My oil heat furnace was functioning normally on an old mercury switch thermostat, but I wanted to replace it with a programmable Honeywell 5+2 one. There were Y, W, G, R, and B wires screwed in on the old stat (see photo underneath, I took one after most of the wires were off but I assure you they were correctly labeled). The system does NOT cool. Maybe at one point it was hooked up to something, but it does not cool and there is no heat pump.

I wired up the thermostat according to the directions, but the blower fan would not turn off. I then read in the instructions that I should not connect the "B" wire, so I took that off, but the blower fan continued to run on "auto", and would turn off when I turned the fan to "on."

Any idea what's going on? I'm almost thinking the new thermostat may be defective, or I'm defective and can't figure out a basic wiring diagram.

This is the old thermostat wiring, with everything tagged correctly except for the B wire which I was yet to take off.

Old thermostat wiring

Sorry the photo of the furnace wiring is so bad. It's in a half basement and it's very dark.

enter image description here

  • Hello, and welcome to Home Improvement. Is that the thermostat end of the cable, or the furnace end? And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Oct 18 '19 at 16:41
  • It's the furnace end. Sorry Daniel, I uploaded a picture of both the thermostat end and the furnace end, but it seems that imgur didn't agree. I will update it with the photo of the original thermostat. – Steven Swyryt Oct 18 '19 at 19:38
  • BTW, big props for labeling wires; so many newbies quickly disconnect everything and then wonder why they can't figure out how to connect it back up. – Daniel Griscom Oct 18 '19 at 19:52
  • Thank you! I was able to get a hold of my friend who works in HVAC, and he wondered why there were so many wires asthe system no longer has AC. There used to be a system years ago, but the unit is long dead. He suggested to try wiring it up solely with R and W. If that works, I will also try inserting G and see if that fixes the fan switch behavior. Additionally, I'm going to remove the jumper that came in the new thermostat from R to Rc, as there is no need to power Rc. I'm wondering if having all these cooling wires in, but no cooling system, is causing the issue. – Steven Swyryt Oct 18 '19 at 20:13
  • If this works out for you, would you mind posting an answer documenting your success? You can then mark it as "The Answer"; that way future visitors will benefit. – Daniel Griscom Oct 18 '19 at 21:05

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