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The heat gain of the home due
to conduction, solar radiation,
infiltration, appliances, people, and pets.
Burning a light bulb, for example, adds only
sensible load to the house. This
sensible load raises the dry-bulb
temperature. |
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The
temperature measured by a
standard thermometer. |
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The net
amount of moisture added to
the inside air by plants,
people, cooking,
infiltration, and any other
moisture source. The
amount of moisture in the
air can be calculated from a
combination of dry-bulb and
wet-bulb temperature
measurements. |
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When a wet
wick is placed over a
standard thermometer and air
is blown across the surface,
the water evaporates and
cools the thermometer below
the dry-bulb temperature.
This cooler temperature
(called the wet-bulb
temperature) depends on how
much moisture is in the air. |
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Cooling loads
vary with inside and outside
conditions. A set of
conditions specific to the
local climate are necessary
to calculate the expected
cooling load for a home.
Inside conditions of 75°F
and 50% relative humidity
are usually recommended as a
guideline. Outside
conditions are selected for
the 2.5% design point. |
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Outside
summer temperatures and
coincident air moisture
content that will be
exceeded only 2.5% of the
hours from June to
September. In other
words, 2.5% design
conditions are outdoor
temperatures historically
exceeded 73 our of the 2,928
hours in these summer
months. |
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The capacity
of an air conditioner is
measured by the amount of
cooling it can do when
running continuously.
The total capacity is the
sum of the latent capacity
(ability to remove moisture
from the air) and sensible
capacity (ability to reduce
the dry-bulb temperature).
Each of these capacities is
rated in Btus per hour
(Btu/h). The capacity
depends on the outside and
inside conditions. As
it gets hotter outside (or
cooler inside) the capacity
drops. The capacity at
a standard set of conditions
is often referred to as
"tons of cooling". |
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Air
conditioner capacity is
rated at 95°F outside with
an inside temperature of
80°F and 50% relative
humidity. Each ton of
air conditioning is
nominally 12,000 Btu/h (this
comes from the fact that it
takes 12,000 Btu to melt a
ton of ice). While an
air conditioner may be
called a three ton unit, it
may not produce 36,000 Btu/h
in cooling. There is a
wide variety of actual
capacities that are called
"three tons". |
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The Energy
Efficiency Ratio is the
efficiency of the air
conditioner. It is
capacity in Btu per hour
divided by the electrical
input in watts. EER
changes with the inside and
outside conditions, falling
as the temperature
difference between inside
and outside gets larger.
EER should not be confused
with SEER. |
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The Seasonal
Energy Ratio is a standard
method of rating air
conditioners based on three
tests. All three tests
are run at 80°F inside and
82°F outside. The
first test is run with humid
indoor conditions, the
second with dry indoor
conditions, and the third
with dry conditions cycling
the air conditioner on for 6
minutes and off for 24
minutes. The published
SEER may not represent the
actual seasonal energy
efficiency of an air
conditioner in your climate. |
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Manual J is a
widely accepted method of
calculating the sensible and
latent cooling (and heating)
loads under design
conditions. It was
jointly developed by the Air
Conditioning Contractors of
America (ACCA) and the
Air-Conditioning and
Refrigeration Institute
(ARI). |
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Manual S is
the ACCA method of selecting
air conditioning equipment
to meet the design loads.
It ensures that both the
sensible capacity and the
latent capacity of the
selected equipment will be
adequate to meet the cooling
load. |
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Manual D is
the ACCA method for
designing duct systems.
Contractors often find it a
laborious process and most
duct systems are just
installed, not designed.
The amount of time necessary
to design a duct system is
certainly warranted in tract
construction where the
design is used repeatedly
and for custom homes where
the total cost of the home
warrants a proper design.
In short, designing a duct
system is essential for
proper equipment performance
and customer comfort. |
