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  • Neil Comparetto

Air Filters, Static Pressure, and Airflow

I get asked frequently “what type of air filter should I use?” Typically my response is “the best one that your system can handle!”.

What can your system handle?

First let’s have a quick discussion on airflow. Air conditioning and heating systems have airflow requirements for proper operation. The amount of required airflow varies depending on the size (capacity) of the system. Since it’s cooling season let’s talk about air conditioners (and heat pumps in cooling mode). They require about 400 cubic feet per minute (CFM) of air per ton (tons are how the air conditioner’s capacity is measured. My unit is 2.5 tons, so it requires 1000 CFM of airflow.)

The furnace or air handler is designed to move the required amount of air up to a certain amount of resistance in the duct system. This resistance is called static pressure. If the static pressure is too high the equipment may not be able to move the required amount of air. This can lead to many issues, from increased noise to complete failure and everything in between. Static pressure is often compared to blood pressure. If you have high blood pressure you are at a higher risk for many health complications. (By the way, if you don’t see test ports in the ducts close to your equipment, your A/C guy isn’t checking static pressure)

The easiest way to manage static pressure is by air filter selection. That’s pretty much the only thing you, as a homeowner/renter, can control. A good rule of thumb is 2 CFM maximum per square inch of filter surface area. If you have one 20”x 20” filter grille it can accept 800 CFM, or a two ton unit.

Where people run into problems is when they want to improve indoor air quality (IAQ) with their air filter but don’t have the available surface area to do so. Typical duct systems are not designed well, so the static pressure is already high. Then, the homeowner goes to the big box store and buys the best air filter they have, because IAQ is important, and now that maxed out duct system can’t deliver the required airflow. I see it all the time.

So what should you to do?

Have your HVAC contractor measure static pressure. Not just the total want to know the pressure drops across all the components (supply ducts, return ducts, indoor coil, and air filter). This is really useful information that they should know, not just to select an air filter. After doing this he or she can determine where any airflow deficiencies in your system are, and can come up with a targeted approach to correct them. For what it’s worth, I find that the air filter and the return duct system are the biggest perpetrators.

I must warn you that the most common high efficiency 1” air filters are very restrictive. Without testing 1 CFM per square inch is probably a safer target. When in doubt the $1 “rock catchers” are a safe choice.

If you are filtering the air at the equipment (the air filter has the same footprint as the furnace/air handler) a high efficiency 1” filter is almost certainly too restrictive, go with a rock catcher.

For those with a single filter grille, or multiple smaller ones, there is a high efficiency that may work for you. Instead of being 1” it is 2” deep...way more surface area, less restrictive, great retrofit option for those that want to improve IAQ. One issue with these is that they require a minimum depth of 7” in the filter grille box, sometimes that is not available.

My favorite option is a high efficiency filter media cabinet located at the equipment. These typically house a 4”- 6” deep MERV 11 to MERV 16 air filter. High efficiency, low restriction, minimal air bypass. Some of these air cleaners even have enhancements like UV lights, and electrostatic charges that increase their air cleaning abilities. The main issue is that these can be costly to retrofit, some duct modifications will be required.

In conclusion, the impact to static pressure and airflow needs to be considered when selecting an air filter. The better a filter is at trapping particles the more restrictive to airflow it will be. More filter surface area is less restrictive. That can be achieved with a deeper filter than the standard 1” type, or multiple filters located at the return air intake.

Hope you found this helpful, as always feel free to follow up with any questions.

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