I'm trying to swap out a circa 1996 Honeywell Trol-A-Temp thermostat with a new Honeywell Vision Pro8000 (TH8110R1008) thermostat. It a conventional furnace system with dampers controlling the 2 zones. The old Main thermostat has six wires. G, R, W, Y, B, O . No common. There's a slave thermostat for the zone upstairs with just three wires. Problem is with the "B" and "o" wires. The new thermostat has a combined B/O terminal (supposedly for heat pump system). The same terminal is used for the W heat in a conventional system. Here's a link to the wiring diagram http://hwll.co/q/VPROlit . Page 117. I called Honeywell support to decipher the puzzle but they wouldn't even talk to me because I wasn't a licensed professional. I couldn't even get out of them whether this thermostat would work on this system. "No Comment" . Any help would be appreciated. I can't believe this thermostat wouldn't work with all it's programmable technology.

 

It'll work if you add a single pole double throw relay wit a 24 volt coil.

Supco 90370 General Purpose Fan Relay, 12 A Load Current, 24 V Coil Voltage, Single Pole Double Throw Contacts: Amazon.com: Industrial & Scientific

I prefer the bigger stronger double pole double throw relay. It is a little louder but more dependable.

Supco 90340 General Purpose Switching Relay, 24 V Coil Voltage, Double Pole Double Throw Contacts: Amazon.com: Industrial & Scientific

 

I have an extra wire to use as the common "C". Why do I need the fan relay and how do I wire it and the thermostat? Thanks.

 

A relay can be used to overcome the issue with a single terminal for O and B but it will not help with the fact that you have a single stage TH8110 thermostat.

The TH8320 or TH8321 could be used with a relay but the heat setpoint would be 2 degrees off.

I would look into White Rodgers thermostats for your application.

 

Looks like White Rodgers also uses an O/B terminal with their WiFi stat...

http://www.emersonclimate.com/Docume...or_website.pdf