" compressor, "R" to "W" for furnace. A hard wired tstat need "R" and "C" to power itself.
Good, and thank you. It's nice to have verification. I think I got it right now.
 
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Old 01-09-10, 07:47 AM
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