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Richieboy67 · Jun 13, 2005, 08:29 AM

Hi all, great site!

My problem is that we only have 3 wires coming up to our thermostat. We are installing central air and I was wondering if it is possible to wire the thermostat with only 3 wires??

Thanks :D

labman · Jun 13, 2005, 08:43 AM

With 3 wires, you can have any 2 of heat, air, and continuous fan. Give up the continuous fan, and you can have heat and air.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

If you have a red, green white cable, you can use the green for air instead of fan.

labman · Jun 13, 2005, 08:18 PM

In thinking some more about this, you might want to pull in a standard 5 wire thermostat cable. That would give you the continuos fan option if you ever wanted it. It would also give you the common blue wire you would need if you ever go to a programmable thermostat. Depending where furnace is, it might not be much of a job, using the existing 3 wire cable to pull in a 5 wire one. If you want instructions on pulling wires, let me know.

One disadvantage of living with compromises like I first suggested, is that if you, or your heirs, ever sell, you can be made to pay to have it redone. May as well do it now.

Richieboy67 · Jun 14, 2005, 05:30 AM

Thanks,

I am not sure if it would even be possible to run a new wire here. Pulling the wire through may not be an option because I believe the wire is nailed in...

As long as we have heating and cooling that should be fine...

Thanks again for your help. :)

Rich