Air Filter Static Pressure Drop

I'm a big dummy when it comes to my own air conditioning maintenance. I talk about the importance of changing air filters to customers and techs, but I never stay up on replacing my own.

Yesterday, I walked into my mechanical room, and my 2-ton air handler sounded like a vacuum cleaner about to implode.

My filter was nasty—nasty to the point that I wasn't willing to leave the filter in. So, I pulled it out and thought to myself, “I'll just grab a filter from the office tomorrow.” Well… I forgot, and I live 35 minutes from my office.

So, today, I grab a filter from my nearby hardware store—a common brand—and pull it out of the plastic wrap to install it. Sure, it was a MERV 11, but that was the only option other than the cheap, spun fiberglass “bug catcher.”

I know what you're thinking, I should have known better.

I've got to give it to this filter manufacturer for actually printing the static pressure drop on the filter (shown above).

My system is set up for 350 CFM per ton, so it's required to run at right around 700 CFM. On my system, this filter will add 0.26″wc of extra static to the return side of the blower.

With most systems being rated at 0.5″wc TESP (total external static pressure), this makes up more than half of that—before any ductwork, grilles, registers, balancing dampers, or coils, in the case of furnace systems.

On a PSC blower motor, this extra static from this filter would result in lower airflow, poor system performance, and poor air distribution.

With an ECM motor, this extra static can result in higher blower motor power consumption, condensate drainage issues, and difficulty maintaining the trap.

While some systems may be able to deal with the extra static at a cost, many will have issues, ESPECIALLY on older systems with PSC motors and furnaces with coils.

This is why larger filter cabinets with lower pressure drop filters often make sense or oversized filter back return grilles.

When choosing a filter, remember that airflow (pressure drop) is just as important to consider as filtration (MERV rating), and just because a filter fits with a system doesn't mean it's the best filter for the system.

—Bryan

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