Identify G and C Terminals

Our 1980's ICG gas furnace is hardwired and doesn't have R,W,C and G terminals, so looking for help in identifying C and G so as I can install a humidifier. I've enclosed the wiring diagram below with my best guesses marked. R and W are currently connected to a NEST thermostat. Help please!




More images are at Google+

Many thanks

NB I did post this previously but guess I was too detailed in my background and not clear in the request.

 

Welcome to the forums.

The requests are coming in faster then we can handle. Since we are volunteers and most of us work during the day the nights get pretty busy.

Anyway..... you do have the R, W and C illustrated correctly. You don't have a G terminal as you have a line voltage fan/limit blower control.

Why do you need the G terminal ? Is it so that the Nest will activate the humidifier and blower ?
I don't believe in letting just the blower run to humidify without the burner firing. Cold air doesn't carry humidity well and it's not a cost effective way to humidify.

If you really want to add that feature, it is do-able. You would need to purchase a single pole normally open relay with a 24vac coil. Any generic HVAC control relay will work in this application. The wiring is fairly easy and I'd be glad to diagram it for you.

 

Yes , your assumptions are correct on the R , W & C.

Now, the question --- can the humidifier just use the *W* wire ?
By using only the *W* wire the humidifier should work only when the furnace is running heat.

 

Many thanks guys!

PJMax: The main reason for G is so that the nest can activate the fan during the night to circulate air, as it's not well balanced at present and a couple of rooms get much colder. However it was also on every one of Honeywell's diagrams in the TrueEase HE150 installation guide.

Canuk: I also found an addenda this evening that shows G and W as interchangeable, though for a different thermostat. Would this work?

As this is only a temporary installation till we can get the furnaces replaced next summer I'm not worried about perfection, just safety. My intent for now was to use the supplied HumidiPRO to manage the humidifier, rather than the Nest unless you've any suggestions?

 

Look at it this way -- Many many many years ago ( 1979\80 ) there was a new push for energy efficiency which created a program for improving the insulation in peoples homes. This resulted in the UFFI fiasco and out of that a new science of buildings evolved --- commonly known as building science ( not to be confused with the private firm using that name ).

The NRC and CMHC here in Canada were and still are heavily invovled with many aspects of the building science . In studying a new concept , known as indoor air quality , they discovered that running the furnace fan continously can improve air quality and comfort.

As for comfort , the conditioned air becomes stratified when the furnace is not running. The thought is having the blower continously running to constanly mix the air so as not to have these layers of cooler levels along the floor and warmer levels up near the ceiling.

In some cases this can be benifical , however, there is the human factor. When it's winter drafts will make you feel chilled . Even though the drafts coming from the supply vents is the relatively warm conditioned air ( approx. 68F -20C ) of the home being recirulated , it's not the same as feeling the moving heated air ( 100F - 38C ) when the furnace is heating.
More often than not ,many people were living in poorly insulated drafty homes with less than optimum heat distirubution ducting , rarely ran the blowers continiously.


The vintage of your furnace has a switch labeled *Auto* \ *Manual* probably located on the left side of the furnace.
Simply -- this *manually* turns on the blower to circulate air whenever you desire rather than relying on when the furnace is supplying heat.


Pretty much every next generation HVAC systems after your's usually has a *G* connection available to manually turn on the blower remotely. The thermostat manufacturers were including a switch to remotely select this option with connecting to the *G* wire.

A similar evolution took place with humidifiers. The simple method was merely having a rotating drum of some type of material to pickup water from a tray below. A humidistat measures the humidity in the home and when required would turn a motor to rotate the drum.
This problem with this method the humidistat may call for the drum to turn but if the furnace blower wasn't running , then no humidty was being delivered to satisfy the humidistat.

You will find most modern humidifiers have the ability to function more efficienty with greater flexability.
The simplest being that humidifier only operating when the furnace is heating.
Having a *G* wire connection available allows the humidifier turning on the blower whenever it detects a need to manage the humidity. Saving the need to leave the blower running continiously


Yes , that should work fine. The humidifier will only run when the furnace is heating.

Yes, I believe that is your best option with what you have to work with.


Sorry for the wordy post --- just in one those moods I guess.

Hopefully this helps.

 

Wife and I spent some time working through the wiring, and the system is now up and running thanks to your help! Humidity is up much yet, but that will take some time.

We have to do the other furnace tomorrow as well then I'll try and get some picture uploaded tomorrow for anyone else who needs to do this in future.

Help is greatly appreciated.

Andrew

 

Glad to hear things are working out.