Old EWC unit 3 wire thermostat

[slieblich]

have a 3 wire EWC thermostat,part of 2 zone system with damper control board from EWC. Installation 1990. There is a remote switch for heat/ac/off along with fan auto/on
I'd like to change to a smart thermostat, and note 3 wires in my thermostat: Y6; R5; W4
are there equivalents of these wires to what is the R, fan and Heat wire? I assume the remote switcher causes the heat wire to change to AC wire when that switch is thrown.

[Erik W.]

Your remote switch does not switch the heat wire to a/c when the switch is thrown it actually energizes either your O or B terminal which switches the thermostats between heat and cool. R is 24 volt hot, Y is for cooling and W is for heating. You do not have a fan wire hooked up, and it is not needed because both your indoor and outdoor units will bring on the fan themselves upon a call for heat or cool. You only need a G wire for continuous fan only operation. When installing your new smart thermostats you are going to only need to connect the same three wires, a fourth wire can be added to the common terminal to run the thermostats on 24VAC instead of battery power.

Erik

[slieblich]

Thank you for the explanation and info; Appreciate it!
Not sure what is meant by the O and B terminal, but I assume I need to bring in a common wire since the Nest doesn't have a battery. Does that need to come off the same 24v transformer as is serving the R wire? I see 2 transformers on my air handler.
thanks again.
Stu

[Erik W.]

The board is supplying the power to the thermostats from the R terminal on the thermostat terminal blocks and you will be hooking up your common wire to the C terminal on the board at the thermostat terminal block, you do not need to do anything with the transformer in your unit.

[alfredalfred]

There are equivalents of these wires to what is the R, fan and Heat wire? I assume the remote switcher causes the heat wire to change to AC wire when that switch is thrown???

[Paul Z.]

Yes you are correct.