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.
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
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
Mark Schofield ( email@example.com) 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.
"Never ascribe to malice what can equally be explained by incompetence."
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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