[dermanj]

Please help.

I want to replace an old Honeywell CT3600 thermostat with a new touchscreen RTH8500 on a gas hot air furnace w/AC. I think my confusion is what to do about the O and B wires (changeover HEAT and changeover COOL, as I understand it).

Here is manual for new RTH8500: http://www.honeywellcentral.com/ssi/...-69-1902ES.pdf

Here is manual for old CT3600: http://www.instructionsmanuals.com/u...-CT3697-en.pdf

Terminals used on the old thermostat are RWYGOB. The old thermostat is wired to a Trol-A-Temp MM-2 Mastertrol Mini-Zone-2 at the furnace: http://customer.honeywell.com/techli...0s/69-1361.pdf

Here is a quote from the MM-2 manual:

"The Zone1 thermostat requires a changeover subbase with O and B terminals. The O terminal must make with R when subbase is switched to COOL mode. The B terminal must make with R when subbase is switched to HEAT mode."

After some back-and-forth with Honeywell customer service, they tell me that the RTH8500 will not work for me -- but that was at the end of a 20-minute phone call that started with them telling me that I cannot possibly have my current thermostat (CT3600) wired correctly with both an O an B terminal, so I'm not convinced they gave me good information.

I would like to use either an RTH8500 or a TH8320. Honeywell customer service suggested that I could use a TH4110D1007, but I really don't want that. I asked if there was a touchscreen model that will work with my system and the guy said no.

Can anyone tell me if there is perhaps an appropriate way to wire my system either with an RTH8500 or a TH8320?

[hvac1000]

Trol-A-Temp MM-2 Mastertrol Mini-Zone-2

It is my understanding that the oldfashion Trol-A-Temp takes a thermostat designed to be used with it.You see like a knowledgeable guy. The O_B terminals are usually used on a heat pump to control the reversing valve.
You will have to experiment to see what might work.

BTW have you tried this in your post

"The Zone1 thermostat requires a changeover subbase with O and B terminals. The O terminal must make with R when subbase is switched to COOL mode. The B terminal must make with R when subbase is switched to HEAT mode."

[dermanj]

hvac1000 asked if I tried this (from my post):
"The Zone1 thermostat requires a changeover subbase with O and B terminals. The O terminal must make with R when subbase is switched to COOL mode. The B terminal must make with R when subbase is switched to HEAT mode."

The old thermostat/subbase has separate O and B terminals. The new thermostat subbase, however, has a single O/B terminal -- and it also looks to be the same terminal as W.

I really thought the RTH8500 (or at least the pro model TH8320) was compatible with nearly everything. I can't help but think there is a wiring configuration that will work properly. I just have little faith in what tech support guys tell me over the phone.

[hvac1000]

Your old stat had a separate circuit for heat and cool change over O and B
The new stat allows you to only choose one and that is done in the set up area of the thermostat program.

The one honeywell mentioned will work.

http://customer.honeywell.com/techli...it/69-1849.pdf

[dermanj]

I understand it will work with the TH4110D (given separate O and B terminals), but I was hoping there might be some practical way to make it work with an 8500 or 8000 series. I really wanted a touchscreen model.

[hvac1000]

It could probably be figured out but I just do not have the time do it and truthfully if a customer of mine years ago wanted me to do it I would have refused. It might take a relay (reverse acting) to make it work. The problem is the change over status. The old stat sent a separate signal from O or B depending upon what you wanted the Trol system to do. Good luck getting it working.