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’s 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. It will always take the path of least resistance. When a large filter-rack opening close to the equipment is available, air will always flow through that opening instead of through the intended return grille(s). 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 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 building.

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 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’s had to clean a secondary heat exchanger knows this is a painful and time-consuming task.

Air handlers used with heat pumps are a bit different because the first component dirt and dust accumulate on is the indoor coil. This can create a variety of problems, as dirt and dust not only affects proper airflow, but also proper heat transfer. Dirt and dust function very well as an insulator for the refrigerant located inside the indoor coil.

DEPRESSURIZATION IMPACTS

In the cooling mode of operation, the fan in an HVAC system is typically set at its highest speed. If an open filter rack is present, there is a greater potential to create excessive depressurization in the area near the HVAC equipment.

If 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 could take place 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 can become an unintentional makeup 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 vented crawlspace, this depressurizing effect can lead to an increased moisture load on the cooling coil and subsequent air quality issues. Moist air from the crawlspace is pulled through the open filter rack and mixes with return air in the duct system.

This excessive moisture load can often overwhelm a cooling system’s 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, which 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°F 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 THE PROBLEM

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 can also be accomplished 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 measured after repairs are complete to assure pressure drop is below budgeted values.

Here is an opportunity to better serve your customers and make system improvements 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.

Publication date: 9/28/2015

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