Brian Whatcott makes a very good point that HVAC units in the USA are
often meausured in tons. However, his estimate of a 5-ton AC unit for a
2000 sq ft home seems high to me.
I am cooling about 3000 sq ft in northwest Ohio with less than 2 tons.
My house is an old Victorian house with walls that are three bricks
thick, so it is fairly well insulated, and also has a lot of thermal
mass. Since it has hot water heat, there is no duct work, so I am using
window AC units which fortunately have gotten much quieter, more
efficient, and lighter. I have four 6000-BTU units. At 12,000 BTU per
ton, that means I have exactly 2 tons. However, I only run two units
(one upstairs and one downstairs) unless the temperature gets over 85 F
oustide. Today it is about 90 F and quite humid, so I have all four
running.
If I were to install central air, which I have considered doing, I think
I would only install a 2-ton unit; not a 5-ton unit. I think AC is
often oversized, and then it cycles on and off. The problem with that
is poor dehumidification, which is important in Ohio. I get good
humidity control by running the required number of units continuously.
If one of the units begins to cycle (because the temperature setting has
been reached), I just turn it completely off.
I'd like to have a two-stage central air system constructed with two
one-ton units. I would run one continuously and bring on the second as
needed.
The science building at Bluffton University just had its 28-year-old AC
unit replaced. A 25-ton compressor was replaced with a two-stage unit
consisting of a 10-ton compressor and a 15-ton compressor. For most of
the summer we have been running the 10-ton only and continuously. It is
cooling fine and keeps the humidity much lower than when we ran the
previous 25-ton that cycled on and off. Only on a few days when the
outside temp went above 95 F did the 15-ton stage kick in. We're saving
electricity and keeping drier with the two-stage unit.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu