Air Stratification


C

Question -

  I was wondering if an engineer could resolve a debate I'm
having with a friend. The debate is this: Is it more energy efficient (in
terms of cooling cost) to have a high-ceilinged house than a
low-ceilinged one? One view is that a high ceiling will allow the heat to
accumulate at the top of the house, be more comfortable at the floor
level, and require less cooling (lower cost for air-conditioning). The
other view is that a high ceiling (with the same floor space) would
result in a larger volume of air that needs to be cooled - that the air
will naturally circulate - and so drive up the cooling cost.

We have also argued over whether a flow through or vent at the top of the
house is energy efficient since the removed air would have to be replaced
and ultimately would have to be cooled.
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The parameters you specified above are not complete enough to answer yes
or no. How well the ceiling and walls are insulated is going to be a major
factor. Whether by "cooling" you mean the entire room, regardless of
shape, is at the same temperature, or whether you are going to be
satisfied with a temperature gradient between the floor and ceiling (and
if so how large a temperature gradient). What is the temperature
difference between the outside of the house and the inside. What is the
air "turn over" between ceiling/floor and inside/outside, and  what is the
heat transfer between the floor and the ground (that is, does the room
have a basement below or is it sitting on the ground) and in that case
what is the ground temperature. You have not specified the relative
humidity. A significant factor in heating/air conditioning is controlling
the humidity in the room. This is very energy intensive. So there are a
lot of undefined parameters that could change the heat balance in room.

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