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I was replacing my 2 thermostats controlled by a 2 mini zone system (Trol-A-Temp). One of the thermostat has 6 wires/terminals (Y,W,R,G,O,B). The other one has 3 wires/terminals (Y,W,R). I was planning to use an ecobee thermostats but don’t know where to connect the terminals O and B of the first thermostat. Also if I will only connect the 3 wires of the second thermostat, will it be able to access the fan when there’s no wiring provided for it? Thanks.

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  • Can you post photos of your zone board wiring please? – ThreePhaseEel Sep 29 '20 at 11:41
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You'll need a different zone board for this

The bad news in your situation is that your existing zone board (assuming its the Honeywell/TrolATemp MM-2) is incompatible with the Ecobee, at least without parts to adapt it, because it relies on both O and B being available as separate signals from the thermostat, and one of them being energized at all times. Smart thermostats generally only provide a single signal that's configurable to be O or B, so you'd need a 24VAC SPDT relay to adapt the Ecobee's O/B signal to work with your board.

Worse yet, your goal of fan control in both zones gets thwarted completely by the...half-hearted fan call implementation in the MM-2: it calls for furnace fan when it gets a fan call from the primary thermostat, but doesn't do anything for the dampers, causing fan-only operation to simply send all that circulating air down the bypass and back to the furnace in an endless, useless loop.

As a result, I strongly recommend replacing your zone board with a new board; modern zone boards will handle fan-only calls correctly and also work with standard thermostat controls without the need for O or B wires.

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  • Thank you very much ThreePhaseEel. I really appreciate your answer. I agree with you. I think I really need to replace my zone board and run 5 wires to the thermostat with 3 wires to accommodate the new thermostat. Have a good day. – Efren Oct 1 '20 at 14:10

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