Understanding Pressure Drop Across a MERV 11 Filter: An Expert's Perspective

High-MERV filters can cause a significant decrease in pressure within the HVAC system from before to after the filter. According to Air Conditioning Contractors of America (ACCA) protocols for HVAC design, this pressure drop is assumed to be 0.10 inches from the water column (i, w, c).David Springer, from the Davis Energy Group, conducted research on the energy use of fans, compressors, and specific filters tested. The focus of this study was on the pressure drop in the filter and its effect on air flow. All filters tested had the same width and height (16 inches x 25 inches), but the depth varied from 1 inch to 4 inches.

The facial speed used was 492 feet per minute (fpm), as required by ASHRAE filter standard (52). This resulted in an air flow of 1367 cubic feet per minute (cfm).The research showed that HVAC systems with high MERV* filters had a greater pressure drop in the filter than those with low MERV filters. The average MERV was twice as high and the high MERV was three times as much. Airflow in the high-MERV filters was reduced by 7% and 11% in the two HVAC systems compared to the low-MERV filters.

Similarly, medium MERV filters also showed a decrease in airflow relative to low MERV filters, this time 3% and 8% less in both systems. The effect of this pressure drop on airflow depends on the type of fan used in the HVAC system. In a system with a PSC fan, airflow decreases and energy consumption doesn't change much. However, for those with an electronic switching engine (ECM), the controls on those motors generally increase engine speed as pressure increases, so a relatively constant airflow is maintained. Blowers with ECM can be more efficient than those with PSC motors when operating against the pressure for which they are designed. When the pressure is higher, they can end up consuming more energy than the PSC fan.

To measure this pressure drop between filters, tools such as Corbett's PROOF IS POSSIBLE PBS series and Greenbuild's study of IAQ chemistry can be used. Tim Taylor from Home Improvement would suggest “more power” with the widespread use of ECMs in modernizations. ECMs cost more up front but are usually able to solve the problem when properly sized. They operate at a reduced cfm up to 90% of the time when combined with two-stage equipment or better. Educating consumers about the state of residential air conditioning industry could be beneficial as it would allow market forces to act. Variable speed ECM motors (not X-1) that can compensate for restrictions can generally solve the problem but may not be embraced due to space issues. Dan, who has worked for three different residential furnace manufacturers, has observed a direct correlation between faulty ECM modules and high return static pressure.

The only solution he sees is the right filter size, which often requires dual filters. Most residential HVAC systems have PSC motors that run the fan but increasing filter size will only help a little and it could be difficult to adapt in a generalized way due to excessive static in existing duct systems. To ensure ECM reliability, whole-house surge protection must be boosted and ECM power must be gradually increased to reflect reality of duct installations in the U. S.