Media Air Cleaners

Media air cleaners are used to clean the air in force air furnaces. These air filters are typically found in the return air ductwork close to the furnace. Dust particle collection on these units varies depending on the types of filters used. Outlined below is a Honeywell F25F Media Air Cleaner.

The F25F media air cleaner includes cabinet, access door and pleated media filter. This unit can be used with gas, oil, and electric forced air furnaces. This unit requires no electrical connections, mounts in any position, requires no maintenance except periodic media filter replacement. Media filter is easily replaced by homeowner.

Air Filtration - One of the best methods of controlling airborne dirt, debris, and allergens in the homes is using good quality filters to remove particulates from the forced air furnace airstream. The quality of filters used varies greatly. The four most common filter media are the following:

Low Grade filters MERV 1-4 - Filter efficiencies are very low and have MERV ratings of 1 to 4 as rated by the ASHRAE Standard 52.2-1999. Most residential homes with forced air furnaces use these types of filters. Use of these filters allows large quantities of dirt, debris and microbials to be present in the air and inside the home. These types of filters/systems are not recommended for homes or offices. These filters typically will only filter out about 1% of fungi and bacteria in the air.

The ASHRAE Standard 62-2001 Section 5.8 states that air cleaners having a minimum rating of a MERV 6 must be installed upstream of all cooling coils or other devices with wetted surfaces. Since this ASHRAE standard is frequently adopted into state building codes, this would make the use of low grade filters illegal in buildings complying with the state building code.

Medium Grade Filters MERV 5-8 - These filters are typically called pleated filters although some panel filters or multilayered filters can also meet this standard. These filters are reasonably effective at filtering dirt out of return and outdoor air streams. This is the lowest quality filter that should be used in homes or office buildings. Normally these filters come in one, two, and four inch thick sizes with the two inch filters used the most often. A two or four inch thick filter has a lower pressure drop then a one inch thick filter. The thicker filters have a lower resistance to air flow and the resistance to air flow increases slower with increasing dirt loads then the thinner filters. Normally a two inch thick filter will last three times longer than a one inch thick filter and a four inch thick filter will last nine times longer than a one inch thick filter. We routinely recommend the use of 4 to 7 inch thick filters for residential homes as a replacement for the typical 1 inch thick low grade filter commonly used. We have seen a lot of problems when 1 inch thick MERV 6-8 filters are used because these filters have such a high initial pressure drop and the pressure drop increases rapidly with dirt loading. These filters will typically filter out about 70% of common airborne bacteria and fungi.

High Grade Filters MERV 9-16 - These filters are typically used in health care facilities where very clean air is desired. A large number of new offices built today are starting to use these types of filters to improve overall air quality. Some of the higher end residential air filters can fall into this category. Some studies have shown that a typical MERV 12 filter will collect in excess of 95 percent of the common airborne fungi and bacteria present in the air.

HEPA - These filters typically can not be used on home furnaces because the pressure drop across the filters is too great. These filter will collect 99.97% of 0.3 micron size dust particles. These filters typically will filter out all the airborne bacteria and fungi in the air.

The filters described above become more efficient at collecting airborne dirt as the filters become dirty, however their resistance to air flow also increases. Because of this, as the filters become dirtier, by pass around the filters will become more pronounced with increasing dirt loads. Filters need to be changed regularly and this will vary to some extend. Residential homes using 4 inch thick pleated filters should changes these filters once or twice a year.

The cost to convert a typical home furnace, which is presently using one inch thick filters, to use 4 to 7 inch filters will vary typically from $100 to $400 depending on the furnace and the spacing around the furnace. Replacement filters for these units will cost between $20.00 and $40.00 dollars depending on the type of filters. Prices subject to change without notice.