Furnace Filters

Mechanical Furnace Filters

Flat, pleated, or ultra high efficiency

Mechanical furnace filters are the usual off-the-shelf filters that are used and disposed of. They draw air in and trap particles. Some are washable / reusable and, with regular cleaning, can last one to five years. They are usually much cheaper than gas phase orelectronic furnace filters, but most don't work as well.


Mechanical furnace filters come in a variety of sizes:

  • One inch - This size of mechanical filter is generally going to have a MERV rating of one to four.
  • Two to four inches - MERV ratings can range from one to 12.
  • HEPA - The best-performing mechanical filters, HEPA filters can have MERV ratings well over 13 and are the most expensive to purchase. They can't be used on some furnace systems because they cause a reduction in air flow.
  • Furnace vent filter - For extra filtering power directed at one room, vent filters are a cheap consideration. Costing only about $10 for a pack of four, they are placed under the floor vent and replaced every one to three months.

Factors increasing MERV ratings

  • Coating - Mechanical filters can be covered with a sticky, oily substance that helps them trap more particles.
  • Electrostatic charge - Some filters, like many 3M furnace filters, have fibers with a permanent positive or negative electrostatic charge that attracts small particles like a magnet.

Flat filters

Sometimes called panel filters, flat furnace filters can be composed of mats of glass or cellulose fiber, wood fill, animal hair, synthetic fibers or even slit-and-expanded aluminum. A common flat-filter material is fiberglass. Flat filters typically remove an insignificant number of small particles and are mainly used to protect the furnace components from damage caused by large dust particles.

Pleated filters

Pleated filters provide a big increase in filtering capacity by increasing the surface area of the filter. When stretched out, they are several times longer than a regular filter, and when in their compact form, they can catch and hold more particles than a flat model. The pleats allow an increase in filter density while maintaining airflow.

High Efficiency Particulate Air (HEPA) filters

HEPA furnace filters have an efficiency rating of at least 97.97 percent for all particles of 0.3-micron diameter. They are the top of the line in mechanical air filters but are rarely used in residential HVAC systems because of their size, price (up to $100) and high electricity requirements. Beware of less-efficient products billed as "HEPA-type" filters - their effectiveness at capturing particles and smoke can be much lower.

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