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Thermostat issue

Im stumped and have to resort to calling an Air condition tech ( which I am Not) I only do plumbing and heating unfortunately. My customer had a standard honeywell thermostat that was Air conditioning only. three wires connected RC,G, And Y. So I attempted to install a Honeywell T9 "Smart" thermostat and in the process I Ran new thermostat wire directly to the boiler (NAvien NFC) so that the new thermostat would do heating and air conditioning. On the heating side I connected R,W, And C all wired directly to the boilers control board. The thermostat will not power on unless I disconnect the Air condition and move the R,RC jumper to 1-wire. My control board on the boiler is correct and not faulty. Im assuming something with the air condition wiring is causing the thermostat to not power on for whatever reason? Ive never had this issue before.
Note: the air-conditioning appears to be wired to a fan relay at the air handler? I am not sure though


  • ratio
    ratio Member Posts: 3,544
    Not positive about the T series, but the Vision Pro series needed the C to come from the Rc transformer.
  • mattmia2
    mattmia2 Member Posts: 8,834
    Are you sure the c on the boiler is actually connected? Sometimes boilers use fan centers wired in creative ways.
  • mattyc
    mattyc Member Posts: 44
    @mattmia2 Yes I triple checked that all my wiring on the heating side was good. To test, I disconnected the AC wires, flipped the jumper on the thermostat to 1-wire and the thermostat turns on and works perfectly. Then I re-connect the AC wires and flip the jumper back to 2-wires and the screen is totally dead. it wont even turn on. My AC guy is thinking that maybe since its technically getting power possible from two transformers (1 on the AC side and 1 on the heating side) that its too much power and causing it not to work? Hes going to try and pull a common wire off the AC side instead and see what happens. He also thought maybe a resister would help? Ill have to wait and see AC guys are swamped with work right now so It'll be a minute before he gets there.
  • JohnNY
    JohnNY Member Posts: 3,223
    edited August 2022
    I recently couldn't get an Ecobee to work until I moved my R wire to the Rc terminal. Now it works perfectly. No idea why. Steam boiler, heat only.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • Jamie Hall
    Jamie Hall Member Posts: 22,100
    Not quite sure what you mean by saying flipping the jumper from 1 wire to 2 wires etc. I presume what I am reading, though, is that you have an R wire from your AC, and an R wire from your heat and a C wire from your heat? If so, it should work if there are no jumpers and the right R switch is down.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 8,834
    Do you measure ~24vac between r and c on the boiler?
  • ratio
    ratio Member Posts: 3,544
    The T series has a switch to separate (or combine?) the heating R & the cooling R. Try putting the cooling R on the Rh terminal & the heating R on the Rc terminal & see if that lets it power up.
  • 109A_5
    109A_5 Member Posts: 1,001

    Two things, you have to power the thermostat and if there is two 24VAC transformers (one for the heating equipment and one for the A/C equipment) you do not want to bridge them together since there is no guarantee they are phased correctly (how would an average homeowner know ?). Often the Low side, 'C' wire of the 24VAC transformers is bonded to Equipment Ground. So if you connect 'R', 'Rh', 'Rc', together at the thermostat you have paralleled the two transformers. Not good.

    Apparently the Honeywell wants to power the T9 between the 'R' and the 'C' terminals in 'One Wire' mode, and it does when you connect it that way, you proved it. In 'Two Wire' mode they MAY want to power the T9 by the the 'Rc' and the 'C' wire from the A/C equipment. If you are actually using the 'C' wire from the heating equipment and the Equipment Ground bonding does not exist between the two transformers the result is the thermostat is not powered. Effectively the 'C' wire is open for the purpose of powering the thermostat.

    In other words apparently in 'One Wire' mode the switch connects 'R' to 'Rc' (actually or effectively) to power the thermostat. In 'Two Wire' mode 'R' to 'Rc' are not connected. The thermostat wants power between 'Rc' and 'C' in 'Two Wire' mode.

    Some heating equipment removes power from the R, or Rh, with some safety interlocks, fuses and sensors. So in an effort to better supply power to the thermostat with a dual transformer scenario getting the power from the A/C equipment may be a better strategy.

    Try a 'C' wire from the A/C equipment. Operating current(s) really should not be going through the Equipment Ground.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • mattyc
    mattyc Member Posts: 44
    @109A_5 Thanks for that info that makes perfect sense. The AC equipment didn't have a straight forward computer board with Clearly labeled terminals. It looks like some kind of fan relay so I'm waiting for the AC guy to handle wiring the thermostat. I'm not risking accidently blowing a transformer on Air conditioning during a heat wave haha And @JohnNY I hate these new wifi thermostats. They always seam to be a problem. Especially on long island, The customers never have the proper wiring to accommodate the new stat. Job security I guess