Why do a/c thermostats have 5 wires?

[Aaron Fude]

I would guess they need three: one for cooling part, one for the fan, and one neutral.

So why 5?

Thanks!

[Mikepier]

You need 5 if its a combination heating/cooling system, otherwise if its strictly for cooling, you only need 3.

[Rick-Meister]

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 stat

[Aaron Fude]

Actually, my thermostat is strictly for cooling and it still has five wires. Thus my question.

Thanks.

[CJT]

Hot, common, fan, cool makes four. Perhaps the fifth isn't really functional. What are the terminals to which they're connected labeled?

[Stormin Mormon]

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.

[Stormin Mormon]

Fifth wire can be transformer power, to supply power to the electronic stat.

[Stormin Mormon]

And the guy with that email adress is solidly PO'd at you for spamming him with all your group replies.

[CJT]

i.e. what I called "hot"

[Mark Lloyd]

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 changeover valve.

[Moe Jones]

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

[Mark Lloyd]

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 compressor only.

[Moe Jones]

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 stat.

[EXT]

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.

[Mark Lloyd]

Yes.