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BIGGER IS NOT BETTER: SIZING
AIR CONDITIONERS PROPERLY
It is generally
accepted that "the right way" to
specify an air conditioning system
is to calculate the loads and select
a piece of equipment that will
provide comfort to the customer in a
wide variety of conditions.
Unfortunately this is rarely
practiced.
A colleague of ours
(we will call him Bill) approached
us at a conference seeking advice on
selecting an air conditioner for his
renovated home. Our
recommendations included, "Be sure
that the cooling load is calculated
and that the air conditioner is
sized to that load." When Bill
attempted to follow these
instructions, only one of the four
contractors would submit a sizing
calculation (two others just wanted
to know how many square feet there
were in the house). Bill hired
the contractor who did the
calculation and installed a
high-efficiency four-ton unit.
Is this a success story? Not
really.
The contractor
calculated a total cooling load of
37,580 Btus per hour at 105°F
outside and 70°F inside. While
the cooling load he calculated could
have been met by a three-and-a-half
ton air conditioner, the contractor
convinced Bill to buy a four-ton
unit "because then you will always
have plenty of cooling."
Bill's air
conditioner short-cycles (runs for
shorter periods of time than it
should) even during the hottest
weather and removes very little
moisture from the air. What
went wrong? Four things:
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The design
temperature for the air is
97°F. The contractor
increased the outside design
temperature by 8°F. |
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The
recommended design indoor
temperature is 75°F.
The indoor temperature was
lowered by 5°F. The
temperature "fudges"
increased the inside to
outside differential by 59%. |
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The
contractor increased the
calculated load by 20% as a
safety factor. |
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The equipment
selected was a half-ton
larger than the next highest
available size to meet the
load he calculated. |
A two-and-a-half ton
air conditioner would have been
perfect for Bill's house.
Instead he paid more for an extra
one-and-a-half tons of cooling.
In addition to costing more to buy,
Bill's air conditioner will use more
energy than a properly sized system,
raising his utility bills. It
won't dehumidify the air as well as
a smaller system would, and chances
are that Bill will be less
comfortable. The utility,
which gave Bill a rebate for his
purchase, will also lose, since the
oversized unit aggravates summer
peak-load requirements.
Selecting the
Right Air Conditioner for the Job
Before one can design an efficient
and effective air conditioning
system, the load must first be
calculated using established
techniques. The Air
Conditioning Contractors of America
(ACCA) conducted an industry study
of residential cooling load
calculations and developed Manual J
to estimate these loads. Manual J
was adopted by ACCA and the
Air-Conditioning and Refrigeration
Institute (ARI), and is the standard
method of sizing loads for
residences.
ACCA has also
produced Manual S for selecting
equipment and Manual D for duct
design (revised in January 1995).
Manual S provides a method to select
air conditioners based on the
estimated sensible and latent load
calculated for the particular house
in the local climate.
If mistakes are made
in the load calculations or the
sizing method is flawed or incorrect
inputs are used, the equipment will
be incorrectly sized and will not
perform as it should. Field
studies have shown that most
equipment is substantially oversized
compared to Manual J specifications.
In the Model Energy Communities
Project, Pacific Gas and Electric
Company (PG&E) found that 53% of the
air conditioners checked were a ton
(12,000 Btu/h) or more oversized and
a study by Pacific Northwest
Laboratories found a third of the
air conditioners to be a ton or more
oversized.
Because of the
efficiency penalty of oversized air
conditioners and because oversized
air conditioners contribute
substantially to utility demand
peaks, in 1994, PG&E commissioned a
study by Proctor Engineering Group
to compare common load calculations
and sizing methodologies to Manual J
calculated values.
What is "Proper"
AC Sizing?
Since optimum efficiency is achieved
at continuous running, it is
important that the air conditioner
be sized to achieve the longest run
times possible. Manual J
specifies use of the 2.5% design
temperature as developed by the
American Society of Heating,
Refrigeration, and Air-Conditioning
Engineers (ASHRAE). For
instance, a 2.5% summer design
temperature of 100°F for Fresno,
California, means that the
temperature generally only exceeds
100°F for 73 hours in the season
(0.025 x 2,928 hours in the months
of June through September). A
theoretical perfectly-sized air
conditioner will run continuously
during those 73 hours. During
the rest of the time the air
conditioner will cycle and operate
at less than its potential
efficiency.
