A common HVAC installation technique encountered in homes
across the country is the use of open filter racks. Open filter racks are
installed to allow for easy filter access by exposing an inch or so of the
filter. The problem with this installation technique is it creates a large
return duct leak in most systems and can be quite restrictive to airflow.
This type of installation is most commonly found in basements,
yet it’s also found in attics, crawlspaces, and garages. As common as this
installation method is, it has some unintended consequences that can plague the
safe and efficient operation of an HVAC system. Let’s look at why open filter
racks can be a problem and the issues they might be creating in the systems you
install, maintain, and service.
Airflow Takes the Path of Least Resistance
As airflow gets
closer to the fan in HVAC equipment, static pressure increases. On the return
side of the fan, this pressure is negative and has a drawing force for the airflow
that is moving through the duct system. The fan in the equipment is always moving
airflow and it doesn’t care where this air comes from.
Air is lazy and will
always take the path of least resistance. When a large filter rack opening
close to the equipment is available, air will always go through that opening
instead of through the return grille as intended. This might sound simplistic,
but it’s easy to forget and its impact gets diminished.
Filter Bypass
An open filter
rack allows a lot of unfiltered air to directly bypass the filter. Air is pulled
around any gap that it can find to move easily around the filter. When unfiltered
air bypasses the filter, dirt and dust in the air have the potential to be
distributed by the HVAC system into the home.
Dirt and dust will
accumulate on the first surfaces they encounter as they bypass the filter
through the open filter rack. In a gas furnace, the vanes of the blower wheel
are the first surfaces that dust and dirt will accumulate on. If you have a condensing
furnace, the secondary heat exchanger right above the blower wheel accumulates
dirt and dust as well. Anyone who has had to clean a secondary heat exchanger
knows this is a painful and time-consuming task.
Air handlers used
with heat pumps and water coils are a bit different as the first component that
dirt and dust accumulates on is the indoor coil. This can create a variety of
problems as the dirt and dust not only affects proper airflow, it also affects
proper heat transfer. Dirt and dust function very well as an insulator for the
refrigerant located inside the indoor coil.
Depressurization Impacts
When the fan in an HVAC system runs in cooling mode, it’s
typically set at its highest fan speed, moving the most airflow. When this
occurs, and an open filter rack is present, there is a greater potential to
create excessive depressurization in the area near the HVAC equipment.
When there are combustion appliances, such as a natural
draft water heater, located in the same space as the open filter rack, a
competition for airflow begins between the fan in the equipment and the natural
draft water heater. The water heater will lose this battle for airflow nearly every
time, as the fan in the furnace is a more powerful driving force. The flue for
the water heater ends up becoming a make-up air duct as the space is pulled
into a vacuum and an unsafe situation is created.
When
an open filter rack is installed on equipment located in a crawlspace, this
depressurizing effect can lead to an increased moisture load on the cooling
coil that can lead to air quality issues. Moist air from the crawlspace is
pulled through the open filter rack and mixes with return air in the ducts.
This
excessive moisture load can often overwhelm a cooling systems ability to remove
moisture and impact its ability to drop the space temperature. The system tries
to compensate for a moisture load it was never designed to handle, but often
comes up short. Combine this with the other air quality issues crawlspace air
adds as it’s being pulled in and you have a real recipe for a poorly performing
system.
When
an open filter rack is installed on equipment located in an attic,
depressurization can lead to an increased heating load during the cooling mode
of operation and that can also lead to air quality issues. It’s common for
system capacity to be cut in half when a large portion of return air is coming
from a 130° attic space. Just as with a crawlspace, the air in an attic
isn’t something you want your customers to be breathing.
How To Correct It
Steps need to be
taken to ensure the filter rack is as airtight as possible to prevent any
unintended bypass or depressurization effects from occurring. Before any
repairs are made, you’ll want to check the pressure drop of the filter to ensure
it’s sized properly for the equipment’s airflow needs.
If the filter
that’s currently installed is sized properly, and has an acceptable pressure
drop, you can retrofit an airtight filter access door to the existing open
filter rack. This will keep the air in the duct system where it needs to be.
If filter pressure
drop is found to be unacceptable, it’s time to look at upgrading the filtration
system to a lower pressure drop filter that can handle the proper amount of
airflow. In many cases this may mean adding an additional return duct and
filter, or increasing the size of the existing filter by adding additional
filter surface area. This may also be done by installing multiple return air
filter grilles.
Any repairs that
are made should be verified to assure they function as designed. Make sure the
new filter rack is airtight and that all airflow passes through the filter
media. Filter pressure drop should also be taken after repairs are complete to ensure
pressure drop is below budgeted values.
Here’s an
opportunity to better serve your customers and make an improvement to their
system when this type of defect is uncovered. Help your customers understand
the effects this type of installation presents and that you have the solutions
to correct it.
Original Article Published: http://ncidavid.blogspot.com/2015/05/the-problem-with-open-filter-racks.html
