topic Nest Thermostat Wiring Compatibility - O and B wires but no heat pump in Nest Thermostats

Nest Thermostat Wiring Compatibility - O and B wires but no heat pump

Morendhil — Sat, 30 Mar 2024 17:14:22 GMT

Hello, I'm trying to replace my current thermostat with a Nest Thermostat, but there is both an O wire and a B wire, rather than a single O/B wire. From the zone controller wiring diagram, it looks like the B wire is part of the fan controls and might not be necessary? But I'm not certain if I'm interpreting it right.

Photos of thermostat wiring, zone controller wiring (zone 1), and zone controller diagram.

This is an oil furnace / AC forced air system.

Thank you so much for your assistance.

Re: Nest Thermostat Wiring Compatibility - O and B wires but no heat pump

David_K — Sun, 31 Mar 2024 13:05:08 GMT

Sometimes both the O and B wires are connected at the thermostat but the wires are not actually attached to anything at the other end on the system control board, so it's worth checking that.

Strictly speaking Nest Thermostats are not directly compatible with systems that need both O and B wires. Those wires control when your heat pump cools or heats your home. It is possible to add a relay to resolve this, usually at a zone control panel, in order to make it compatible but for that, I'd recommend consulting professional help. I'll link below guidance on how to do that.

Get installation help or HVAC service from a professional - Google Nest Help

Re: Nest Thermostat Wiring Compatibility - O and B wires but no heat pump

Morendhil — Fri, 05 Apr 2024 04:02:56 GMT

Thank you for your reply. I have photos of the the zone control panel in my original post. Are you able to interpret the wiring diagram (photo 3) for what the O and B wires correspond to?

Re: Nest Thermostat Wiring Compatibility - O and B wires but no heat pump

David_K — Fri, 05 Apr 2024 17:05:13 GMT

No as I'm not a HVAC engineer I'm afraid.