G’day,
I’m very interested and am planning to do a similar control scheme for my system. Maybe we could work together 
I’ll have a close look at your thermostat control which I’m sure will come in very handy for what I’m trying to achieve.
In my case, with my new house I’ve inherited a zoned, ducted refrigerated reverse cycle Fujitsu HVAC system. The house is two storey with high ceilings and large voids which connect the first and second floors. The outdoor and indoor units work, but due to the layout of my house and the fact there’s only one temperature sensor upstairs, the system is nowhere near optimal and I’m wasting $$$$'s in electricity. It’s worth noting that in Australia this sort of problem is common, so if we came up with a solution I’m sure it would be popular.
First floor has one zone only. Second floor has three zones, one of which covers all three upstairs bedrooms, the second zone is for a home theatre, and the third is for an open activity room next to the void. Master bedroom are currently serviced by a central evaporative cooler, but I’m looking at converting the ducting over to the refrigerative system later.
During the summer, with all interior doors closed the bedroom furthest away from the HVAC indoor unit does not cool down enough, while the other two closer bedrooms get too cold. The theatre room gets way too cold, and the activity room gets too warm because the cool air falls down to the first floor. Basically the system was designed badly and I’m sure I can fix it for MUCH less than a new zoned commercial system would cost (to put a new system in now would cost >$30k, and I’m sure still won’t work properly).
What I’m planning to do is give each room it’s own zone and temperature sensor, possibly with ceiling fans placed above the voids. I’ve found a Modbus based interface which will allow commands to be sent and retrieved from the indoor HVAC unit. With this interface I can select fan speed (low/med/high/auto), an overall temperature setpoint, and operating mode (heat/cool/dry/fan/auto). I can also send to the unit a temperature reference to make use of the HVAC unit’s internal temperature control loop.
I can see my design consisting of the following:
-New dampers installed so that every air outlet can be switched on or off
-Temperature sensors in all rooms
-Modbus interface from OpenHAB to the HVAC indoor unit to command mode, setpoint and fan speed
-Relay board which opens/closes dampers as necessary
-Software thermostats for each room, for automatic control
I plan on making a proof of concept initially with the Modbus interface, relay board and one temperature sensor. Once this is proven to work I’ll install hardware for all zones to allow for full manual control. Then I can consider automating. This is where it gets tricky, and I think this is the sort of problem you have in mind to develop a solution to. I’ve been googling for days and haven’t found anyone doing a home automation based zone controller.
Here’s how I imagine the basic control narrative:
-User chooses which rooms are to be conditioned and the desired temperature setpoint of each room
-OpenHAB would then set the HVAC unit mode and setpoint temperature based on the most extreme setting (i.e. in summer the coldest temperature setpoint would be sent to the HVAC unit, in winter the warmest temperature). Unsure if the inbuilt Auto mode should be used here, it will require some testing to see how it behaves.
-Dampers are then actuated on and off to keep the temperature in each room to be within a certain band (say 2 degree C plus or minus)
-Fan speed would be adjusted based on how many dampers are open (the more dampers open the higher the fan speed). The inbuilt Auto fan speed mode may be used, but again I’ll need to test to see how it behaves.
Once this is achieved, some advanced functions could be looked at. For example:
-Presence detection for full smart control
-Void ceiling fan control - in summer the fans would pull cool air upwards to assist cooling the second floor, in winter the fans would push warm air downwards to ensure proper heating of the first floor
-Ceiling fans in all rooms to reduce temperature differential between the floor and the ceiling
-Humidity monitoring for each room - if the humidity gets too high OpenHAB could command the HVAC unit to operate in Dry mode, if it gets too low it could activate the central evaporative cooler for a few minutes to give the house a blast of moist air
-CO2 monitoring - the evaporative cooler could be used to provide an occasional blast of fresh air as required to keep CO2 levels in check
Apologies if this was long winded, but I’ve put it in black and white for my benefit also so I know what I’m working towards 
