whole house central AC multi-zone

After having a contractor do manual J to determine the tonnage for AC in two bedrooms and a large family room, I would like to know the problems and issues if the system is to be zoned with individual stats and motorized dampers to control the bedrooms and/or the large family room, either individually or together. The tonnage was calculated for the entire load. If only part is being cooled, would a multi or variable fan be called for? Would the higher or lower air flow affect the refrigerant pressures? Would the capacity for the whole system be too much for only one zone thus affecting the dehumidification capabilities. Hopefully someone from alt.hvac will have some input. This will be a split system for cooling only. Heat is oil fired hot water. Thanks for any thoughts. Mark S.
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Would
Dehumidification works best with a cold interior coil, a 25 degree split or more. Slowing the fan will help dehumidification but a colder coil works better.
Hopefully someone from alt.hvac

You do not mention the sqft/cubic feet involved. Zoning a/c is done in my area for only the extreme rich. It adds a level of complexity for the project that a lot of installers are not accustomed to.
Are your walls between the zones insulated? If not then there is little reason to add a lot of complexity. Do you have supply and return ducts in for the zones?
Listen to your installers and choose the one that makes the most sense. I think your trying to trip over a penny and will end up will dollars flying out the window. According to the link supplied your looking at ~400 hours annually for cooling. Where I live it is more like 2000 hours. A high seer unit installed by a good contractor with some performance numbers after the install and you will be money ahead. IMO http://198.147.238.24/ac_calc/default.asp
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Mark Schofield ( snipped-for-privacy@sbcglobal.net) said...

Air conditioners are sized to provide cycles with somewhat long duty time. If an air conditioner is oversized, then it would short cycle as the cooling power would satisfy the thermostat in short order. The problem with this is that in addition to providing cooling, the A/C is also there to provide dehumidifying capabilities. Short cycles do not give it the chance to remove much moisture from the air, so you feel cool but damp.
I suspect that by zoning off a system that is sized for the whole house, there may be some of this effect, but I also suspect that it would not be as bad as if the unit were just oversized for the whole house. The short cycling issue would vary depending on the difference in the temperature settings for different zones.
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You want to cool these three rooms in combinations but not any other room in the house. Sounds like a recipe for inefficiency but it really depends on your climate and how much you actually do need to use the A/C.
If use is infrequent (part of the day for 20-30 days per year or so like in northwest) it may be reasonable to do what you propose. If you use A/C daily for 9 months (like in the southeast) then I think you should just cool the whole house and keep it cool full time. Turning A/C off when you are out seems like it saves energy but if conditions are right you could end up using more energy to cool all the objects in the house to a comfortable temp over and over again than if you just let it stay stable.
Consider the thermal mass of the solid objects in the house as well as the thermal mass of the air you are cooling them with. The contractor probably only sized for the volume of air which assumes you keep it cooled most of the time. If you are turning it off and on often, maybe the oversized A/C for the zone is a good thing to compensate for the warm objects and walls.
PS similar logic applies to heating systems
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to answer a few questions.1- the home is in Durham, CT. and 2- a high probability that either one or the other zone would be on, Infrequently both at once.
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Mark Schofield wrote:

variable speed blower. The evaporator will need to be fed by a TXV (Thermostatic Expansion Valve).
This is what I use: http://www.udarrell.com/airconditioner_current_temperature_btuh_charting.html
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