Ok here is my solution to a cost effective solution for single to dual zone conversion. Some assumptions are that the AC/Furnance is big enough to heat the whole structure. Must say I have not done this but my engineering logic and Handyman experience says this would be cheap and effective... You HVAC guys please feel free to challenge this design.
Steps would be:
1) Find out where your upper and lower (output) vents are connected and install a motorized damper.- Damper just toggles between upstairs and downstairs ductwork and does not allow blockage of both.
2) Locate where the input (Cold air return) upper and lower ducts are connected. Install motorized damper that toggles between uppper and lower but never blocks both.This is the missing critical piece:
3) Install appropriate sized duct just before your motoried damper on both input side and output side. (Size would be determined by the amount of backup pressure you may to handle.) I would start with 6in and monitor system for backup. 4) Install adjustable spring loaded pressure release damper. Not sure the term is correct but it is a simple damper that opens when it sees too much pressure or too much vacuum (Same damper only reversed to be pressure or vacuum sensitive. 5) Hook onto this damper and go into home. Not important where since this is just to prevent CFM backup in your air handler.Note: The above setup handles variables such as someone shutting all the vent on a zone or one zone with less flow than the other. My experience tells me any project that requires high tolerances is doomed to fail and expensive to maintain.
6) Buy or build your 24VAC controller to run thermostats and dampers. 7) Locate your thermostats at the proper locations, normally near center of zone and near cold air return so the sample it is taking represents the rest of the zone.8) Run system and adjust backflow dampers to eliminate backup pressure or vacuum that would be under manufactures specs of minimum air flow requirement.
Notes: The cold air return control would be just as important as the heat/cold output. I would think the best performance would be to close lower input and output while the uppper is open or visa versa.
I cannot imagine, if this was DIY application, that you would be spending more than $500.00 but that is just a guess.
Steps 3-5 are probably the most important to keep the cfm regulated correctly.
I think the issues with blocked CFM are that your A-Frame (AC) can ice up if flow is restricted or the Heat transfer unit (heating) can get too hot if the flow is not there.
All modern furnances have a heat sensor near the heat transfer area and will shut down the flame if it reaches unsafe temperatures. Usually flame cycling is a good indicator of air flow problem.
Disclaimer: This is the opinion of an analytical Handyman. This process or system has not been verified so use at your own risk. My two story home will be dual zone with the above process in the next year, and I will give feedback on sucess or failure.