Hi there.

I want to replace my current White-Rogers 1E56W309 thermostat with a new Honeywell RTH7500D. The problem is that the tools that installed my high efficiency furnace didn't install the wall plate with my thermostat (it was a new house and the builder contracted these guys). Instead, they wired the two wires directly to screws on the thermostat. The dilemma that leaves me is that I have no terminal labels for the current wire placement.

From what I've read, I'm pretty sure that the two wires I have are for terminals R and W. One wire is red and one is black. However, if I look at the wire connections to the current thermostat, the red wire is connected on the left to a screw that has another wire running to the mercury bulb. The black wire is located on a screw to the right and does not have any other wires connected to it via the front of the thermostat.

So my questions are this:

1) Do you agree that the two wires will be R and W?
2) My theory is that the red wire on the screw that has another wire connecting to the mercury bulb is my R wire which, through process of elimination would mean my black wire is my W. Do you agree with my theory?

I don't want to find that I've mixed up my R and W wires while trying to replace a simple 2 wire thermostat. Any confident advice would be greatly appreciated.

For a truly simple 2 wire thermostat, it doesn't matter. You could go to the furnace and see if the red wire is connected to R or the transformer, and the black to the W or the gas valve. If so, you are correct, red to R and black to W. For heating, the thermostat switches power from the transformer to the gas valve or whatever the heating plant has. Which wire is which only becomes important when you add continuos fan and A/C, or a digital thermostat draws power from the transformer.

Both wires would have connected to the mercury switch. If the switch has 3 contacts, the red from the transformer should have been connected to the common. I would trust them to be the right places.

That was great advice.

After reading your post I followed the wire to my furnace again. What had me additionally confused at first is that my furnace has a computer board where the wires to/from the thermostat attach to. However this time, I gently lifted the wires and found that the terminal descriptions for the wires were located on the board. The verdict was that R was red and W was black just as speculated. However, now I could hook up my new programmable thermostat with confidence.

Thanks very much Labman!