Hi everyone. I currently have a hunter thermostat thats digital but am replacing it with a newer honeywell rth5100b. Heres my question:

The old thermostat has the standard 4 R G W Y terminals with their respective wires connected to them. However, there is a 5th wire ( blue in this case) connected along with the red to the R terminal. When looking at the wiring at the fan control, it appears that the blue wire is connected to the left most TH terminal on the gas valve and to nothing else. All other wiring seems to match up, its just this blue wire.

should i just match the old thermostat with respect to the wiring?

I would hook them up the way they came off. Unless you feel like tearing the wiring apart to find out exactly what they are doing.

If you have an older furnace, this blue wire may be to relite the pilot if it should go out , they did this on older furnaces with a standing pilot but normally would connect it to the "B" terminal which is made when there is a call for heating, by connecting it to the "R" terminal it would relite the pilot any time the pilot goes out, not just in the heating mode.

The furnace is from aprox 1984 or so. However in 1990 or so central air was added. since the R terminal supplies 24V im assuming this was just some wacky way the installer decided to connect the TH terminal. the middle terminal of the gas valve has a white wire, connected to the W terminal on the fan control, and then off to the thermostat of course. there is also a 2-wire thermostat cable going to the compressor for the a/c, one wire connected to Y at the fan control which makes sense and the other is connected to the C terminal on the control (i guess this is just a way for them to get 24V to the ceompressor.


oh and the gas valce is a white-rogers 36c03 type 241 gas valve. the furnace itself is an ICG which i beleive is essentialy keep rite now. the fan control is made by honeywell.