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THERMOSTATS PAGE THREE:
Let's look at the field wiring of a thermostat.
Here is the older round thermostat by Honeywell--the T87 and one of the older rectangular
ones as well. Don't worry if your thermostat is newer than these. The new digital and programmable ones wire about the same. Either can be
used as most of the information is similar:
This example may have the same terminal designators or a few might be
different from yours. Don't worry. You will probably have what you need.
The application determines the terminals in use or available. Most commonly
they are R,G,W, and Y. And if you are lucky, the wires connecting to them
are Red, Green, White, and Yellow. Others will have an RC (red for cooling)
and an RH (red heating). Most of the time these two will be wired together
with a jumper.
BUT, if you have a dual transformer type system, you must not wire these two terminals
together. Some manufacturers like Carrier are particularly fond of using a
control transformer in the heating unit and another in the outside
condensing unit as well. Both are involved in the control system and may or
may not require additional wires to the thermostat.
The Red wire (and terminal) is usually one side of the transformer--the
voltage source. The
thermostat acts like a switch and as needed will switch this wire on to other
terminals as well to accomplish tasks. If you turn the fan switch to the
on position, then the thermostat (acting just like a light switch) will send
that red wire on to the green wire and it will in turn activate one side of
a relay that already has voltage to the other side of it and that makes the
relay close its contacts and "poof" the fan comes on. That isn't too
difficult to see. Is it?
If you set the System Switch to heat position and the temperature setting is
high enough, then the mercury bulb will flop over and make contact between
the red wire and the white wire and the white wire will activate a gas valve
or an electric heater relay and "poof" again. The heat comes on.
Sometimes, systems are designed where the red wire will also be
switched to the green wire (fan) at the same time as the white wire (heat). This
usually is in cases where there is no relay in the heater for the heat activation
to turn on the fan at an appropriate time, so the thermostat must do it instead.
On heat pump thermostats, red will almost always switch on the fan instantly.
The same is true in a call
for cooling where the contacts are made through the mercury switch to switch the
red to the yellow (compressor contactor) and the green (fan) wires at the same time.
If you have a "C" terminal ("common"--other side of the transformer versus R for red) on your thermostat it usually is only for a
little light bulb indicator. It won't likely perform any other actions. However, on some of the
new electronic thermostats it will be there to perform memory and operational functions.
Sometimes there are two yellow wires for cooling Y¹ and Y² or for heating W¹ and
W². In this case you will have more than one mercury switch for heat or
cool and it is just more of the same process. The Y terminal will usually
send the voltage of the R terminal out to the outside unit and makes it come
on by energizing the contactor (relay).
If you have two Y terminals, and thus two mercury bulbs for cooling, the second one will turn on an additional compressor within the same condensing unit or it will turn on the higher speed of the single compressor. If you have two W terminals, it will either turn on more heat strips within an electric heater, a second stage of a gas valve within the furnace or if in the case of the heat pumps, it may be for the heat strips to augment the heating from the heat pump or to turn on an additional amount of electric heat during the defrost cycle of the heat pump. Sorry, it got a little complex there.
So, simply it will be:
- RED--is one side of the transformer voltage.
- GREEN--goes to a fan relay.
- WHITE--goes to a heat mechanism.
- YELLOW--is to turn on the outside unit.
- COMMON (C)--is the other side of the transformer voltage.
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It is usually thought by the non-professional that the "W" wire turns
on the heat and the fan. No-No. This is not the most common usage. Most
commonly, the WHITE wire will turn on an automatic gas valve inside the
gas heater that flows gas and ignition occurs. The chamber will heat up to a specific temperature and another thermostat inside the gas heater will turn on the fan. |
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This is that thermostat. Newer heater and furnaces don't have this control at all. They use various sequential relays or printed circuit board timers and relays and thermal discs instead. But this one we picture here is a little bit different appearance, but still very much in use. The tubular probe is the temperature sensor that senses the temperature of the air between the heat chambers and a relay inside the front portion of the fan/limit switch. It will then activate the fan as the chamber temperature reaches about 130°F or so. Additionally, the fan/limit contains a relay that will turn off the burners if the chamber temperature exceeds the allowable limits. This is, of course, the limit portion of this control. One more thing. When the thermostat on the wall turns off the gas flow, the chambers will cool eventually and the fan/limit will then turn off the fan in the furnace when the chamber temperature cools to about 100°F. |
Thus, the fan always comes on much after the flame and will delay considerably after the flame is off. This prevents bursts of cold air in the heating mode cycle. Newer units do all this with the pc board timers and thermal discs both for high limit temperature control and fan operations.
Note that this method of fan control is different from what happens in the cooling mode. In cooling, the thermostat on the wall turns on the fan.
In the newer models that now have small computers controlling everything, there is no need for the fan/limit switch because all the functions of the fan, burners, ignition and delay actions are controlled from timers versus temperature of the chamber.
If your heater is an electric unit, then a sequence of relays will be turned on by the
WHITE wire from the thermostat to the heater, and in turn will
turn on the fan. To confuse you more, some electric heating units require
thermostats that turn the G (fan terminal) on at the same time it energizes
the W (heat) wire. Fortunately, this is not very common. This is not my preference because this type
of control usually will blow "cold" air for as long as a minute before the heating
elements can sequence to full capacity to allow the air to be fully warm. Drafty feeling.
Note:
If you are replacing a thermostat, make sure at the time of purchase that you buy the correct control for your system. If the fan is turned on from the thermostat during (electric) heating, you will need to purchase one that does that. If you don't, then the fan will not turn on during heat and you run the risk of burning out the heating elements and, of course, you will have to purchase a second new thermostat.
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