How Much Does a Filter Reduce CFM? An Expert's Perspective

Take the CFM target used to find your filter and combine it with the nearest fan rating. It is advisable to consider a 10% to 25% drop in air flow due to the tensile strength through the carbon filter. The Air Conditioning Contractors of America (ACCA) protocols for HVAC design assume a pressure drop of 0.10 inches of water column (i, w, c). David Springer, from Davis Energy Group, published an excellent article with a lot of detail about what they did and what they found, including the energy use of the fans, the use of compressor energy and the specific filters they tested.

They tested all filters with the same width and height (16″ x 25″), but the depth varied from 1″ to 4″. They used 492 feet per minute (fpm) as facial velocity because that is what the ASHRAE filter standard (52) requires. With that nominal velocity and dimensions, the air flow is 1367 cubic feet per minute (cfm). Flow is equal to area multiplied by velocity; see my article on the continuity equation for more information on this.

A PSC motor is the one that powers most HVAC fans. Variable speed blowers have electronically switched motors, ECM. The average MERV is twice as high and the high MERV is three times more. They also found that air flow in high MERV filters decreased by 7% and 11% in the two HVAC systems compared to low MERV filters.

Similarly, medium MERV filters also showed a decrease in air flow compared to low MERV filters, this time 3% and 8% lower in both systems. Research shows that, in general, HVAC systems with high MERV* filters have a greater pressure drop throughout the filter. What happens to airflow depends on the type of fan the HVAC system uses. In a system with a PSC fan, airflow decreases and power consumption doesn't change much.

The Stephens article cites a 2002 study that shows that 90% of all residential HVAC systems had PSC fans. Undoubtedly, that number has declined over the past 16 years, as high-performance homes and high-performance HVAC systems have become more popular. But I'm sure the vast majority of homes still have PSC motors that run on the fan. For those with the other type, the electronic commutation motor (ECM), the controls on those motors usually increase motor speed as pressure increases, so a relatively constant air flow is maintained.

Blowers with ECM may be more efficient than those with PSC motors when operating against the pressure for which they are designed. But when the pressure is higher, they can end up using more energy than the PSC fan. Tools for measuring the pressure drop in filters include static pressure tests and duct leakage tests. It would seem that increasing the size of the filter will only help a little and physically it might be difficult to adapt in a generalized way.

There is still excessive static in existing duct systems, which is also difficult to resolve. So, as Tim the Toolman would say, this may be the time for “more power” with the widespread use of ECMs in modernizations. Of course, ECMs cost more upfront, but a properly sized unit usually solves the problem. In addition, they operate at a reduced speed per minute up to 90% of the time when combined with two-stage or better equipment.

However, to gain ECM reliability, manufacturers of HVAC systems must boost surge protection throughout the home and gradually increase the specific power of the ECM to reflect the reality of duct network installations in the U. S. Finally, better consumer education about the state of the residential air conditioning industry could be the ultimate solution, allowing market forces to act. The only solution I see is the right filter size, which often requires double filters.

However, homeowners and certainly builders don't accept it because of space issues. Dan, I've worked for 3 different residential oven manufacturers so far, and they all have ovens that can go up to 1″ ESP. I don't want to mention brand names on this website. I guess I didn't know that we even put ESP limits on a nameplate, so I'll have to check that.

I know that we would all like our customers to design systems with an ESP of 0.5″ or lower, but we also know that many don't. So maybe the nameplate is a matter for CYA (Cover Your Assets). As for why we're limiting maximum ESP values, it's not because of risk of controller or motor failure; PSC motors are automatically discharged at high ESP values and ECM motors have electronic protection that will limit their speed.