03-06-11, 11:07 AM
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#2
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Master EcoRenovator
Join Date: Mar 2009
Location: Western Wisconsin.
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Quote:
Originally Posted by S-F
why does my furnace have 5 terminals?
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Your link explains why.
Quote:
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.
Cooling
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
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.
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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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