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Old 03-05-11, 08:01 PM   #1
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Default Thermostat wiring question

I just picked up a new thermostat which works with my Z-Wave set up. The plan is to be able to control the furnace a little better. It’s programmable and I could turn it down super low when I’ll be gone for an extended period of time and then remotely set it to slowly turn up in small increments several hours before I get home. So I go ahead and remove my old thermostat only to find that there are only 2 wires. The new unit needs 24 VAC to run as it isn’t battery powered. Apparently the 24 VAC wire is usually the blue one (C) but it looked like my wire bundle only had 2 wires. So I went to the basement to check out the furnace. Lo and behold the red wire is going to the white screw and the white to the red. They are proper on the thermostat for the record. The furnace has all 5 screws. I don’t know what any of them do aside from C which apparently is common and where I need to get the juice from. Does anyone have any idea what’s going on here? Do you think I could just get a new 5 wire line and match up the letters?

EDIT: Isn't the white wire for the fan? My furnace fires up before the fan does. Is that normal?


Well I just did what I should have done first and asked Google my questions. I came up with this highly informative page right off the bat. It seems that all I need for the furnace are W and R. R being the voltage that gets connected to W by the thermostat to fire up the whole process. I don't think that it matters that they are crossed with an old mercury thermostat as it mechanically connects the two based on the the temperature sensitive motion of a bi-metal strip and making the connection via the mercury. Honestly it might not matter for a digital thermostat also. Just as long as the connection is made. So all I need to do to get things up and running is to get a new 3 wire line, tape it to the old one and pull it through. Man! I'd be doing that right now if Hampden-Zimmerman (the electrician supply place a block away) were open at 10 PM. Still though....why does my furnace have 5 terminals?

Last edited by S-F; 03-05-11 at 08:52 PM..
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Old 03-06-11, 11:07 AM   #2
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Originally Posted by S-F View Post
why does my furnace have 5 terminals?
Your link explains why.
Terminal designations
The usual terminals are:
G – Fan, usually a green wire
R – 24 VAC usually a red wire (Rc an Rh are discussed later)
C – 24 VAC Common
Y – Compressor, usually a yellow wire
W – Heat, usually a white wire
The thermostat is a set of contacts where G, W and Y are connected to R based on the following rules.
If there is a call for cool, R is connected to Y (compressor). In air conditioning mode the thermostat controls the fan, so G (Fan) is also connected to R. For cooling, the furnace usually selects the high speed of the blower.
[edit] Heating
In conventional heating which is oil or gas, the furnace controls the fan, so only R is connected to W (heat). For electric heating, the thermostat controls the fan (G), thus in this case both G (Fan) and W (heat) are connected to R (24 VAC). For heating applications, a lower blower speed is used and the furnace selects it.
The furnace controls the fan because oil and gas heat is not instantaneous. In order to avoid a blast of cold air, the furnace controls the fan. It can do this with a simple delay or by temperature set by the limit switch in the plenum.
Humidification is a heat only function. Humidifiers are controlled by the W (heat) terminal.
Rc, Rh and R
Rc and Rh have been added to accommodate separate transformers for heating and cooling, thus for single transformer systems Rc and Rh are jumpered together and assume the function of the R terminal. Some systems use R and Rc, so consider R to be Rh in this case. Rh is defined as R (heating) and Rc is defined as R (cooling).
The Fan (G) terminal
The Fan (G) terminal is usually connected to Rc when Rc exists on the thermostat.
The Common terminal
The common terminal has been a recent addition because early thermostats created the switches with the liquid metal mercury. Mercury is a hazardous material and must be disposed of properly. Electronic thermostats need power to operate and it would like both a hot and a common terminal to get power or both ends of the transformer secondary need to be available to the thermostat.
There is a technology called “power stealing” where unopened switches can power thermostat. The current available is limited by the resistance of the furnace relays.
Power can also be provided by a set of batteries. When the batteries die, you then have no air conditioning or heating.
And it's done this way because if you have Air conditioning in your house it's often hooked up to the furnace, also if you have a wood stove but want the air in your house filtered and circulated so that the heat is distributed around your house then you turn your thermostat to "fan".
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Old 03-08-11, 08:17 PM   #3
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Yes! You are right! I never thought about using the blower for anything aside from moving air heated by the furnace.

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