Air filters and air cleaners

Your filter sits where air heads back to the furnace or air handler. Filters are mounted in the return air duct or plenum and work whenever air moves through the ducts. On the return side of the air handler, a filter protects the motor and improves indoor air quality. Furnace and HVAC filters only clean the air while the system is running. Filters are also used in fresh air intakes to clean outside air brought into the home.

So a filter does two jobs at once: it guards the equipment and it cleans some of the air you breathe. Both depend on the filter being clean. If a filter is dirty and overloaded, it won't work well.

Source: ASHRAE · U.S. Department of Energy · U.S. EPA

The number on the filter's frame is its MERV rating. MERV, short for Minimum Efficiency Reporting Value, reports how well a filter captures particles between 0.3 and 10 microns. The higher the MERV, the better the filter traps particles of certain sizes. Filters of MERV 6 and above can trap pollutants like pollen, dust mites and mold spores. Filters of MERV 13 and above must show they remove at least 50% of the smallest particles tested.

EPA suggests choosing a filter rated at least MERV 13, or as high as your system will take. Another EPA page says to use the highest efficiency filter your system's maker recommends. You may need an HVAC technician to tell you which high efficiency filter will work best for your system. Pleated media filters and electronic air filters are higher-efficiency choices, but these filters may have higher static pressure drops, meaning more resistance to air. A higher MERV does not always mean more resistance, because higher MERV filters often use more filter material, with more pleats and more thickness.

Source: U.S. EPA · U.S. Department of Energy · ASHRAE

Changing the filter is the easiest care you can give your system. All filters need replacing from time to time to work properly. Makers typically recommend a new filter every 60 to 90 days. ENERGY STAR suggests checking, cleaning or changing filters once a month in a central air conditioner, furnace or heat pump. If your filters clog, change them more often.

A neglected filter costs you in more than one way. A dirty or clogged filter can sharply reduce airflow, making the furnace run longer and adding wear on the motor and energy use. It can also raise energy costs and damage your equipment, leading to early failure. Filters for the system should also be kept dry.

Fit matters as much as the rating. Make sure the filter fits snugly so air does not leak around it, and that you do not have to bend or crush it to make it fit. Check that no filter is missing and that air cannot slip past through gaps around the holder. During dusty renovation work, seal off supply and return registers and do not run the system until the dust is cleaned up.

Source: U.S. EPA · ENERGY STAR · U.S. Department of Energy

An air cleaner adds filtering on top of what your furnace filter does. Portable cleaners are compared by their clean air delivery rate, or CADR: the higher it is, the more particles the cleaner can filter and the larger the area it can serve. Most filters are designed to catch either particles or gases, not both. Filters in your furnace or ducts filter the air only while the system is operating.

Running the fan longer to filter more has a cost. Longer run times increase electricity costs and may also make humidity control less reliable in the cooling season. A technician can help you weigh that for your system.

Source: U.S. EPA

It helps to know the limits before you buy. No air cleaner or filter will remove all of the pollutants in your home. Portable air cleaners and furnace filters do not address the cause of mold and musty odors. The most effective ways to improve your indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air. Filtration can be a useful addition to those steps.

Ductless mini-splits are a special case. Many have washable mesh filters with very low capture of fine particles, chosen to keep airflow free. A typical maker's advice is to clean those mesh filters every two weeks. If you have a ductless system, the mini-split page covers its care.

Source: U.S. EPA · U.S. Department of Energy