03-30-2024 10:14 AM
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.
Answered! Go to the Recommended Answer.
03-31-2024 06:05 AM
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
03-31-2024 06:05 AM
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
04-04-2024 09:02 PM
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?
04-05-2024 10:05 AM
No as I'm not a HVAC engineer I'm afraid.