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Filter Face Velocity and Why it really Matters

Coming off the COVID wave, indoor air quality is a hot button well worth discussing. Realistically 50-70 of the jobs that you will likely encounter will have excessive filter face velocity making this simple measurement an invaluable tool to increase revenue per ticket by recommending additional filters and improving returns. Paired with products like HAVEN you can offer a total solution to improving your customers IAQ, and converting services calls into additional revenue.

So how do you calculate filter face velocity?

Just like the speed of a car is measured in miles per hour, you are measuring the speed of the air going across the filter media in feet per minute (FPM).

FPM= CFM/Area OR Ft³per min/Ft²

For example:

We have a 16″x 25″ filter and we want to determine the FPM to make sure it is in range for the equipment.

We have to first convert it from inches into square feet for the denominator of our equation.

Here’s what we know

A. 16″ x 25″ = 400 in²
B. 1 Ft² = 144 in²
C. The airflow is set to 800 Ft³/min

400 in²/144 in² = 2.78 Ft²

Then from there, we can plug in the rest of the equation 800 Ft³ per min/2.78 Ft² = 288 FPM

288 FPM falls within range between 250 & 500 so it is a good filter size.

What happens if the FPM is 230? A lower FPM is not really going to hurt anything, it’s just probably a little more filter media than you might need.

Now, once you go above 500 FPM what happens is you get sifting of the air and sifting of the dirt through the filter, so the dirt molecules are going so fast that they don’t even get trapped by the filter media and they go straight through the filter and end up on the blower, the circuit board, and the coil.  You want to make sure that you have enough filter media that the air velocity is going slow enough that it actually traps the dirt in the filter media.

Now, what should we pay attention to when looking at different types of filter media? A 1-inch filter has a lot lower pressure drop because the media is like a thin cotton media and it doesn’t really slow the air down that much or create that much pressure drop.  A 3-inch filter is a lot thicker of a media that is a lot more restrictive so in order to have these two filters have the same pressure drop, the 3-inch needs a  lot more surface area and that’s why they have big pleats in them.

You will also want to make sure the return air is flowing through the media equally among the filter face, avoiding heavily congested spots on the filter, which will slow down the airflow.

You also want to make sure no air is going around the filter.  Air will take the path of least resistance, so if there is a gap between the side of the filter and the filter door, air will take that easier path and avoid the filter all together. 

How can you be sure your filtering is working properly? By monitoring indoor air quality. When we’re talking about indoor air quality, there’s only a couple things we can do; control the source of particulates or dirt, add ventilation, and add filtration. The best thing you can do for good indoor air quality is to trap dirt and throw it away.

But how do we make sure those filters are working well?  We recommend using an in-duct monitor device such as the HAVEN Central Air Monitor. It monitors air in the duct before the filter and is watching the temperature, relative humidity, volatile organic chemicals, air velocity and particulates.  So we know how much dirt is in the the air, temperature and the relative humidity of the air. Bundle it with the Central Air Controller and take action on what the monitor is picking up.  It can control the fan, adjust temperature, humidity and introduce outside air if needed.

Why is humidity control so important for indoor air quality? Excessive amount of humidity can lead to mold issues and outgassing of a lot of organic volatile chemicals.  Not enough humidity can make your textiles, like carpeting, and wood fibers break down quicker than normal, creating a lot of dust and particulates. So while the filtering can grab those and what we really want to do is just stop the source from happening in the first place.

You can also visually verify that filtering is effective by looking at the condensate trap. The water in the condensate trap should be crystal clear and when it’s not, then there’s a clear indication of dirt bypassing the filter.

Filtering not just protects the equipment from condensate line blockage, electronic component failure, reduced airflow and evaporator blockage. More important than that, there are huge opportunities to improve IAQ and protect the building occupant.

Check out the products we recommend for filtering and IAQ, click on them to learn more.

HAVEN Monitor and Controller bundle

General Filters

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This blog was transcribed by Valerie Buckles based off the video “Filter Face Velocity and Why it really Matters”, to watch the original video, click on the button below.

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