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help upgrading 2 wire system thermostat!

Hi all,

A few months ago, my husband banged his shoulder into our old, round Honeywell thermostat breaking the little plastic bubble cover. I've thought about replacing it with a programmable, and used the accident to do so.

Bought a Honeywell programmable 5-2 at local big box store. Our system has oil heat and central A/C and I searched the internet for correct hook up for the black and white wires I have. Found a lot of info, all different! Finally settled on the white wire to "W", black wire to "R" with a jumper between "R" and "Rc". Turned the furnace on, got heat, programmed it and proudly showed my hubby my work when he got home.

Proud until today. 84 degrees in the house when I got home from work. Turned little switch to "cool" setting to cool the house off for the first time after this long winter, and, nothing. Lowered temp to try to get it to come one - nothing.

I then thought about my choice of wiring, as I was very confused. If it's wired wrong, could the heat have functioned this whole winter, but the air won't function on this mis-wired tstat?

Can someone help me? First, I need to know the correct wiring, but second, I'd li,e the VERY dumbed down version of what happened/how heat works by not air?

Thanks so much! Time for a cool shower and bed....hopefully it will be a bit cooler til I can figure this out...


  • RJ_4
    RJ_4 Member Posts: 484

    Typical heating/cooling single stage t-stat wiring is  R (red) = 24volts  with Rh and Rc jumpered on 1 transformer systems.      W (white) = heating.   Y (yellow) =cooling.   G (green) = indoor fan.     and in alot of cases there will be a common wire which could be black, blue or brown  this would need to be verified. Not sure how your oper. heating and cooling with 2 wires.    do you have a sep. cooling t-stat or control ?   probably time to call a pro.
  • R Mannino
    R Mannino Member Posts: 434
    Got Any

  • Jamie Hall
    Jamie Hall Member Posts: 17,170
    I have to admit...

    that I'm having real trouble visualising how you got cooling off a two wire hookup to an old round Honeywell -- probably T87 or something of the sort.  Because... they are a simple switch; on below a certain temperature, off above it (well, a little fancier than that, but that's about the size of it).  Closing the switch told the heating system to run.

    Your programmable hooked up the way you have it is doing just about the same thing, only electronically.

    Are you sure there isn't either another thermostat somewhere?  Or another heating/cooling changeover switch somewhere?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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