Hello, I have a 15 year old gas fired furnace. I recently changed the old White-Rogers thermostat to a digital non-programmable one.. I connected all the wires (5 wires) White to white, yellow to yellow, red to RC, and Black to R, and green to green. The problem I'm having is being able to run only the fan. The blower comes on when the furnace fires up, but when I place the switch to make the fan run, nothing happens.

Any suggestions

What is the function of the black wire? I would expect the thermostat to need power from the transformer on the R terminal and the heat wouldn't work either if the black wire didn't supply it. What about the red wire? Normally with a single transformer, the red wire is jumpered to RC and RH. The thermostat then switches the power to the green wire for fan only.

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Originally Posted by labman

What is the function of the black wire? I would expect the thermostat to need power from the transformer on the R terminal and the heat wouldn't work either if the black wire didn't supply it. What about the red wire? Normally with a single transformer, the red wire is jumpered to RC and RH. The thermostat then switches the power to the green wire for fan only.

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As far as I can figure out on the old thermostat the black wire was labelled #4-I followed it down to the furnace and it is attached to the side of the furnace on a junction box, labelled C. I thought this was the common wire. I actually didn't connect it the first time, as per the thermostat instructions, but the stat didn't seem to be working properly. I removed the jumper between RC and R and connected the black to the R terminal also as per the instruction... these instructions... ah.

The black wire (labelled #4 on the old stat) has to do something with the air conditioning. I tried removing it, and the new stat would only display heat not cool. Maybe I need to reset something on the limit switch?

I still can't figure that black wire. Here is how most systems work:

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.

The Stat I bought is a honeywell (RTH5100B). The terminals that the stat has are labelled, RC, R, Y, Not USed, O/B W, and G. It doesn't seem that I have a blue wire. I'll have to check which wires go back to the gas valve and try to follow which ones go to the AC. Thanks for you help.