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What's needed to wire 3 zone thermostats to Weil McLain water boiler?

JBerndt Member Posts: 1
I am replacing my CGa-3 boiler with an almost identical one (but 30 years newer). The old boiler was always on (Thermostat circuit in boiler always closed) and the three circulators (each with their own thermostats and relays) worked independently.

I want to wire the new boiler to turn off when no thermostats are calling for heat (my understanding is that this is how it is supposed to operate). I can get that to work with a single thermostat/relay/circulator. The manual makes no provisions for multiple thermostats with independent circulators, but it says that if multiple zone valves are used, an isolation relay is required.

If I wire all three thermostats to the corresponding wires in the boiler, one thermostat would turn on all three circulators.


  • Jamie Hall
    Jamie Hall Member Posts: 23,283
    You'll need three relays. One for each circulator, which will power the coil. Wire the normally open contacts of the relays in parallel and connect that circuit to the boiler T-T.

    Or... control the boiler with an aquastat.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • offdutytech
    offdutytech Member Posts: 133
    A zone control panel is the way to go. It provides 24 vac to your zone valves isolating them from your boiler transformer. It provides easy connections and a light to show you what zone is calling for heat. Then it's two simple wires to your boiler to turn it on and off. I typically install taco zone control panels. Usually I buy one with an extra zone just in case a relay goes bad it's a quick 5 min fix of moving wires. Example if you have 2 zones buy a 3 zone panel.