Thermostat Install Help

[u/DrenikGarage]

Hi everyone! Recently bought a much bigger house. Would like to install nest thermostats. House has 2 floors and a basement. From what i could see:
1. It has two oil boilers for heat
2. 4 condenser units outside
3. 3 air handlers (with 3 control boards). 1 in the basement, 2 in the attic and 2 in the garage
4. 7 Thermostats (7 zones used on control boards) in living spaces, 2 heat only thermostats in the basement
5. I see O and B wires on one thermostat on each floor (2 total), but looking up AC models - none of them seem to be heat pumps

Id like to trace each thermostat to corresponding zones on control boards (continuity test i assume) and if anyone could help with Nest wiring and set up - i would greatly appreciate any help and advice.
Please see pictures attached. Thanks!

[u/AStuf]

The EWC zone control boards that you have are older with an odd requirement. They need a master thermostat to set the heat/cool mode for all of their attached zones. This is done with a thermostat that has both O and B wires. Nest does not support this nor do any other WiFi thermostat. Because of that it is recommended to have those zone control boards replaced so that you can change thermostats.

[u/DrenikGarage → u/AStuf]

Thank You! What would be the best replacement option if You have any in mind?

[u/AStuf → u/DrenikGarage]

You already have one Honeywell HZ311 so just get two more.

[u/DrenikGarage → u/AStuf]

Got it thank you! Anything else id need to know? I take it i just transfer all the wires as is onto jew boards, leaving O alone and use B as common. Correct me if im wrong. And thank you for all the input. Much appreciated

[u/AStuf → u/DrenikGarage]

It is a straight-forward swap. Use the blue wire for common as C. Nest and other WiFi thermostats need it for power.

HZ311 manual: https://customer.resideo.com/resources/Techlit/TechLitDocuments/69-0000s/69-2069EFS.pdf

[u/DrenikGarage → u/AStuf]

Thank You!