Removing air born dust and dirt benefits the people breathing the air and the machinery moving the air. Dirt accumulates on bearings and motor windings and gradually restricts air flow through a heating or cooling coil and settles in ductwork.
Collecting the dust from before it passes through the furnace or air helps to protect the machinery, and the filter should always be installed in the return air duct.
Mechanical or adhesive filters are the least expensive but the least efficient. Constructed of spun fibers shaped to form a mat, they are framed with cardboard to specific sizes. As air is moved across the fibers, dust is attracted and sticks to an oil film or glue sprayed on the fibers. This type of filter collects between 6% and 8% of the dust passing through when new. Efficiency will increase as the filter gets dirtier, but air flow will be restricted as well. Available at many hardware and retail stores, they are inexpensive, and should be replaced monthly if the system is in constant use.
Some manufacturers include a new filter with the equipment that is rubber coated and washable. Coarse fibers are rubberized to bind them together and make them sticky. As air passes through, dust collects on the fibers. A blast from the sink sprayer or garden hose will rinse the dirt away and ready the filter for re-use.
A better mechanical filter has been developed, called a media filter, that begins its life removing more dust (about 75%) and gets more efficient with use. It approaches 99% eventually, but also becomes too restrictive for proper air flow. This filter is made of paper or synthetic fabric, woven just loose enough to allow air to pass, but tight enough to trap particles as small as pollen.
Anywhere from one to five inches thick, the media is pleated to allow as much surface area exposure as possible. A cardboard frame keeps the filter in the proper shape and gives it enough rigidity to prevent collapse from air pressure.
The one inch thick pleated filters need to be replaced as often as the spun filters, but the three to five inch thick pleated filters can last 6 months to a year. Dirt build up is visually noticeable; frequent inspection will help determine life expectancy. Replacement media filters can be found at HVAC and some plumbing supply stores. The one inch thick pleated filters are designed to replace the spun filters, and are available at retail and hardware stores.
The simplest filters are made of fiberglass and are disposable. Others are washable screens.
High performance media filters use pleated paper or spun materials to collect microscopic dust particles as the air passes through.
Electronic air cleaners put an electric charge on the dust particles as they enter the grids and collect the dust on plates that are also charged. In the process of putting an electric charge on the dust, ozone is created, which helps kill bacteria and odors.
Electrostatic filters use the motion of air across a weave of synthetic threads to create a static charge on the dust and collect it.
Electronic air cleaners lose efficiency as they get dirty, and should be washed monthly when in constant use. Loud snapping noises that continue through the entire heating or cooling cycle are signs that the grids need cleaning.
Non-electric filters will increase in efficiency as they get dirty, but also decrease in airflow. The balance between cleaning efficiency and proper airflow is difficult to define. Experience and familiarity with the systems performance is the best guideline.
Full air flow is critical to the performance of a central air conditioning system; clean filters are a must. The wet cooling coil will collect a great amount of dust particles as the air passes through.