When I first moved into my house about a year ago, for a place this size I thought it was odd it had two thermostats, especially since they were only about 10 feet away from each other.
I guessed correctly before even speaking with the former owners that one was for the heater, and the other was for the AC. Part of the reason for this was because when it house was built it did not have air conditioning. So about 15-20 years ago they added a central air system to house in addition to the boiler based base board heating system that was installed when the house was built. So the heating and cooling systems are on completely separate systems. At the time they did the upgrade it appears they installed a couple of digital thermostats.
The reason I assume they were installed at the same time was they were exact same matching units.
The heater control unit was in the living room and the AC was in the hallway, of course it was in exactly the wrong place directly under the cold air return, but that is beside the point.
So twice the hardware, twice the number of batteries, twice the system that needs to be set up and programmed, and you have to remember to turn the one off, before you turn the other one on or they just sit there and work against each other. Why you might be asking? Well, because they were thermostats that were set up to run either individual systems like they were in my house, or in a home that have a combined heating and cooling forced draft systems. So the wrong thermostat for my house. The irritating part of this is the previous owners probably spent more on the two wrong thermostats then they would have if they had just bought one more expensive unit which would have controlled both systems.
So I decided it was time to fix a set-up that never should have been there in the first place. So after a little looking around I located a thermostat that had all the features I wanted, and was able to control two separate systems like I have in my house.
Instead of trying to make the existing wiring work for my needs I decided to run all new thermostat wires for the AC and the Heater.
So I removed all the old thermostat in the living room and patched all of mounting holes. Then I mounted the new wiring block wall plate and used the old heater thermostat wires to pull the new wires up from my utility room in the basement. In this case if you look above the wire counts do not match my description of my heating and cooling in my house. That is because my thermostat also shipped with an outdoor temp sensor so my thermostat will not only control both systems, it also displays the current temp outside. Hence the three separate sets of wires.
After everything was pulled and cleaned up and the panel was wired, it seems a little less messy.
On a side note, do not attempt something like this if you are not familiar with data and electrical flows, have never replaced a thermostat in your life, you are not good with tools, or are afraid to actually take the cover off of the main AC and heating control units. Instead of splicing into the old wires, I actually ran completely fresh wires from inside the heater controller, and the AC blower unit and of course all the way from outside for the temp sensor. You can also splice into the old wires, but was something I wanted to avoid because I was looking to try and clean up my existing system, and not add more to the mess.
The final system up and running, and the program is active. It has the ability to program a different set up for every day of the week if you want. It is showing all of the main temps: Current indoor temp, outdoor temp, and the temp program is set try and maintain it at. And it is all controlled by a touch screen interface.
Now all I have to do is repaint the living room and hallway and there will be no evidence there ever were two separate thermostats in the first place.
BTW: If anyone one wants to purchase a couple of old working thermostats I would be willing to let them go really cheap. *laughter*