Hi,
It helps us to know what make/model old AC roof unit and control box you have along with the old T stat. Or even better, a picture of the wiring diagram on the side of the AC roof unit, the inside control box and the old T stat.
We also need to know what type (make and model) of new digital T stat you are using. There are many types, some will work with the older analog control boxes, some won't.
What little you told, it is a Duo Therm AC, which points to a Dometic. Most of the older Dometic's have a 6 wire white plug from the roof unit to the control box. Is that correct?
But you did say you had a large wire harness, that points to the ducted system and Duo Them points to Dometic
I'm going out on a limb here guessing what you have, does the control box in the ceiling below the roof unit looks like this?
Does the inside that metal box looks like this?
And does the wall thermostat look like this? With the cover plate off.
If the answer is yes to both of those questions, you have the older Dometic ducted air analog control system.
To upgrade the T stat to a digital into that older system, you need to buy a new T stat that has dry contacts in it. Dry contacts meaning a relay with an normally open contact that will close to create a call for heat signal. The digital type of T stats I'm referring too often have batteries (2 AA batteries etc) in them to get the power to run the T stat.
If your furnace is also controlled by the same T stat now, then new digital ones can also control the old furnace. But again the system needs to have a new digital T stat that has a relay interface.
Give us a little more to go on with what you have, and we can help better.
I have done many upgrades on the older Dometic analog systems with Hunter digital T stat, approx $30.
Hope this helps
John
