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Thermostat upgrade... Loud buzzing noise from furnace xfmr


I have a zoned system with a tankless hot water heater.

Zone 1 - Original house - radiators
Zone 2 - Addition - hydronic furnace

A Taco SR502 2 zone controller takes inputs from the two thermostats.

The old thermostat in the addition used R and W wires only. The new tstat requires a C connection for power, so I ran a new 3 wire cable from the furnace for this purpose. The R and W wires then continue from the furnace to the Zone Controller which controls the boiler. The cable run was old and made up of several patches and I re-ran a new cable.

The furnace powers up on its own and powers the tstat OK. However, when power is restored to the zone controller the transformer in the furnace starts buzzing loudly (no call for heat). I tried running the tstat from the zone controller common instead of the furnace, but still get the same buzzing noise (no call for heat).

Although heat appears to work, the zone light in the zone controller is much dimmer than usual. In addition, the power light in the zone controller flickers when the furnace is switched on, but the zone controller is off.

Any ideas? The wiring is - as far as I can tell - identical to how it was previously with the old mechanical thermostat.



  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    Check your voltages everywhere. I'm going to bet on a bad connection with high resistance.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 10,278
    edited November 2015
    Does the zone controller have it's own transformer?
    Your boiler has it's own transformer?

    You could have them out of phase..best guess would be to put it all back exactly the way it was. Then see if you have the buzzing sound. If so then reverse the wires on the first transformer. Figure out which is really the R wire connection and then try to connect the common to the other side of transformer.
  • peterb01
    peterb01 Member Posts: 11
    It looks like both the zone controller and furnace were trying to power the R wire. I left R, W and C connected on the zone controller, thermostat and furnace and simply disconnected the transformer in the furnace.