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    Higher MERV Filters May Demand Bigger Return Ducts

    Nov. 12, 2020
    As an HVAC professional, it’s your responsibility to see beyond the single request for a high MERV filter and prescribe additional solutions to the potentially negative reactions the new filter may cause.

    Improved air filtration appears to be an immediate solution to ease the spread of Coronavirus in homes, offices, and institutional buildings. Unfortunately, using Minimum Efficiency Reporting Value (MERV) filters creates a new wave of HVAC system issues that produces unseen liabilities for many HVAC contractors. Let’s take a look at the cause of and solutions for some problems caused by installing high-efficiency filters.

    Sir Isaac Newton
    In school you learned Newton’s Third Law; “For every action, there is an equal and opposite reaction.” 

    Newton’s law is still in force today. When you make any change in an HVAC system, it’s your job to account for the equal and opposite reaction solutions. Today’s subject is how to compensate for airflow, ventilation, and mechanical problems higher efficiency filters may cause. 

    There is no doubt your customers want higher efficiency filters. They need cleaner indoor air more than ever before. As an HVAC professional, it’s your responsibility to see beyond the single request for a high MERV filter and prescribe additional solutions to the potentially negative reactions the new filter may cause. 

    What is the Equal and Opposite Reaction to High MERV Filters?
    Every HVAC system fan is limited to a maximum resistance to airflow it can handle. When you add a higher MERV filter to a system, it can increase airflow resistance above the fan’s limit. The resulting negative effects may include: 

    • A 25% reduction in system ventilation
    • Up to a 35% overall decline in system performance, and
    • A potential 50% reduction in system life.

    Compare an incomplete filter prescription to a doctor who prescribes a patient an incomplete prescription and treatment. What if the result is a 25% breathing reduction, a 35% decline in the patient’s overall health, and a 50% loss of lifespan? Malpractice is the outcome in medical terms and that doctor’s career would be over.   

    Typically, the more efficient the replacement air filter, the greater its airflow resistance.

    A typical fan in a residential system will move the required system airflow at a maximum-rated static pressure of .50 inches of water column (in. w.c.). Field data collected by National Comfort Institute confirms the average resistance to airflow (or pressure drop) of an air filter in a well-performing residential HVAC system is no more than 20% of the maximum-rated static pressure. The percent budget for commercial filters is similar.

    Twenty percent of the pressure budgeted for air filter pressure drop will leave enough pressure to overcome the resistance of the system coil and distribution system and allow the system to deliver required airflow. 

    When a fan is rated at .50 in. w.c. the filter pressure drop should not exceed 20% of .50 in. or .50 in. x 20% = .10in.  A commercial system example is 2.0 in. x 20% = .40 in. Normally, if you install a high MERV filter, the system operating air pressure will exceed fan-rated pressure and corresponding system problems begin to multiply. 

    The Numbers
    Filter manufacturers publish pressure drop and efficiency data for each filter they make. Although this data is often sketchy and requires study to understand, you can interpret filter specifications and prescribe solutions. Remember, the solution will frequently include system modifications in addition to a new filter.

    Here are some average measured pressure drops of low, medium, high, and very high MERV rated filter pressure drops. Since air velocity through a filter has a direct impact on the filter’s pressure drop, the average pressure drops listed below are from filters with an average velocity of 300 to 400 feet per minute (fpm).

    MERV efficiency generally reflects the filter’s ability to capture a percentage of particles measuring .3 to 1.0 microns and higher in size. The typical Coronavirus particle measures .8 microns.

    MERV efficiency generally reflects the filter’s ability to capture a percentage of particles measuring .3 to 1.0 microns and higher in size. The typical Coronavirus particle measures .8 microns.

    • Low MERV air filters, near MERV 4, are about 20% efficient, average a pressure drop around .10-in. w.c. Without system modification, this filter pressure drop matches a maximum fan pressure of .50 in.
    • Medium MERV air filters, near MERV 8, are about 80% efficient, average a pressure drop of around .20-in. w.c. Without system modifications, this filter would suit a fan rated at 1.00 in.
    •  High MERV air filters, near MERV 12, are about 90% efficient, average a pressure drop of around .35-in. w. c. Without system modifications, this 90% efficient filter could be used with a fan rated at 1.75 in.
    •  Very high MERV air filters (HEPA or High-Efficiency Particulate Air filters) are near MERV 16, are above 99% efficient, average a pressure drop of around .80-in. w.c. Without system modifications, this HEPA filter would require a system fan rated at 4.00 in.

    The bottom line, simply replacing a low MERV filter with a high MERV filter may be malpractice unless you modify the HVAC system to compensate for the increased airflow resistance.

    What’s Happening in the Field Today
    Edgar Andrade, owner of Bay Area HVAC is a California HVAC professional who specializes in ventilation upgrade projects and solutions. Edgar agreed to share his experience and insight with me. His typical scope of work to upgrade system ventilation includes evaluating a building’s performance and compliance with industry standards and code requirements. It calls for increasing fresh air into a building by installing economizers and controls, plus installing higher efficiency filtration. And finally, the scope includes adding needed system modifications. 

    Your residential customers are making the same mistake by buying high MERV filters as they wander down the aisle at their favorite home improvement store. 

    His studies document increased productivity, including higher student scores in classrooms and many other specific benefits following his company’s ventilation upgrades. 

    He reports since the Coronavirus pandemic, several of his customers have used their in-house maintenance staff to replace standard or low MERV air filters with MERV 13 filters. He says they intend to protect occupants from airborne infections, which at first glance appears good and noble. Your residential customers are making the same mistake by buying high MERV filters as they wander down the aisle at their favorite home improvement store. 

    Unknowingly, when they install higher MERV air filters, some of his (and your) customers, have imposed immediate and long-term damage to their HVAC systems and conditioned environments. 

    Edgar sees common issues surfacing that include:

    • Reduced system airflow and ventilation rates
    • Occupant discomfort
    • Shortened blower motor life
    • Increased utility costs
    • Cracked heat exchangers
    • Damaged compressors
    • Outside air decreased below code requirements
    • Negative changes in building pressure.

    Prescribe a Complete Solution
    When your customer requests high MERV filters, install them. But in addition to replacing the filter, it’s your responsibility to prescribe a complete solution. This includes essential modifications such as: 

    •  Additional return or supply ducts from the equipment to the conditioned space to relieve the added static pressure caused by the new filter.
    • A high MERV replacement filter with the lowest possible pressure drop matching the capacity of the system fan.
    • Multiple filters in a new filter housing or several return air filter grilles to increase filter surface area. Added filters will significantly reduce filter pressure drop.
    • An outside air inlet or economizer to reduce return-side pressure. This will also improve indoor air quality by adding fresh air to the building and creating a healthy positive pressure in the building.
    • A return air fan to offset the high MERV filter pressure drop. This is being done in commercial as well as residential HVAC systems.

     Since increased filtration efficiency is wanted and needed by your customers, you can and should provide for their wants and needs. Using the ideas in this article coupled with your increasing knowledge and experience, you can avoid malpractice by prescribing complete and effective air filtration solutions matching each system and building.

    Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC professional interested in a free procedure to measure filter pressure drop, contact Doc at ncilink.com/ContactMe or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, downloads, and current training opportunities.