The Importance Of Proper Filtration: An Overview

Written by Jim Bergmann

May 13, 2024

What do dirty blower wheels, low airflow, failed circuit boards, plugged furnace secondary heat exchangers, cracked heat exchangers, high stack temperatures, plugged evaporators, plugged condensate drains, elephant snot, failed blower motors, dirty supply ducts, dirty registers and poor IAQ, low efficiency and low capacity all have in common? Air filter bypass.

Filter Bypass:

Air Filter air bypass is one of the largest and most expensive failures in equipment installation. Additionally, it is one of the most overlooked issues in preventative maintenance and service. It can easily be prevented. In short, if you are cleaning a blower wheel, evaporator, fan motor etc. you have filter bypass and it urgently needs corrected.

Air filter bypass happens in two ways, air and dirt going around the filter or air or dirt going through the filter. Air going around is typically a result of improperly sized filters, open filter slots, a lack of filter gasketing, poor filter slot design, and duct leakage downstream of the point of filtration. Dirt going through the filter is a function of the media type and the velocity of the air as it enters the media.

A Look Back: The Durability vs. Efficiency Trade-off:

Fiberglass filters of old were designed to stop rocks and bumble bees. Most manufacturers of the old furnaces like Williams had at least two 16” x 25” filters if not 20’ x 25” filters in an “A” frame rack. The systems were designed with low airflows, high temperature rise, and low filter face velocity. Fiberglass filters protected the equipment from large objects entering the blower wheel like hair and large dust particles.

Older systems were simple and durable, but not efficient. Not efficient to operate, but also not efficient to manufacturer, ship, or install. Size wise they were monsters and required assembly that took the better part of a week as the furnace could not easily be carried down the stairs in one piece like that of today.

In the older systems blower wheels got dirty, coils dirty over time and system efficiency quietly eroded. Typical systems did little to nothing for IAQ. High mass (cast iron or rolled steel) heat exchangers, and A/C systems with large fin spacing and high latent capacities helped. Furnaces could cycle on the limit without danger of cracking the heat exchanger, and the evaporator coil for the most part was self-rinsing in everything but the most arid climates. If the system was fully charged with refrigerant, the evaporator coil would stay decently clean. If not, the top part of the coil, starving for refrigerant would never get wet, never rinse, and would plug. Condensate drains for the most part needed cleaned annually.

The Evolution to Modern Systems:

Today’s high efficiency systems by comparison are highly efficient when installed properly, compact, light, easy to ship and install, but also in a different vain, complex, and fragile, with features like low mass heat exchangers that transfer heat more efficiently, but that cannot continually cycle on limit. Furnace airflows are substantially higher for same sized equipment meaning that significantly more filter furnace area is required. On the A/C side. evaporator fin spacing is much tighter, coil temperatures are warmer resulting in dryer coils. Refrigerant charge much more critical. This means better and more filtering is required to prevent evaporator coil fouling and system failure.

Transitioning from Filters to Air Cleaners:

It’s also important to remember that we have transitioned from furnace filter to “air cleaners”. Filters were to protect the equipment and air cleaners do that as well as to protect the occupant and better manage IAQ. While they are still filters, they are distinctly different and require more carful engineering when applied to the system.

High efficiency air cleaners incorporate a filtering media made from microscopic fibers woven into an intricate web that traps and holds airborne particles of dust, pollen, animal dander, etc. The spaces between the fibers are so small that particles as tiny as 0.01 micron are easily captured. By comparison, a particle 300 microns in diameter (30,000 times larger) would fit easily through the eye of a sewing needle and a fiberglass filter.

Technical Considerations for Optimal Filtration:

Filter face velocities should be below 500 FPM and ideally 250-300 FPM in a perfect world. Remember, filter face velocity has little to no bearing on the filter thickness. Filter thickness (increased surface area) is needed to mitigate pressure drop due to more restrictive types of media. In other words, for a HEPA filter to have the same pressure drop as a typical 1” fiberglass filter (20”/25” or 3.47 Ft^2) they need about 78Ft^2 of media. This means then the need for carefully controlled pleats and about 5” of thickness for the filter housing.

The Problem with Over-the-Counter Filters:

The standard 1” filters from big box stores often fail to strike the necessary balance. They combine high-efficiency media with insufficient surface area, leading to excessive pressure drop—even when clean. If you’re going to use restrictive media, you need more area. This means more filters in parallel. 1” pleated filters collapse and or bow when the media starts to load creating a path around the filter for air and dirt to flow around the filter circumventing and benefit that they had. The filter which already may have already had excessive face velocity is now further handy capped by restrictive media. Keeping the equipment clean but scarifying airflow is like stopping breathing when the pollen count rises. You solve one problem but create another that is much worse.

Applying The Science

For a more in-depth look at the essential best practices for maintaining your residential HVAC filters and ensuring your system’s longevity and efficiency, check out our detailed video where Jim reviews his Zephyr (aka Elite Air) filter after 6 months of operation.

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