Old Thermostat Has O & B Wires Separate

Problems related to residential installations.

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Old Thermostat Has O & B Wires Separate

Post by uconnjtl » Tue Dec 20, 2011 10:14 am

Hello everyone,

I am buying my parents a new programmable thermostat, and have run into an issue with compatibility. My parents have a zoned forced air system, which means the existing thermostat has the following -


The issue is that the O and B wires, which I guess control the zone damper, cannot be separately hooked up to O and B terminals on the new programmable thermostat. It only has an O/B connection.

I have read varying bits of advice on this problem across the web but can't seem to get a clear answer. One solution I heard was to essentially connect and set up the thermostat as if it were a heat pump system, with the O connected to O/B terminal and B connected to C terminal. Unfortunately, the person on that post didn't follow up about whether this solution actually worked.

Any advice would be greatly appreciated! Would love to make sure this thing can function before giving to my parents!

Thanks in advance.

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- Old Thermostat Has O & B Wires Separate

Post by heatseeker » Tue Dec 20, 2011 1:13 pm

well I'm sure others can give you advice on this better than me, so with that in mind You should not mess with the t-stat wires unless you know exactly where they go. If there is a separate zone would there not be a separate t-stat to control that zone also or maybe a duct stat? You are treading thin ice on this one and could end up frying a transformer, with all of that said you need to know exactly what equipment you are working with before you take action.

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- Old Thermostat Has O & B Wires Separate

Post by Freon » Tue Dec 20, 2011 2:22 pm

At the thermostat, write down the wire color and what terminal it connects to... red wire to R terminal etc. Then go to the furnace and verify the same wires from the thermostat terminals connect to corresponding terminals at the furnace.

What is the existing thermostat... make/model? What is the furnace make/model?

O and B are usually old heat pump wires so try and see exactly how the zone valves are activated, assuming there really are zone valves.

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