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mportobello 03-21-2010 12:00 AM

Unexpected wiring found on existing thermostat
1 Attachment(s)

I am replacing my thermostat. See attached photo for the existing wiring and new wiring.

1. Why is the existing thermostat wired backwards (R --> W and W --> R)?

2. Why is G (fan) controlled by a relay and switch located in the basement rather than the thermostat? Is it for mechanical 2-wire thermostats?

3. Is the new wiring reasonable?
R --> R
W --> W
G --> G
C --> C

Thanks for any help!

Other details:
Trane XE80 natural gas forced air furnace with White Rodgers 50A65-475 ignition control board
Current thermostat is a LUX 1500
New thermostat is a HAI RC-80
My place is a townhouse condo built 1999 in Calgary, Canada
If attachment doesn't work:

beenthere 03-21-2010 04:02 AM

Somebody just didn't know how to wire.

Just connect to same terminal designations on both stat and furnace as you have drawn. And it will be right.

yuri 03-21-2010 08:47 AM

The relay is probably for turning the furnace fan on because you have a central exhaust fan (building code) hooked to a de-humidistat and bathroom fan switches somewhere. When that fan gets turned on it has to turn the furnace fan on to prevent a negative pressure in the house. I lived in Calgary and that fan system is building code in lots of Canada.

richiemoe 03-21-2010 09:57 AM

You better make sure there is not ajunction box somewhere on your stat wire. Not that there would be but you never know.

yuri 03-21-2010 10:54 AM

GOOD point. Especially in homes over 30 yrs old you may have splices in the tstat wire where someone added a a new section when A/C was added etc etc. What starts of as RBYG may end up being RWYG or whatever. Then you have to trace it with an ohm meter.

HandyManMarc 03-21-2010 02:48 PM

Let me start off by saying that I am just an HVAC tech in Georgia so I don't know how things are done up there in Canada. All previous replies make perfect sence though the wiring does not. The relay looks to be controlling the fan in heating mode. In the case of gas heat, when there is a call for heat the thermistat should not send voltage to the furnace to bring on the fan, just voltage on the W terminal to bring on the heat. Once the temperature has reached a certian point within the heat exchanger the fan is brought on by the furnace itself. This prevents too much cold air from being blown into the house before the air gets hot enough. Once the call for heat from the thermistat has stopped, the furnace will shut off the fan when the heat exchanger has cooled down enough. You may not need the relay with the new thermistat if the new stat can be set to gas heat which will allow the fan to be controlled by the furnace. There is a jumper between rc and rh on the old stat incase you live in a mobile home with dual low voltage transformers. I'm assuming since you live in a condo your stat is located close to the return which is not far from the furnace so trace out the wires, if there is a juction not what color becomes what and then hook color to color from stat to furnace. Just make sure not to cross the 24vac and the common or you will blow that little fuse on the trane board. Here is a list of what colors are suppose to control what functions.

(R) Red---24vac (Y) Yellow---cooling (W) White---heat (G) Green---fan
(C) common which is what ever color the installer/manufactuer chooses (I prefer brown)

mportobello 03-21-2010 11:49 PM

Yuri, yes you on the right track, thanks. The relay is connected to a make-up air intake fan. This fan is interlocked with a large hood fan over the stove. I believe this is to ensure the hood fan doesn't pull exhaust air from the gas burner. Missed this first time around because I was testing the hood fan with the furnace service door open.

So is it reasonable to leave the relay connected to G as well as the thermostat?

Thanks for the help beenthere, richiemoe, and HandyMark.

yuri 03-22-2010 06:14 PM

DO NOT use the G wire from the tstat. When the fan relay pulls in it will send 24 volts from the R to the G at the tstat and will backfeed thru the tstat and start the A/C. The larger range hoods and exhaust fan have to be interlocked with the furnace fan to prevent negative depressurization in the house. The manufacturers have not built a protection circuit for this as it is unique to Canada and the technology comes from somewhere uh south of us. LOL:laughing:

HandyManMarc 03-22-2010 06:37 PM

LOL ya'll do things wierd up there in Canada!

yuri 03-22-2010 06:50 PM

Yep. Our Igloos are too airtight so we need lots of venteelation. Especially on "chili"nights.:laughing:

mportobello 03-29-2010 10:26 PM

Thanks, I am up and running with the new thermostat.

How are range fans usually interlocked with the make-up air fan?

The range fan does not start the make-up air intake fan anymore. While tracing wires I did short a switch that starts the make-up air fan. After resetting the breaker the switch still turns on the fan. Perhaps this short damaged something else.


beenthere 03-30-2010 04:53 AM

Is the switch wired to turn on both fans at same time.

Might have damaged half of the switch.

yuri 03-30-2010 06:11 AM

Usually there is a relay with a 110 volt coil. The 110 to the range hood also goes back downstairs to the relay which is mounted on the furnace. The relay has a NO normally open set of contacts between R and G to start the furnace fan. If it is a house central exhaust fan system they usually put a pressure switch in the suction pipe of the fan to close contacts between R and G. There are other variations but 90% of them are done this way.

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