Hi there,
Just got a Honeywell RTH230B programmable thermostat to replace the old one. I have a gas furnace.
I uninstalled the old thermostat and found three wires:
Red, connected to terminal marked "R"
White, connected to terminal marked "W"
Green, connected to terminal marked "B"

I was a bit confused with the "B" wire, since this letter did not appear on my new themostat. It only had Rh, Rc, W, Y, G.

I connected as follows:
Red wire to Rh
White wire to W
Green wire to G.

Problem is, once I turned the new thermostat on, the furnace did not seem to work unless I put the "Fan" switch to "On" (rather than "Auto"). But then the furnace also continued to work even after the home temperature was warmer than the thermostat setting.

Can anyone help with this? What am I doing wrong? (I put the JP2 jumper at the HG position, which they say is for gas furnaces).

Thanks.

There are a lot of questions here on that thermostat, and neither anybody else or I seem to have the answers. I know how some furnaces are wired. Others are different. I keep trying to put the pieces together. You have 2 problems. The green wire must be connected to the blower relay as is standard. Normally, the B terminal is the opposite side of the transformer or ground that some thermostats need for power. Normally furnaces have a limit switch or timer that delays the blower after the gas comes on and off after it. I guess your system could have had that function in the old thermostat. Not sure how to best fix that. Connecting the green and white wire both to the W terminal would start and stop the blower the same time as the gas. Might study the directions and see if there is any provision for the blower delay.

As for the shut off, I have less of an answer. Somebody recently reported their furnace needs power to be switched from the white to another wire. However, the green wire couldn't be both the blower and the shut off. I think a couple of people got it working by changing the battery. Some questions I can give a good answer on. I am sorry I don't have a better answer to yours.

Thanks very much for this. Turns out that, although the white wire was connected to the W terminal of the old thermostat, downstairs it was connected to... nothing at all! The Green wire, on the other hand, was connected to the common side of the transformer. So all I really had was a simple two wire system, and connecting the green wire to the W terminal on the new thermostat did the trick.

I guess the lesson is not the assume the wire colors actually mean what they're supposed to, and just go check where they actually lead to on the furnace itself.