When and why should I change my HVAC air filter?

Written by Air Filter Expert Jon Holmes

Dirty, dusty filters; no one likes changing them, but they play such an important role in tenant comfort and building efficiency.  Unfortunately, many times building operators aren’t getting the best value for their filter spend because air filters are either changed too soon, resulting in increased labour and filter costs, or some overextend the filter’s life to save filter dollars but pay an energy penalty to do so.

“I change my filters when they ‘look’ dirty”

an image of a dirty air filter with a white filter frame on top of a cleaner looking white filter.

All too often filters are changed prematurely because the filter “looks dirty,” but many still have service life left.  As a comparison, fast lube shops show customers the outside of the car’s air filter to upsell a replacement, but the dirty upstream side of a filter is a poor indicator of when to change it. No person visually, no matter how experienced, can accurately estimate how much life a used filter has left. If any visual metric is to be used, the downstream (clean air) side of the filter should be the only visual cue, but pressure drop is the only accurate way to properly determine filter performance and life expectancy.  As long as a filter maintains MERV-A efficiency, air quality does NOT improve with more frequent air filter changes.

“I change my filters based on time”-Should I always change my air filters on a set schedule, or is there a better way?

a woman is holding up a clock with bells on top of it with one hand and pointing to the time with her other hand. She looks stressed-out

The pollutant challenges that air filters face change year to year, depending on seasons, construction, wildfires, etc.  As such, changing filters based on time could lead to either prematurely changing filters, or keeping them in the air handling units too long and restricting airflow.  Depending on the air handling equipment, this will either reduce airflow or increase energy consumption as fans must work harder to pull air through a clogged filter. Many operators use a hard rule of “every three months” to determine when filters are changed, but like in the fast lube shop example, technology has advanced so that the car’s oil should be changed based on the number of kilometers instead of time as it is a more accurate representation of the effectiveness of the oil’s ability to protect the engine.  Air filters are no exception.

So how do I know when I should change my filters if not based on visual interpretations or time?

It depends on what the end goal is – energy or time savings? The first step is to either get a pressure differential gauge, or ensure existing ones are in working order. As the saying goes, “If you don’t track it, you can’t manage it”.

Energy: If the facility has variable frequency drives, it is typically cheaper to replace the filter at approximately double the initial pressure drop rather than waste the energy to push air through a pressure-loaded filter. For example, at $0.10/kWh, at 500 FPM velocity, operating for 12 hrs/day it will cost approximately $17 in energy per 1/10th of an inch of water gauge per filter opening per year. For a building with one hundred 24″ x 24″ pre-filters, this equates to $8750/year in energy at just a ½″ pressure difference. Often times building operations will leave an air filter in longer to reduce filter costs. However, for example, to save a $6 air filter by running it to 1″ w.g., it costs the facility an extra $87.50/filter in energy usage. Instead of saving money, they are actually spending too much.

Here is the engineering formula to justify the quantification:

Labour: On constant speed fans, there won’t be as much energy savings specifically from changing the filter at double the initial pressure drop, but there will be an increase in airflow. There may be energy savings as the coils won’t have to work as hard to maintain temperatures because of the increased airflow in the facility. Five minutes per filter of labour can be reallocated to higher priority items if the filter was changed at a maximum 1”w.g. should airflow demands be met. 

How can you afford not to consider sustainable high performance air filtration as part of your net zero plan?

Which way should I install the air filters?

Pleats and pockets should always run vertically (up and down). When ordering filters with different height and width, it is important to ensure  the pleats and pockets are running along the vertical size.  For example, if the frame where filters are installed is 24H x 20W, but the filters ordered are 20Hx24W, they can be installed, however, the pleats and pockets would be oriented horizontally, leading to worse performance and potential for lower quality pocket filters to oscillate enough to cause holes in the filter.

“Great information, but what’s in it for me?

a gauge that reads max and has R.O.I. printed above it. This symbolizes great return on investment when using premium air filters

Every day, building operators are tasked with more and more projects, in addition, everyone is under pressure to reduce costs. To retain tenants, building operators are also tasked with the challenge of keeping the air temperature and quality at a perfect level for everyone, a near-impossible feat with economy air filters.

Bottom line . . . changing both pre- and final filters based on pressure drop measured by a Magnehelic, rather than an arbitrary time or visual metric will result inhappier tenants, lower filter costs, lower energy bills, and lower waste costs while being able to reallocate labour. Staff can focus on tenant engagement and other more value-added projects.

Ultimately, the best value air filter is one that maintains the lowest AVERAGE pressure drop/resistance to airflow, taking longer to reach the 2X initial pressure drop while maintaining air quality through sustained MERV-A ratings.  A $10 filter that lasts twice as long as a $5 filter provides a lot more value than meets the eye.

Read more about high-quality air filters

To consult with an air filtration expert on the topics covered in this post or for further information, please use this contact form and an air filtration expert from Camfil Canada will answer any questions you may have.

About Camfil Canada Clean Air Solutions

For more than half a century, Camfil has been helping people breathe cleaner air. As a leading manufacturer of premium clean air solutions, we provide commercial and industrial systems for air filtration and air pollution control that improve worker and equipment productivity, minimize energy use, and benefit human health and the environment. Read more about Camfil Canada


Media Contact:

Phillip Ilijevski

Camfil Canada Inc.

T: 437-929-1161

-Air filtration expert Jon Holmes, Camfil Canada

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