Are you considering high-efficiency filters for your HVAC system? Perhaps you've already made the investment and wonder why you're not seeing improvements you expected. Whether it's to improve air quality or reduce change-outs, there are several products on the market with higher Minimum Efficiency Reporting Value (MERV) ratings that claim to help you meet both objectives. However, a filter with a high MERV rating may not protect your HVAC system and building occupants from airborne particles.
Warped frames, pinched pockets, missing or deteriorated gaskets and incorrect installation lead to air bypass, which can cause higher MERV rated filters to lose up to 6 rating classifications. The MERV rating was developed by the American Society of Heating, Refrigeration and Air Conditioner Engineers - ASHRAE. MERV values vary from 1 to 16. The higher the MERV value, the more efficient the filter will be in trapping airborne particles. Another consideration is air flow through the your HVAC system. Leaving a dirty air filter in place or using a filter that is too restrictive may result in low air flow and possibly cause the system to malfunction.
An improper filter seal can cause gaps which may seem small and insignificant on the surface, but even small gaps can have a surprising effect on filter performance. For example, an air gap as small as 1mm on a system with MERV 15 filter can reduce its efficiency to MERV 14. A gap of 10mm can decrease performance all the way down to MERV 8!
Because higher efficiency filters typically have a higher pressure drop, bypass tends to have a larger effect on high-performance filters. The dust load on the filter also has an effect on bypass flow. The smallest amount of bypass flow occurs when a filter is clean and can increase by as much as 10 percent when filters are dirty.
When air bypasses the filter, airborne dust and particulates can cause fouling of the coils and fans of the HVAC system. Fouling of coils reduces airflow through the system and thermal transfer in the coils, which can add up to a significant increase in energy costs. Fouling of fans reduces the ability of the fan to move air causing the system to work harder to keep up with demand.