Nest / EWC-ST-2D Controller

[sagosto]

Nest 3rd Gen
York A/C H2RA060S06D
York Furnace P4HUC20N09201A
EWC Zone Controller (EWC-ST-2D)

My zone controller requires the dreaded O/B terminals. To power my 1st floor Nest (primary), I moved the "O" terminal from wiring block under "ZONE-1 TSTAT" (3rd from top on right side) to "ZONE-1 TSTAT" "C" terminal for COMMON wire power. On the 1st floor Nest (primary), I connected the "O" (now represents common wire) to the "C" terminal. To lock the zone controller in heat mode, I used a jumper (e.g. paper clip) between the zone controller's R and B terminals. This gives me heat on both zones.

I cannot get A/C to work. If I turn A/C on, I get the fan and A/C does not turn on. I moved jumper from "B" to "O" and I now have A/C on 1st floor but I cannot get A'C to work with Zone #2. How do I lock the unit in A/C similar to what I did for heat?

[John B.]

Sagosto,

The ST controller's O and B terminals operate like a Summer/Winter switch. It prevents Zone 2 (or higher) from demanding Cooling during Winter time and vise-versa...They have nothing to do with a Heat Pump.

Your "paper clip" jumper MUST connect from the "R1" terminal to the "B" terminal, in order to lock the ST controller into HEAT mode, which then allows any zone to demand Heat.
On the flip side, you MUST remove the jumper from the "B" terminal and connect it to "O" instead (R1 to O). That will lock the ST controller into COOL mode, which then allows any zone to demand Cool.

You can purchase the part #MCS-DXB switch, that will switch the "O" and "B" circuits, so you don't have to...Or upgrade to a model NCM300, which eliminates the O and B switch over.

I have attached a wiring diagram that shows switching from the ST controller to the NCM300.

John

ST to NCM300 wiring.pdf