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Discussion Starter · #1 ·
I have a GMP100-4 furnace unit and just got a thermostat to replace a very old inefficient one. The problem is that the new thermostat requires a "C" wire connection, apparently to power the unit (no battery option). Is there a simple fix for this? Can include pics of furnace board if it would help.
 

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R=power +
C=power -

Depending which series board you have.
You'll need to wire directly to the transformer common.
 

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The C teminal of the stat connects to the C terminal of the board, or transformer.

Its the same as an A/C condenser needing to be connected to the C terminal of the board or transformer.

You may be thinking of jumping the RC and RH terminals together.
 

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been , You got it..:thumbsup: Just looked at mine.. That's exactly what I was thinking.. I deleted the other so he wouldn't get confused.... Thanks been
 

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If it needs a C wire then another wire would have to be run to the tstat from the furnace ( a huge pain). I would return the tstat and buy a Honeywell Vision or FocusPro as they are battery operated and very good. I saw a very slim Robertshaw type w/o battery and after a power shutdown it was very slow/weird on startup. Did not impress me.
 

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Based on my experience those are ABSOLUTELY horrible/unreliable and I hate them:censored:. When they go haywire/on the fritz they can damage a circuit board etc. Stay away from them if at all possible or you may be sorry.
 

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Discussion Starter · #10 ·
More Info...

I have actually run a wire from the furnace to the thermostat, but simply don't know where to connect to the furnace board. Thermostat that my wife purchased has a remote (she wants that). Are there any battery powered models that don't require the C hook up? Thanks for all of the responses. Again, I can provide a pic of the board if that would help.
 

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Based on my experience those are ABSOLUTELY horrible/unreliable and I hate them:censored:. When they go haywire/on the fritz they can damage a circuit board etc. Stay away from them if at all possible or you may be sorry.
Hey Yuri, you mean the "Priduct"? Yeah I hate that too when they fail.

Which Priduct do you hate more the Honycomb brand or the Black Dodgers
brand:laughing:
 

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You can provide a pic. Some of the Goodmans I ran into don't have a C terminal. You have to trace the leads from the 24v transformer and use wire connectors. The 2 Honeywells I recommended are the best I know of and available at HDepot. I recommend the Honeywell as I am a tech and have seen lots of poor quality thermostats do weird things and can sometimes damage a circuit board. Those 2 require no C wire/terminal.
 

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It has a remote control?

Or a remote sensor?
 

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Clover, We have had terrible results with fastst*ts. Had one turn a A/C contactor into a machine gun.
 

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Clover, We have had terrible results with fastst*ts. Had one turn a A/C contactor into a machine gun.


LOL...what caliber was it firing?:laughing::jester:

Chattering like an SOB I bet. Never tried the product, din't need it or trust it.

I have yet to not be able to get cable to a furnace, and i consider myself quit fortunate for it.

Honeywell has their IAQ stat and there are several wireless product on the market.
 

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I am really curious to see how well all that wireless stuff will pan out. Expensive, and hopefully nothing like the smart/idiot valve.
 

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I will agree with posting of a picture.

If this is a gas furnace, the common wire should easily be found if we assume a single stage gas furnace with AC.

I've made some assumptions:
1. 1 stage heat, 1 stage cool
2. single transformer
3. Fossil fuel system
4. Nothing odd like communicating stats or Carrier's ABCD bus. (That I can safely rule out)

Under these conditions:

1. locate the W wire. It should go to the gas valve.
1a. The other wire should be common (C)
2. Locate the Y wire. it should go to the outside unit.
2a. In that bundle of wires find the other one that goes to the outside unit.
This wire should be Common (C). Both commons should be connected together somehow.

There will be 24 VAC between (C) and (R) at all times.
Check for a voltage between the 2 commons. There should not be any. Turn off the furnace. Measure the resistance between the two (C)'s above.
 

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Most Goodman units have wires hanging near the board. Look at were your t-stat presently connects. The common wire you need to hook up is usualy blue. If you have a A/C the outdoor condenser will be connected to the common you are looking for and the yellow from the t-stat.
 
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