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I recently moved into a new home and wanted to replace the old thermostats with digital Honeywell ones. From what I understand the home has a Rheem 10 Seer (1996) compressor and a Williams oil furnace installed in 2012 (not sure of the exact model). The system is zoned via dampers with a master thermostat downstairs (this one can switch between heat/cool and fan on/off) and a secondary thermostat upstairs (just temperature control on this one).

My problem is that I can't figure out why my main thermostat has a B and an O wires. Full list of wires with labels:

  • B - black wire
  • O - orange wire
  • G - green wire
  • Rc + Rh (via jumper) - red wire
  • Y - yellow wire
  • W - white wire

Master thermostat picture: master thermostat

The secondary thermostat is even stranger with what seems to be incorrect labels/wire colors. It has three wires with what seems to be labeled like this:

  • W/Heat - white wire
  • R/Com - red wire
  • B/Cool - green wire

Secondary thermostat picture: secondary thermostat

Instructions page from the new thermostat for conventional systems: thermostat manual screenshot

Which wires would I connect these on the Honeywell thermostat, and which settings should I choose in the setup?

UPDATE: I called Honeywell's help line and they redirected me to a zoned help line (800-828-8367 option #5). There a very knowledgable sounding person told me that my thermostat is not compatible with zoned systems. He said that the B and O wires here are used for controlling the dampers in the ducts. His recommendation was to get a RTH2410 or RTH2510 (newest models with no screws on front plate). The manuals for these suggest that they do accept B and O wires in non-heat pump situations.

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Do you have the manual for the old thermostat? –  Tester101 Jul 19 at 3:33
    
I do not have a manual. –  ipartola Jul 21 at 14:44

1 Answer 1

You should check the wiring in the furnace. There are no standards for thermostat wire colors, so you can't always trust the color of the wires. Seeing how the wires connect to the furnace, is a sure fire way to figure out what's what.

The O (cool mode) and B (heat mode) terminals are usually used for a reversing valve, in heat pump systems. I'm not familiar with oil systems, so I can't say for sure what they would be used for.

Some thermostats have separate O and B terminals, like this.

Thermostat wiring diagram Thermostat wiring diagram with O/B

Others have an O/B terminal, and use a jumper to select which should be used.

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