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Need help with old TROL-A-TEMP controller and slave thermostat wiring

I am trying to upgrade an old TROL-A-TEMP Mercury Slave thermostat on the second floor. It has Red (power), White (heat) and Yellow (cool) wires, and I plan to add a Common line. The TROL-A-TEMP 422044 3-zone Controller panel wiring to the first floor Master thermostat confirm that Red has the constant (26 VAC) power, and switched to White to heat, and to Yellow to cool. Surprisingly however at this Slave (and also at another Mercury type Slave in the basement) the White wire is powered all the time in heat mode and the Slave thermostat switches it to the Red wire to turn on the heat (I assume when the Controller is switched to Cool mode the Yellow would be powered and would switch to the Red to cool - but have not tested this). The Slave wires are connected properly according to wire colors and connector labels on the Controller panel. Is this normal? I would love to see a schematics diagram of the TROL-A-TEMP 422044 3-zone Controller to understand this better but cannot find it on the web.


  • Jamie Hall
    Jamie Hall Member Posts: 22,934
    Keep in mind that your mercury thermostats are simple switches, and they don't either need power or care which terminal -- if any -- has voltage on it. And electricity is colour blind. What your voltage test is telling you, though (and it's useful) is that the red wire is connected back to a power supply -- so the new common wire will need to be connected to the other terminal output of that same power supply.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 3,605
    Do a Google site search of Heating Help (append 'site heatinghelp.com'), there're a few threads here about them, including schematics & how to use a Trol-A-Temp damper with a modern zone panel (HZ432, HZ311). IIRC, the slave stats switch the neutral—if it were me I'd add an xfrmr & a few isolation relays to simulate the switching action of the slave stat.

    Actually, I'd upgrade to one of the aforementioned zone panels. Steer clear of the Honeywell dampers with the little watch-motor actuators, they have a hard time closing on anything over 0" WC pressure.