Getting
the best performance from your new air conditioning unit
By Marshall Keneipp APS Energy Efficiency Consultant
Planning to install a new heat pump or air conditioner? Research shows the importance
of quality installations to home air conditioning system performance. If you
are in the market to have your air conditioning unit replaced, you'll need to
think about what contractor to use, what brand of unit you want, how much you
can spend and what options to select. You'll also want to know what SEER rating
to ask for in you're new unit. The SEER rating is the Seasonal Energy Efficiency
Rating and is a measure of the efficiency of the unit in the cooling season as
measured in laboratory conditions. The higher the number, the more efficient
the unit is. But did you know that the SEER rating on the unit and the actual
as-installed SEER rating can be very different depending on the quality of the
Installation? In fact, how the system is installed can be more important to the
comfort and efficiency of the overall system than the factory rated SEER level.
An air conditioning system is a complicated assembly of components including
the air conditioner itself, the ducts and the controls. All of these components
work together as a system. Efficiency and performance is a function of how the
whole system works together, not just the efficiency rating of the unit itself.
So, if you're going to be sure that you get the best value for your dollar when
it comes time to install a new system you need to become an air conditioning
expert. Right? Not really, but it will help if you know what to look for when
talking to your contractor. |
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Installation
and its effect on your system:
- Does
the system have the right amount of refrigerant?
- Does
the system have the recommended airflow?
- Does
the duct system have acceptable levels of leakage? (no system
is 100% airtight)
- Is
the unit appropriately sized?
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| Field
studies have confirmed that these are the areas where the most
common field installation problems occur and each of them can reduce
the actual installed SEER value of a system. Lets look at each
of them briefly. |
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| The
amount of refrigerant in the system: |
| Heat
pumps and air conditioners use refrigerant to provide cooling and
each installation requires a specific amount of refrigerant. If
there is either too much or too little refrigerant the system will
not function at its intended efficiency. The amount of refrigerant
is referred to as the "Refrigerant charge." Field studies
have shown that 60% to 80% of units tested do not have the proper
refrigerant charge. It is common to find systems that are undercharged
by 10% to 30%. This reduces the cooling capability and efficiency
of the system. For example, a system with 10 percent less refrigerant
than it should have (10% undercharge) will experience about a 12%
loss in efficiency, and a 20% undercharge will see about a 22%
loss in efficiency. |
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| System
airflow: |
| Air
conditioning systems need to have the right amount of air flowing
through the system in order to cool efficiently and economically.
In fact, manufacturers typically recommend that air conditioning
systems have airflow of 400 cubic feet of air per minute for each
ton of cooling capacity (typical residential systems are between
3 to 7 tons of cooling capacity). Field studies have shown that
airflow is commonly below manufacturers recommendations. On average,
air conditioning system efficiency is reduced by about 5% for typical
airflow problems. |
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| Duct
leakage: |
| Duct
leakage is another very common problem that can have three detrimental
effects on system performance. First, duct air leakage is a direct
loss of cooling capacity. Since most air conditioning systems in
the Phoenix area are installed in the attic, expensive cool air
may be lost in the attic rather than being distributed in the home
where it is needed. Second, return air leaks will often draw in
hot attic air or air from whatever the surroundings are (e.g.,
garage). Third, a difference in supply and return leakage can cause
pressure differences in the home which lead to greater home air
leakage and increased cooling energy costs. Field studies show
consistently high duct leakage levels in typical homes. One study
found an average loss of cooling efficiency of 37% due to duct
leakage. An average efficiency reduction of 21% is a good approximation
for typical situations. |
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| Appropriately
sized units: |
The "size" of
an air conditioner is measured in tons of cooling capacity. Its
natural to think that the larger (more tons) an air conditioner
is the better it will work. But when it comes to air conditioning
systems, bigger isn't better. On the contrary, it's a proven fact
that oversized systems do a much poorer job of providing comfort
than appropriately sized systems. Oversized units turn on and off
more frequently causing more wear and tear, are less energy efficient,
do a poorer job of dehumidification and can be more noisy. Not
only that, they are more expensive to buy, operate and maintain.
Studies show that oversizing by 50% or more is very common and
this reduces the efficiency of a system by about 9%. Thus, a 50%
oversized 12 SEER becomes a 10.9 once installed. The efficiency
you get may be different from what you think you're buying So,
what does all of this add up to? That is, if you think you're buying
an air conditioning system with a certain SEER rating, what are
you actually getting once its installed? Let's say you've purchased
a heat pump with a 12 SEER rating here's what you'll get if your
system is subject to these same kinds of common problems:
- A
20% refrigerant undercharge reduces its efficiency right from
the start to about 9.4.
- Combined
with improper charging, substandard airflow reduces the as-installed
efficiency to about 8.9.
- Duct
leakage alone can reduce the effective SEER to 9.5. Combining
duct leakage with undercharging and substandard airflow reduces
the efficiency of a 12 SEER to 7.0.
- Oversizing
combined with undercharging, poor airflow, and duct leakage
further reduces the as-installed SEER to 6.4.
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That's
a loss of almost 50% of system efficiency due to installation
problems! |
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| Look
for quality in an install: |
| So,
what's the moral of this story? While the SEER rating of your system
is an important indicator of the efficiency and quality of the
machine your buying, don't rely on it alone – make sure you
get quality in the installation. A properly installed unit with
a lower SEER could be much more efficient in the end than a higher
SEER unit that's improperly installed. And that means hiring a
contractor with the skills and knowledge to install it right. Those
skills may include the ability to diagnose other problems in your
home that have nothing to do directly with the air conditioning
system, such as excessive air leakage of you homes exterior "skin".
And if you are having your ducts sealed to prevent leakage, make
sure your contractor knows what he's doing. Improper sealing techniques
can cause home pressure imbalances that can result in health, safety
and indoor air quality problems. |
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Acknowledgment:
The information in this article is drawn from a paper by
C. Leon Neal of Advanced Energy Corporation, and research conducted
by APS and other utilities and researchers across the country. |
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