Recently I replaced an old mercury thermostat with a programmable digital model (a PRO1 T955W).

It is set to operate in Heating mode. The desired temperature is set to 70 and the in-home temperature is in the low 60s. Yet, the oil-burning furnace does not turn on. As soon as I touch the LCD screen, its back-light turns on and shortly afterwards the furnace motor starts running/burning. Then after a few seconds, the LCD dims, and the furnace stops before the desired temperature was reached.

I was initially wondering if I might have switched around the R and C connectors. The thermostat only has those two inputs. After switching them again, the same problem persists.

Any ideas?


Here is a picture of the wiring. It shows the R & C wires. The insulation looks a bit worn. I have made sure that both cables are securely screwed into place. Picture of Wiring

Here is a picture of the back side of the thermostat: Picture of backside

Here is a picture of the thermostat display when connected. Here the display is lit (after I pressed the display), and the furnace immediately turned on. As soon as the display dimmed (after a few seconds of not touching the display), the furnace stopped immediately. Picture of thermostat display

  • 3
    Consider taking a picture of the wiring and adding it to your post.
    – Jay Bazuzi
    Commented Dec 22, 2010 at 6:11
  • I couldn't find your thermostat on Honeywell's website. Googling found it by a different manufacturer (I added a link) but they have an OEM branding program, which would explain things if yours has a Honeywell decal. Please feel free to roll back my edits if I'm way off base.
    – Niall C.
    Commented Dec 22, 2010 at 13:10
  • There's usually an indicator the furnace is "on" (I would guess the flames icon to the right of the temperature in your picture). Does that turn off as well when the LCD dims?
    – gregmac
    Commented Dec 28, 2010 at 23:29

3 Answers 3


What happens when you short out the two wires? A simple mercury thermostat does that to turn the furnace on, so when you do it the furnace should come on.

If that works, then I'd guess that the new thermostat is defective.

  • when I short the two wires, the furnace turns on (with a 2 second delay), and turns off after disconnecting the two wires
    – byneri
    Commented Dec 23, 2010 at 21:14
  • Then I'd return the new thermostat, and try a different one - the problem is in the thermostat, not in the wiring or the furnace.
    – chris
    Commented Dec 24, 2010 at 0:34

From what I remember about thermostat wiring, there are a couple different powers available to use. It sounds like you could be using the wrong power to supply the thermostat relay which is usually located on the thermostat (one which is only available when the LCD screen is on and whatever micro-controller in thermostat wakes up). I would suggest investigating the terminals and see if you need to power the thermostat off a different terminal or power the thermostat relays off a different terminal than the actual thermostat.

  • The outlet only has the two wires. Not sure where else to power the thermostat from. I tried swapping the R/C wires, but that does not seem to make a difference. The thermostat has its own batteries. When they are taken out, the display's backlight turns off and the LCD lettering completely disappears. So I guess the display is only powered by the batteries
    – byneri
    Commented Dec 23, 2010 at 21:16
  • @byneri - When I replaced our thermostat, there was a whole terminal block worth of wires in addition to the two you have pictured, one of which was external power for the device/relays. Perhaps the two wires are backwards? You could (not sure if this would cause damage) have the two inverted as to which should be where. I saw a reset button on the back of your device, have you tried that? There also looked like there was a potentiometer (spinny dial thing) on the back, is that possibly a hysteresis adjustment?
    – Joel B
    Commented Dec 23, 2010 at 23:10
  • I have tried swapping the two wires: same behavior. After pressing the reset button, the display indicated that it was resetting, but the problem persists. You asked about a potentiometer. I guess you're referring to the black round component from the picture of the back side (right hand side). It is a black plastic cover, and does can not be turned or adjusted (as far as I can tell).
    – byneri
    Commented Dec 24, 2010 at 21:25

It turns out that with this thermostat model PRO1 T955W, the "master thermostat" (from the above picture links) is not intended to power the furnace. Instead, one has to connect the "base module" to the C and R wires.

The "master thermostat" (the one with the display, as in the above picture links), senses the temperature and sends a wireless signal to the "base module". It also controls the temperature programming, "heat swing" and other settings. The "master thermostat" can either be wired, or operated with batteries.

In this case, the solution was to wire the "base module" to the old thermostat's wiring, and establish the wireless link between "base module" and "master thermostat". Previously with the "master thermostat" wired to the old thermostat's wiring, the display's back light must have sent a signal to the wire that turned on the furnace for as long as the display was lit.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.