Air Filter Static Pressure Drop

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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 think 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 setup for 350 CFM(594.65 m3/h) per ton so it’s required running at right around 700 CFM(1189.3 m3/h) which means on my system this filter is going to add 0.26″wc(64.69 pa) of extra static to the return side of the blower.

With most systems being rated at 0.5″wc(124.4 pa) 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 and condensate drainage issues / difficulty maintaining trap.

While some systems may be able to deal with the extra static, many will have issues ESPECIALLY on older systems that have 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)

— Bryan

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