All furnace filters are rated by the MERV scale. M E R V is the Minimum Efficiency Reporting Value which was developed by ASRAE- The American Society of Heating, Refrigeration and Air Conditioner Engineers or ASHRAE.
MERV values vary from 1 to 16. For example, a basic 1” Poly fiber filter is rated at a MERV 4, a pleated 1” filter can be rated at a MERV 8, and a 5” filter in a cabinet assembly can reach a MERV 13. For those who want better filtration a higher MERV filter is the answer; however, always be conscious of the resistance a higher filter has on the systems External Static Pressure or ESP.
The simple 1” poly fiber has a resistance of .16” (W.G.) while the 5” filter can have a resistance as high as .2” (W.G.) While that doesn’t seem like much, most residential HVAC systems are only rated by the manufacturer at .5” (W.G.) total external static pressure (supply ductwork, return ductwork, filter, supply grilles, and return grilles.) The most popular big box store filters and they’re not cheap, have a pressure drop as high as a 5” filter and require a monthly change. The 5” filter can be changed annually or semi-annually depending upon the home and its use.
So, when you see someone using a big box store filter knowing it is a drag not only on the HVAC system but also the homeowner’s pocketbook, take the time and explain this story. If a homeowner is conscious enough about IAQ and is willing to spend $13.00 for each filter ($179.00 annually) a Media air cleaner is a better option. Although there is an installed cost, annual filter replacement can be as low as $60.00.