A
coil fouled with dirt and grime cannot supply proper heat
transfer, which results in higher discharge pressures and
greater energy consumption. Equipment operating with dirty coils
can use up to 37 percent more energy than it does with clean
coils.
Accumulating dirt on coils inhibits heat
transfer, which makes the compressor work harder, adding more
heat and raising head pressures as much as 75 percent. Higher
head pressures reduce the system's cooling capacity by as much
as 30 percent, a condition that is most noticeable on the
hottest days when cooling is required the most.
The higher temperatures and operating
pressures caused by dirty coils can shorten the life of the
equipment by leading to the breakdown of the compressor's
lubricant. Acid formation also can occur under these conditions,
leading to an acid burnout. Acid formation and lubricant
breakdown will adversely affect the compressor, leading to
equipment failure. A failed compressor means no cooling and
costly repair.
A
dirty coil reduces the cooling ability of the air blowing across
the condenser coils. If the condensing temperature is raised
from 95° Fahrenheit (F) to 105°F as a result, then cooling
capacity will be cut by 7 percent and increase power consumption
by 10 percent, with a net compressor efficiency reduction of 16
percent. Such performance degradation in a 10-ton unit operating
at 9.0 EER for 2,000 hours per year at 8 cents per kilowatt-hour
wastes about US$250 per year in operating costs (Figure 1).
It takes a technician about an hour to clean the condenser,
which typically costs about $50 and up. This is money well
spent—in this example, cleaning the unit would have a payback of
just over two months, with a net annual savings of $200.
Typically
when a HVAC contractors make the effort to service a unit,
several tasks are performed, including cleaning the dampers and
both the condenser and evaporator coils, changing the filters,
tightening belts, inspecting and servicing the fans, and
verifying refrigerant charge levels as well as damper and
economizer functionality. The cost for an annual maintenance
contract that includes all of these tasks is between
approximately $1,000 and $1,200 for a 10-ton unit. Filter
changes and inspections are frequently performed quarterly, with
the remaining tasks performed annually.
Although the energy savings achieved from
cleaning the condenser coil alone won’t cover the cost of an
annual maintenance contract, the other services performed will
also deliver savings. More importantly, these services will
ensure that the unit cools the building and its occupants as
intended. Also, they will help to prevent or quickly catch
malfunctions before they have a chance to further degrade the
equipment or waste significant amounts of energy. An economizer
stuck in the open position in a climate such as that of Tampa,
Florida, for example, can cause the HVAC system to use 50
percent more energy than a system without an economizer.
The condenser coil is exposed to unfiltered
outside air and elements such as dust, dirt, leaves, grass
clippings, animal fur, animal urine and more. While the
condenser coil does not affect indoor air quality, cleaning it
at the same time as the evaporator coil will result in improved
energy efficiency.
A coil cleaning program should be instituted
when the coils are new and clean, and should be performed with
enough frequency to prevent coil deterioration. This can be as
often as four times a year (monthly cleaning is reported in some
areas).
If they do not already exist, install
easy-to-open panels to gain access to the coils. This will make
the job much easier and, consequently, the job is more likely to
be done when necessary.
For
any system to run at its optimum performance, clean coils are
extremely important. Clean coils positively affect energy
efficiency, indoor air quality and long-term system performance.
With dirty coils, all three of these elements suffer. Here is a look
at how to keep air-cooled systems running smoothly.