C wire for Ecobee3 thermostat in 2 zone heating and A/C

thermostat-c-wire

My previous post was deleted by the Community. I'm incorporating very helpful comments from Stack users here in this updated entry.

I have hydronic baseboard heating that uses a Weil McClain oil boiler. Two zones, one for the first floor and one for the second. I have central A/C with the air handler in the attic and the condenser outside. The A/C also has two zones, one for each floor.

I have two thermostats, one on each floor, with OFF, HEAT, COOL, FAN ON, FAN AUTO options. Each thermostat controls both heating and cooling. My first floor thermostat is currently a Honeywell T8775X (digital round, non-programmable). My second floor thermostat is a Honeywell CT87 (classic round, non-programmable).

Images of thermostat wiring, boiler, and A/C

Please see included images of A/C, boiler, and thermostat wiring in link above.

I plan on installing two Ecobee3, one for each zone. What would be the best way to connect the Ecobee3 – to the A/C or to the furnace? Is it ok to connect both thermostats to one system (i.e., both to the A/C or both to the furnace)?

Thanks!

Best Answer

Looks like there's an extra blue wire in each of the thermostat cables at the zone controller. Simply connect the blue wire from the thermostat cable, to the C terminal of each ZONE# T'STAT terminal block. Then connect the blue wire to the C terminal on the new thermostats. This should give you power to the thermostats.

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However, since it looks like it's using the transformer from the air handler to power the zone controller. You'll likely have to also install a jumper from the C terminal on the 24VAC Transformer terminal block, to the C terminal on the System terminal block.

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Depending on the transformer that's in the air handler, the additional load of the thermostats might be too much for it. So make sure the secondary side of the transformer is fuse protected. If it is, and the fuse blows. Then you might have to upgrade the transformer, or use a second transformer to power the zone controller and dampers.

WARNING: If the transformer is NOT fuse protected, and you draw too much current. You will end up damaging the transformer.

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