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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I'm replacing a thermostat and have some questions
, I

Customer Question

Customer: I'm Ed; I'm replacing a thermostat and have some questions
JA: Thanks. Can you give me any more details about your issue?
Customer: I have a 2 zone system. The controller is a trol-a-temp Mastertrol Minizone 2. The thermostat I'm replacing is a honeywell Chromotherm 3; it has teh following wires connected: O W G R RC B Y,; R & RC ARE JUMPERED. MY NEW THERMOSTAT IS A HONEYWELL TH9320FW; It seems to require R RC Y C W G. Three is no C and nothing about B or O
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Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


If you have a heat pump you need a thermostat with the O/B terminals. if your outside unit runs a lot in the winter you have a heat pump.

Your new thermostat may be good for both a heat pump and a conventional system, look for that in the directions, if it does not have options for a heat pump and show the O/B terminals its the wrong thermostat.

Let me know what you find, we can go from there.

Customer: replied 1 year ago.
I don't have a heat pump. According to the Trolatemp manual, " The zone 1 thermostat requires a manual changeover subbase with "O" and "B" terminals. The O terminal must make with R when the subbase is switched to COOL mode, and the B must make with R when the subbase is switched to HEAT mode. The CHromotherm III had a COOL - OFF - HEAT switch., I assume that it used these terminals.
The otehr problem is the lack of a "C" wire, which the the new thermostat requires
Expert:  Phil replied 1 year ago.

Hello again, I am not at all familiar with that trotemp sub base, and have never seen the standard O/B nomenclature mentioned unless it was in context with a heat pump system, but I suppose it could be employed that way... double check that so far it does not seem right to me.

Meantime I will opt out in case one of our other experts here is familiar with your situation.

Good luck with it.

Expert:  Daniel Plourde replied 1 year ago.

hi, i am dan i can help you with this, you need a thermostat with a separate O and B terminal the C terminal is not too important but the thermostat for your system will not work as the master. you can use it for the slave thermostat.