AIR CONDITIONING
The air conditioners that we are familiar with,
whether window- or split-, are called Compression type, because
the driving force is a compressor, usually powered by electricity. That
compressor is pulling, compressing, and pushing a refrigerant,
a fluid that boils, changing from liquid to gas, at minus 20-40°F,
far below water’s boiling point of 212°F.
In the diagram below, the Evaporator is the cold exchange coil
inside the house, in the A/C air handler. When the household
air is blown over the coil, the moisture in the air condenses
on it, and the air itself is cooled.
The diagram above is copied from Modern Refrigeration and Air
Conditioning, by Althouse, Turnquist.
Inside the Evaporator, the refrigerating liquid, pushed by compressor,
bursts through a small orifice into the enlarged area of the
exchange coil, where it expands to the volume of the coil.
That pressure drop encourages evaporation. The heat that
the exchanger draws from the warm room air completes the evaporation
of the refrigerant.
From the Evaporator, the gaseous refrigerant is drawn into the
compressor. The pressure of compression heats the refrigerant
to a temperature above that of the outside air, and drives the
refrigerant through the outdoor exchange coil. The outdoor
exchanger is called the condenser, because it is there that the
hot refrigerant transfers its heat to the not-so-hot outdoor
air and condenses back to liquid. The compressor continues
to push, driving the refrigerant back through the Evaporator.
TO SAVE ON A/C EXPENSE:
1 Design the house so as to keep
the sun out during the cooling season.
2. Install and use a whole-house fan.
3. Select the most efficient equipment.
4. Perform simple, periodic maintenance.
1. Keep out the Sun in the cooling season:
This important step is detailed in the Design section,
with further explanation under Solar Angles.
2. Install a Whole-house fan: In areas where the
days are hot, but the evenings are somewhat cooler, it would
be worth a lot to be able to turn off the AC before bedtime,
or have you forgotten how pleasant it would be if that compressor
motor was not thrumming all night?
To this end, install a whole-house fan in the attic roof or end
wall (not on the neighbor’s side). It should be able
to replace the air of the entire house in about half an hour. In
a two-storey house with a 3,000sf floor plan, a 1,000 cfm fan
is recommended.
When the outside air starts to cool in the evening, turn off
the A/C, open a door on the North side, and turn on the whole-house
fan. In a few hours, it might bring the house temperature
low enough to get through the night without A/C. In many
areas, use of the whole-house fan may obviate the need for A/C,
even during the day.
3. Buy with Efficiency in Mind: The measure of
an air conditioner’s efficiency is its EER, Energy Efficiency
Ratio. If that number is not clearly stated, or if you
question the value that is given, you can calculate it easily,
as follows:
a). The manufacturer states that this split AC system is
a “three-ton”, or “four-ton”. One
ton of AC will remove 13,000 BTUs per hour.
b). The plate on the outdoor compressor housing rates the
compressor motor as “4KW”, or “5.2KW”,
or whatever.
c). Solve the equation for a 3-ton unit that uses 4 kilowatts
per hour:
Output in BTUs (39,000) divided by Input in Watts (4,000) = EER of 9.75
At this writing, published EERs of 10 or 12 are fairly common. For
buildings that must operate the A/C 24 hours every day, it would
be prudent to buy the most efficient system available. For
older, in-place systems, find or calculate the system’s
EER. Then, study your electric bill, perhaps with
the help of an analyst at your power company, to see how much
it costs to operate your AC for the whole cooling season. If
there are months in which you don’t use AC at all, this
should be easy.
Divide your present EER by the EER of the new unit that you may
be considering, and multiply your annual AC operating cost by
the result:
Old EER divided by New EER
times old AC expense = New AC expense
Now you have another “Return on Investment” decision
to make, bearing in mind that the saving in AC operating expense
will be there every year for the life of the new equipment.
If you would like us to do the calculation, email us: franergy@verizon.net
4. Maintenance: After you buy a new AC system,
while there is still a little time left on the warranty, call
in a representative of an HVAC company to talk about a maintenance
contract. Listen carefully to comments about the
original installation, and about typical maintenance problems.
Whether or not you buy a maintenance contract, there are things
that should be done:
a) The compressor in the outdoor housing must be
kept clear, for good air movement over the exchanger, with no
encroaching bushes, no sheets of plywood casually leaning against
the housing, etc.
b) At the air handler inside the house, there is
a pan under the cooling coils, and a small pump on a copper drain
line, to carry off the condensed moisture. Check occasionally
to see that water is dripping into the pan, and that the pump
is removing it. Check, also, to see if the moisture in the pan
is very dirty, or shows any evidence of algae. This can
be a health issue.
c) Replace the air filter at least once a year, more often
if you sense that the ambient air is dusty.
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