Some systems use two transformers and thus two "hots." RH for heating,
RC for cooling. If the system only has one transformer, the thermostat
has a jumper to go between RH and RC.
So, to get to five wires you'd have
RH Hot for heat
RC Hot for cooling
Yellow for compressor
White for heat
Black for common (to power the clock and electronics in a setback T
In case one breaks. Spare wire within the bundle.
Cause it's what the installer had on the truck.
To generate questions.
Obscure building code.
Cause the installer got a good price.
Any or all of the above.
Most that I have seen use 2 transformers, one for A/C one for heat.
So that would be 2 wires for A/C and 2 wires for heat with a 5th wire for
fan controls. Power for the clock and electronics is picked up from one of
the wires supplying either A/C or heat.
On Thu, 17 Jul 2008 23:30:47 -0700 (PDT), Aaron Fude
It's hot not neutral. The common wire to a group of switches (which
the thermostat is) is normally hot. In the case of normal thermostats,
that's 24VAC 1A max.
How many wires are actually connected to the thermostat? Maybe the
installer had some 5-conductor wire, and used that to save money. The
wire coming out of my furnace is 8-conductor. The other 4 aren't
connected to anything.
I usually see 4: 24VAC supply, heat, cool, fan.
An additional wire might be for a multi-speed compressor, or heat-pump
Years ago I learned the following:
Red = Hot = Power (24 volt) = R terminal
Green = Go = Blower = G terminal
White = Snow = Heat = W terminal
Yellow = Sun = A/C = Y terminal
Blue = Cool = A/C (If there is no Yellow wire) = Y terminal
The 5th wire = Common = C terminal
The wires coming from my furnace are red, white, green, and gray
(could have been blue once?). This is the cable that has 4 extra
wires: orange, black, brown, yellow (not connected to anything).
I think the color code you gave is standard. I guess whoever put this
in had some extra wire like that.
The only system I've seen with C connected to the thermostat was one
where I added that connection because of thermostat I added required
it. On other systems, C (from the furnace) is connected to the
If we are talking 24 volt wiring it is OK to have the extra wiring incase
one of them go bad you can use the extra wiring.
When it comes to the C terminal, you use it on the subbase when the stat
needs the common conection so it sounds like you have a standard cool / heat
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