Making purchases of almost any type of home product or appliance,
from a new refrigerator to living room furniture, can be difficult
because of the many choices, varied options and wide range of prices.
The problem of selecting the right product, however, can be especially
difficult when choosing a new home air conditioner, particularly
for people living in hot, humid climates where the unit is used
so often during much of the year and rising energy costs can make
the wrong choice a major cash guzzler.
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FSEC researcher Don Shirey
prepares
air handler for laboratory testing.
Photo: Nicholas Waters |
A recent research project by Don Shirey and Richard Raustad of
FSEC, assisted by former FSEC staff member Hugh Henderson, provides
insight into understanding and quantifying the moisture removal
performance of cooling coils at part-load conditions (e.g., when
the compressor of a residential air conditioner cycles on and off),
which under certain circumstances can dramatically reduce the system’s
dehumidification ability and possibly even cause indoor air quality
problems.
The researchers conducted detailed performance measurements on
eight cooling coils, collected performance data at seven field
test sites, used an engineering model to look at dehumidification
performance degradation, and reviewed the existing literature on
the subject to come up with some “best practices” that
will help homeowners choose and operate their air conditioners.
One major finding clearly supported the importance of proper equipment
sizing. Single-stage cooling units that operated at less than half
of their full load with the supply air fan running continuously
(fan “ON”) provided very poor dehumidification. Homeowners
need to be sure their air conditioners are not oversized as compared
to the expected cooling loads, or the resulting time operating
at part load can result in higher humidity levels in the home.
They found that having two stages of cooling capacity also greatly
reduced the degradation of dehumidification performance at part
load. And the study also confirmed that operating a system in the “AUTO” fan
mode, with the supply air fan cycling on and off with the compressor,
is the best strategy for improving the system’s dehumidification
performance.
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The laboratory facility
was comprised of a control room (left) and two chambers to
control various air conditions. The chamber used to control
typical outdoor temperatures is shown.
Photos: Richard Raustad |
Shirey noted that the project, which the researchers worked on
for more than three years, came up with some hard data to support
a number of strategies that would help improve an air conditioner’s
performance and maximize its energy efficiency. “We really
wanted to conduct a study like this for years since many ideas
about part-load dehumidification performance and techniques for
improvement have been suggested, but we needed data measurements
under controlled conditions to prove or disprove them,” he
explained. “For example, sizing can clearly make a huge difference,
especially in terms of moisture removal. Continuous supply fan
operation, while the compressor cycles on/off to satisfy the thermostat,
is going to reduce the dehumidification ability of the system.”
Another suggestion for homeowners is to pay attention to how the
supply air fan is controlled after the compressor turns off. “Many
systems have the supply air fan continue running for a certain
period after the compressor is shut off (e.g., 1 to 3 minutes),” he
added. “This is fine when humidity levels are not a problem
and extra cooling left in the system from the cold coil can be
used. But in humid climates like Florida, this feature needs to
be disabled or the air flowing over the wet coil is going to bring
this moisture back indoors.”
A problem in doing this is that while many air-conditioning technicians
know their brand of systems well, they won’t always know
how to disable this feature on other units, and it’s not
always obvious what needs to be done. Florida homeowners should
insist that this “fan overrun” strategy be disabled.
In addition to coming up with suggestions for homeowners, the
researchers noted some strategies for the manufacturers to consider
for future units. Manufacturers need to slow down the indoor fan
proportionally when the system operates at lower capacity. “Some
manufacturers understand the importance of this in terms of dehumidification
performance,” he pointed out, “but not all manufacturers
have gotten the point yet. I suggest that consumers do their homework
and learn about the performance of systems they are considering
before choosing their next home air conditioner.”
One way to do that homework is to read through the FSEC contract
report of their study at http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1537-05.pdf (13Mbyte).
The full 613-page document is a lot more than most people
will need to get the key recommendations. But if you’re concerned
about operating your air conditioner efficiently and letting it
keep the house at a comfortable dehumidification level, you ought
to look through at least the report’s abstract and conclusions:
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1537-05-es.pdf.
It makes it clear that sizing a system for peak performance doesn’t
always result in the performance you want when the system operates
at less than full load.
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