MERV, or Minimum Efficiency Reporting Value, is a number from 1 to 16 that is relative to an air filter’s efficiency. The higher the MERV, the more efficient the air filter is at removing particles. At the lower end of the efficiency spectrum a fiberglass panel filter may have a MERV of 4 or 5. At the higher end, a MERV 14 filter is typically the filter of choice for critical areas of a hospital (to prevent transfer of bacteria and infectious diseases). Higher MERV filters are also capable of removing higher quantities of extremely small contaminant (particles as small as 1/300 the diameter of a human hair). A higher MERV creates more resistance to airflow because the filter media becomes denser as efficiency increases.

MERV filter that their unit is capable of forcing air through based on the limit of the unit’s fan power. MERV 11 is considered highly effective for residential use without creating undue strain on the Air Handler blower motor. Residential duct systems have a direct and significant effect on equipment size, equipment efficiency, equipment malfunctions, envelope infiltration, operating cost, utility demand loads, vent performance, exhaust system performance, indoor air quality, ambient noise, occupant comfort and owner satisfaction. Therefore, the duct system must be carefully designed and properly installed or the potential benefits that are associated with building an efficient structure and using high efficiency equipment will not materialize.ACCA manual D presents the methods and procedures that should be used to design residential duct systems. The subject material includes information about system selection (constant volume or variable volume), system performance characteristics, duct materials, blower performance, airside devices and duct sizing procedures. ACCA manual D also includes information about duct system efficiency and the synergistic interactions between the duct system, the envelope, the HVAC equipment, the vents and the household appliances. Indoor air quality, noise control, testing and balancing also are discussed. Other ACCA manuals that pertain to residential HVAC system design include Manuals J (loads), S (equipment selection), T (basic air distribution) and H (heat pumps).