A properly sized air
conditioner should provide maximum
value to the customer as well as a
reasonable profit and further
customer referrals for the
contractor. If an air
conditioner is cycling even at four
in the afternoon on the hottest
days, it is a sure sign it is
oversized. Incidentally, if
the air conditioner is running
continuously on hot days, it doesn't
necessarily mean that it is the
right size. It is more likely
that the system is oversized and has
one of three big problems:
leaky ducts, improper charge, or low
air flow across the coil.
Oversizing:
Causes and Effect
Customers depend on the expertise of
contractors in selecting air
conditioners. Yet contractors
generally size air conditioners at
least a half-ton larger than
necessary and often oversize by a
ton or more. Even the most
conscientious contractor is driven
to avoid call-backs (or even
lawsuits). An oversized air
conditioner can mask problems from
duct leaks, improper flow across the
coils, and improper charge.
Unfortunately, many customers think
that "bigger is better," so in a
competitive situation, the
contractor proposing the proper size
unit may lose the bid.
Contractors are hesitant to adopt an
unfamiliar method of sizing when the
methods they have developed over the
years have served them well:
"I've done it this way for 30 years
and I've never had a complaint."
It is no surprise then that air
conditioners are oversized; however,
the advantages of a properly sized
air conditioner are so large that
these barriers need to be overcome.
Customers pay a price for oversized
air conditioners, and in many
climates, lose comfort as well.
A properly sized air
conditioner costs the customer less.
Bill's air conditioner cost him more
money because it was too big.
The contractor had the opportunity
to discuss the value of the air
conditioner based on the delivered
efficiency and offer Bill equipment
at a lower cost. He missed the
opportunity.
Short Cycles
Air conditioners are very
inefficient when they first start
operation. It is far better
for the air conditioner to run
longer cycles than shorter ones.
The efficiency of the typical air
conditioner increases the longer it
runs. If the on-time of an air
conditioner is only 5 minutes the
efficiency (EER) is 6.2. If a
properly sized air conditioner half
the size were used instead, the same
amount of cooling would take place
in about 9 minutes, and the
efficiency would rise to 6.9.
This represents a savings of 10% for
the customer. Most of the
cooling season the cooling loads are
well below the capacity of properly
sized air conditioners, and for
oversized units the short cycling is
a substantial problem. Because
of the short cycles, Bill's
high-efficiency air conditioner is
less efficient.
Moisture Buildup
The ability of the air conditioner
to remove moisture (latent capacity)
is lowest at the beginning of the
air conditioner cycle. The
moisture removed from the indoor air
is dependent upon the indoor coil
temperature being below the
dew-point temperature of the air.
The moisture then wets the indoor
coil and, should the unit run long
enough, will begin to flow off the
coil and be removed out of the
condensate drain. For short
cycles, the coil does not have time
to operate at the low temperature
and when the unit stops, the
moisture on the coil evaporated back
into the indoor air. Thus, in
humid climates, a properly sized air
conditioner will do a far better job
of removing moisture from the air
than oversized units. Bill's
oversized air conditioner could not
remove enough moisture from the air,
so his house was cold and clammy.
Noisy Operation
The speed of the air blowing through
the supply registers and the air
being drawn into the return grille
affects an air conditioner's
performance. If the air speed
is too high, it will be noisy and
uncomfortable, and the return grille
filter effectiveness will be
reduced. The speed through the
grilles depends on the size of the
air conditioner (a larger unit has
more air flow and higher air speed)
and the area of the grille (a
smaller grille causes higher air
speed). With a properly sized
air conditioner, it is easier to
have sufficient supply and return
grille area to keep the air speed
low and the noise at a minimum.
Common complaints about oversized
air conditioners are that they blast
frigid air and that they are noisy.
A properly sized air conditioner,
with proper ductwork and grilles,
will provide longer cycles, more
consistent temperatures, and better
mixing of the house air.
If you would like
more information on this topic
please feel free to
contact us any time. |
