I thought it was minimum of 3 zones up to 5 zones. anyway, as to savings... as you mention not heating/cooling some areas like living are at night has a savings. comfort is a great thing with zoned systems because you can control temps by tstats rather than one tstat controlling whole house. eventually all motors fail. motors on zone dampers are not excluded. any early failures would be covered under warranty & you may have to pay labor to change. you'd have to weigh cost of zone equipment control board, motorized dampers & multiple tstats. against cost of install without zoning. factor in things like vs ahu's, two stage equipment etc to get the numbers to compare. another issue is getting a contractor with experience in zoned systems. I've always found a zoned system to be great & homeowners love it...but if the contractor hasn't done a zoned system, or uses a different brand than they are used to..it can be a disaster. in any event, compare the numbers shop for experienced contractors & get opinions/bids from them. it will be easy to tell who understands zoned systems & who doesn't once you talk to a few companies. best of luck.

2 systems would give you redundancy if the system failed. 2 systems would also give you twice as much equipment to take care of and more of a chance of something to break (twice as many parts). Carrier's Infinity zoning system is pretty sophisticated and probably the easiest route to go with a pre-existing setup. Carrier is OK equipment. Since you're zoning I think the Infinity would be a good system for that. I've heard good things about the Infinity system as a whole. Just one question: why not the newest Infinity ICS 3-stage furnace?

Have you already done the easy stuff - having effective weather-stripping around all outside doors, double-paned windows, closing up obvious areas of outside air infiltration and beefing up insulation? These things provide the biggest payback opportunities to provide comfort and save on heating. I too live in the Bay Area and find that our heater doesn't come on overnight other than during the coldest few weeks' worth of overnight temps. Our overnight setback temp is set to the high 50s. By the time we wake up, the thermostat has triggered the heater and the house is nice and warm to start the day.

Sizing can be less of an issue when you have multiple units. Because each unit is controlled independently, provided of course the unit itself is not over sized for the area in which it serves resulting in excessive short cycling. In your case 3 ton plus 2 ton isn't necessarily equal to 5 tons of cooling... unless you drive those thermostats really low. By having two systems with total capacity of 5 tons (which, most of the time this would be oversized for a 1700 - 1800 sq ft structure) has a few advantages. 1. You have the ability to use one system only during low heat days or set the opposing unit higher. The unit set higher only runs if load conditions warrant.... meaning if it's not hot, the thermostat controlling the unit does not call... the unit can not run without a call... 2. Utility bills can be much less... because you rarely use both systems at the same time. You run the bedroom system when you sleep, the other when you're awake. If you have a home office in one the bedrooms you may need to rethink things a bit. 3. Extra capacity for when it's hot out. When it's hot you have the capacity to cool the home. (even while entertaining guests --- additional people add heat) Remember heat waves come, heat waves go... a so call 'properly sized' AC system is essentially properly sized roughly 10% of the time... especially if that system is a single speed AC system. Alot of people I come across like to sleep cool. So for that reason I doubt I would change the configuration of the space. Once you realize what you have, the choices you make are mostly budget type concerns. Essentially a set up like this runs like a 2 stage system. This kind of subject is above the pay grade of many... choose your contractor wisely. I service the Katy, Texas area.