"Mark Schofield" wrote in message
.. .
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.


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