I have searched through this forum but not found a definitive answer yet on my situation
I am replacing old Honeywell thermostat with a new one (RTH300B PROGRAMMABLE). The old one had 5 wires two unmarked (I determined this was for the clock and not needed anymore). Then R was a RED wire, W was a White Wire and B was a black wire. At the other end the same three wires were connected R-Red B-Black and W-White.

Problem: New thermostat says not to use B wire. Only R & W - this does not seem to work, but when I put my multimeter leads to the wires (making sure I had voltage) - if I touch the B and W (at furnace end) it kicks on. The unit does turn off at the designated temperature. makes me think I really do need the B wire. I have read some posts where the B wire was common in older HON systems (I have a simple oil burning furnace only)

Did you read the section of the sticky on thermostat wires? You may have to go to the furnace and track down what the B is connected to. If it is a common, it should go to the transformer and also be connected to the coils of any relays. You may also have to look at the old thermostat and see what it was doing with the black.

Ha -disregard. I remembered when I replaced the blower motor two years ago I had found that whoever owned this before I did had that wiring backwards. I replaced the R with the B and everything works as it is supposed to.

Thanks anyway!!