I'm trying to help a neighbor set up her forced air furnace to run the fan manually to circulate air. The old thermostat has a fan switch, but it doesn't do anything. There are only two wires (red and white) to the thermostat, so I assumed the lack of a third (green) wire, which mine has, is the limiting factor.

Since the furnace closet is on the lower level of the split-level house, I thought I'd mount a new thermostat on the wall adjacent to it, and run a new set of wires straight to the furnace. But it has a 24V transformer, also mounted in the closet, and I can't understand how it needs to be wired. Currently, there are two wires, red and white, running from the transformer into the closet wall. There are two wires, red and white, running from the wall (mid-level floor) to the old thermostat upstairs. And there are two wires, red and white running from the closet wall to the furnace. These are connected to red and black wires on the furnace controller. I don't know what's happening behind the walls!

Since I have the transformer together with the furnace, and I have new wires coming from a new thermostat into the furnace closet, isn't this a scenario for a simple wiring job? I thought it was going to be easy before I realized the 24V was a factor. It probably is easy, if you know the wiring procedure.

Thanks for any help I can get.

George

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. Many have RH and RC to allow separate transformers for heating and cooling. With just one red wire, jumper them. 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.

From the above, you need a wire from the transformer to the C terminal at the furnace, and the other one to the R terminal of the thermostat, plus wires connecting the W & G terminals at each end. It may be fairly easy to use the existing 2 wire cable to pull a 3 wire cable from the furnace to the closet. For the difference in cost, you could pull a 5 wire one for any future upgrades. It is fairly easy to fish wire through an interior wall. Post back if you need help on it.

Ok thanks, this is very clear. But what is the red wire coming into the furnace with the white (from the thermostat)? Does the furnace tap into the 24V line (for its electronics)? If so, there must be a tie-in or splice inside the wall that produces this red wire.

George

I thought I understood the problem until I carefully reread everything. Are you saying there are 2 transformers? If so, I would do away with the one in the closet. Then use 3 of the wires to run from the furnace to the new thermostat, or pull in a new bundle.

Are you sure the transformer in closet is for the furnace? Not the door bell or something?

Sorry if I'm confusing. Closet = furnace closet = small room where furnace is located off downstairs hallway. There is one transformer mounted at the wall switch for the furnace. The transformer has a red wire and a white wire which disappear behind the sheetrock. On the other wall of the closet, apparently heading back from the thermostat, is another set of red and white wires which feed into the furnace. From what you'd said, I'd expect only a white one coming from the thermostat.

The objective is to abandon the current thermostat and mount a new one right outside the furnace closet on the lower level, so I can run new thermostat wires (through 3" of wall) to incorporate a green fan wire. I just wasn't sure about the involvement of the 24V line. I was originally think I could simply run red, white, and green wires straight from the new thermostat to the furnace wiring.

George

Quote:

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Originally Posted by gkidwell

.... I was originally think I could simply run red, white, and green wires straight from the new thermostat to the furnace wiring.

George

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That should work as long a wire from the transformer is connected to the same R terminal at the furnace as the red wire to the thermostat. Then the second wire from the transformer needs to go to the C terminal of the furnace. The old red and black wires should work.

Thanks labman, upon thinking about it, I'm sure your last will work (I don't have frequent access to this home, so I can't check it right now!). Thanks for your help. And FYI, we love Labradors (and Goldens too).

George