[QUOTE=skippedover;13249631 The best solution is to have a computer track the temps in the various zones, the trend of the room temps, the time since the last call for treatment in each zone and how that compares to historic needs for the zone, the known capacity of each zone and use modulating dampers to control the actual airflow into each zone so that minimum established airflow is allowed under all conditions. There is one other possible solution for a very small zone and that's typically referred to as a 'non-voting' zone, meaning it's thermostat is only a limiter and cannot command equipment operation. Thus the non-voting zone gets either heating or cooling only when another voting zone calls the equipment into service.

The third option, modulating zone dampers is available with the Bryant Evolution or Carrier Infinity zone control systems and equipment. [/QUOTE]

Manufacturers? Carrier, and recently American Standard, are the two state of the art communicating systems that I'm aware of.

Skip gets my vote as the top expert on communicating zoning on this site. Carrier has owned this space since 2004, so that's a lot of real life experience. Search his posts and you will quickly build a foundation of knowledge that will help you avoid catastrophic errors.

Assume my duct work will not handle the excess supply for the three zone system I plan to have installed in my 3500 sq ft, two story house. Let's also assume that it is too expensive to upgrade the ducts.
If you think about it, would you rather upsize duct so you can heat a crappy house, or put those resources toward making the house not crappy and as a result fixing the duct without touching it? I've seen a lot of insufficient duct work become sufficient just by down sizing equipment. This might be enough, or you may need to perform some aggressive weatherization improvements to get load down to where you can install small enough equipment that your duct isn't a problem.

You absolutely want a heat pump, whatever the cost of electricity. Dual fuel allows you to control switchover. Cheap gas expensive electric, you simply lock the pump out at a higher temperature. As my grandmother said, take a jacket. You'll regret having it and not needing it a LOT less than NOT having it and needing it. And on expensive electric, if you have much cooling load you'll want the top efficiency pump for cooling anyway.

Do you like the idea of producing the energy used to heat your house? If your electric is really high you might produce it yourself for less. What is your solar exposure? That option gets more attractive by the minute.

Don't use bypass design if at all possible. That's like fingernails on a chalkboard to an energy guy.