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Cannot power up T5+ Stat...

john p_2john p_2 Member Posts: 359
Trying to change one thermostat from a Honeywell Pro 4000 to a Honeywell Lyric T5+ but having an issue powering it up.
I have a hot water boiler for heat and air handler in basement with a condenser outside for AC - so two thermostat wires at this stat location.
I posted a diagram of how I wired it for the Lyric T5+.
Basically it's simple, for heat I connected the Red wire to R, White to W and my common wire (Green) to C. For the AC I connected the Red wire labeled RH to the RC terminal, the white wire labeled Y to the Y terminal and the Green wire labeled G to the G terminal but no luck. All the other new T5+ stats I installed are working. I did use another brand new T5+ to see if I had a bad stat but with no luck. Any help would be appreciated, thanks


  • SuperTechSuperTech Member Posts: 1,392
    Is your air handler doing heating? I don't see why a boiler would have a common. If it was a boiler without hydro air I would just expect to see two wires, RH and W. The air handler would supply 24 volts to power the thermostat at RC and Common, as well switch the fan and compressor with terminals G and Y.
  • john p_2john p_2 Member Posts: 359
    Super Tech, the boiler has a Taco zone control panel and thats where my common wire originates. My air handler only does cooling/ boiler for heat.
  • john p_2john p_2 Member Posts: 359

    Here is the terminal pad for the Honeywell Wifi stat
  • john p_2john p_2 Member Posts: 359
    I'm thinking maybe eliminate the RC terminal as it probably isn't needed if because I'm using the R and C from the heating side.
  • ratioratio Member Posts: 2,563
    Looks like the C is coming from the wrong system—stats typically take power from the cooling side.

    You do need the RC wire, it's the power source for the cooling calls (Y, & the fan [G] as well).

  • john p_2john p_2 Member Posts: 359
    Ratio, confused worse than ever now. So you saw my diagram I posted? If I eliminate the Common wire from the heating side I dont understand where I get a Common from on the cooling side as all the wires are behind the walls, the wires I have are included in the diagram
  • SuperTechSuperTech Member Posts: 1,392
    You only need one common to power the thermostat, it typically comes from the cooling side, but it doesn't have to. I would try that. You definitely need to use RH and W for the heating, RC, Y, G for cooling. Try adding the common from the air handler. Do you have a volt meter?
  • john p_2john p_2 Member Posts: 359
    Yes I have a meter but not sure I have the extra wire to grab a common.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 802
    @ratio and @SuperTech are on the right track. My experience is to power the Honeywell thermostats with a common wire you must use the transformer in the air handler... if you only have 3 wires from the air handler you may need to run a 4 conductor from the air handler

    this is what you need to do.

    if that is not possible then think outside the box like this

  • john p_2john p_2 Member Posts: 359
    Ed, you got me thinking I might be able to "think outside the box " like you say because the boiler and air handler are very close together.
  • ratioratio Member Posts: 2,563
    This is a common difficulty when upgrading to smart thermostats. It sounds like you'll be able to make something work, but sometimes it's Really Hard™.
  • HVACNUTHVACNUT Member Posts: 3,517
    Check for a blown fuse in the zone board or air handler.
  • john p_2john p_2 Member Posts: 359
    I forgot to mention I got an error code in my Taco PC700 outdoor reset when I connected the wifi stat that didn't power up. Once I reinstalled the old stat the error code went away.
  • john p_2john p_2 Member Posts: 359
    you were 100% with your "think outside the box" comment.

    My "C" common wire on the AC side was a brown wire in my air handler that was clearly documented in the literature that came with the unit.

    Luckily it is not too far from my heat system. I brought that wire over to the thermostat termination box just above my Taco zone control panel and connected it to the common wire brought down from the new wifi stat on the first floor.

    Everything now works as it should, thanks to all for the help.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 802
    edited June 16
    Glad to help. WOW ! like the pic. Someone has too much time on their hands to have diamond plate boiler room wall treatments to attach piping and controls to!

    Really like the look!
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