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Honeywell WIFI T-Stat shorting control fuse in cooling mode

I have a hydro air system I installed about 15 years ago I was using a battery operated t-stat therefore didn't need to connect the C wire. With the new stat when I connect the C wire to the B terminal at the AHU (Tran TWE031E-SF-1B) it shorts the control fuse only when the stat calls for cooling . I am using a relay to activate the boiler which is wire from the W at the stat and the B terminal on the AHU. I am also using a aqua stat to delay the fan by connecting it in series with the W from the stat to W2 at the AHU.


  • ratio
    ratio Member Posts: 3,654
    Is this a two transformer system? Check for a short between Y and COM on the stat wire itself, unhook it at both ends. Look carefully at them, if it's pinched at one end or the other you may very well clear the fault when you take them off the terminals.
  • dbucciero
    dbucciero Member Posts: 3
    Had concerns because it is a two transformer system and was missing the obvious. Ended up being a short between Y & COM in the wall somewhere. Lucky I have a spare wire. Thank you.
  • John Mills_5
    John Mills_5 Member Posts: 952
    Bad coil in the contactor isn't uncommon.