Return to Heating and Ventilation Articles
Whole House HEPA Air Cleaners
and Filtration Systems for Allergy Relief
by Gary Cerantola, P.Eng., MBA
Increased levels of contaminants in the air can even affect the well-being of fully developed healthy adults.
The higher the level of contaminants in the air, the quicker a person can reach their tolerance level. Then the contaminants could start to damage their respiratory system and their general health. Significant reduction of contaminants in the home can help the body recover from constant exposure to contaminants outside the home. We like to say this reduction in the stress to our respiratory system can be compared to a good nights sleep for a tired athlete.
What the heck is a "HEPA" filter?
HEPA is a filtration standard created by the U.S. Atomic Energy Commission. HEPA stands for “High Efficiency Particulate Air”. Some providers refer to HEPA as “High Efficiency Particulate Arresting”. Both terms refer to the same technology. HEPA filter media is proven to be 99.97% efficient @ 0.3 microns (a micron is 1/1,000,000 of a meter or 1/25,000th of an inch). Due to their extremely high efficiency, HEPA filters have become widely used in medical, electronic and industrial applications. In particular, HEPA is the air purification method used in operating rooms, clean rooms, space research and isolation chambers.
The graphic below shows the relative size of particles that can be removed by air filters. HEPA filters can remove particles finer than smoke! Amazing!!
HEPA filters are composed of a various sized borosilicate fibers, which are pressed together to form a netlike structure with openings large enough for air to pass through but too small for most particulates.
HEPA air cleaners are effective in controlling:
- Household dust
- Animal dander
- Pollen
- Bacteria
- Dust mite feces.
Good HEPA air cleaners incorporate both activated carbon and/or zeolite for odor and gas control, in addition to particle removal.
Carbon Volatile Organic Compound (V.O.C.) Blankets:
Like Carbon Pre-filters, the Carbon Inner Blanket is placed in the air purifier as an additional purification media. It is a form of charcoal which has the ability to remove impurities from air, gas, and liquids. When various impurities come into contact with the activated carbon, the constituent molecules become trapped in tiny capillary passages on the surface of the carbon in a process called adsorption. Due to differences in molecular structures, some impurities are more readily adsorbed than others.
Carbon/Zeolite V.O.C. Cartridges:
V.O.C. cartridges can also make up part of the cleansing capability of the air purifier. These cartridges capture contaminants the same way as a Carbon Blanket, but they use solid carbon and zeolite pellets which have a comparably larger surface area. This makes V.O.C. cartridges the filter of choice for situations where V.O.C. removal is a prime objective.
Since HEPA filters coupled with additional air cleaning capability have a very high air resistance, they cannot be installed directly in-line in a Heating and Ventilation and Air Conditioning (HVAC) system. Instead, whole house HEPA filtration systems must be installed in a by-pass configuration using a separate blower motor system.
Figure 1.0 HEPA Whole House Filter By-Pass
Configuration
Because HEPA units work on the air exchange principle - air is cleaned by continually pulling air from the room into the filtering/cleaning device and exhausting clean air - their effectiveness is determined by calculating the number of air exchanges per hour in a given area.
The greater the number of air changes in a home that occurs, the cleaner the air. To calculate air changes per hour (ACH) for your particular home and particular HEPA unit you can use this formula:
One air change occurs in a room when a quantity of air equal to the volume of the room is filtered by the HEPA filter unit. Air change rates are units of ventilation that compare the amount of air moving through a space to the volume of the space. ACH is the volume of air (usually expressed in cubic feet) moved every hour divided by the room volume (also usually expressed in cubic feet).
Airflow is usually measured in cubic feet per minute (CFM). Your HEPA filter blower will have a CFM rating assigned to its blower. This is multiplied by 60 minutes to determine the volume of air filtered through the unit per hour (in cubic feet)
To calculate room volume (in cubic feet), multiply average room height (in feet) by the room area (in square feet). Room area is the room width (in feet) times the room length (in feet).
ACH = airflow per hour/room volume =CFM X 60 minutes/cubic feet
It is important to be aware of how air contaminants can affect our health and to adopt corrective measures that will improve indoor air quality in our homes. Whole House HEPA/air cleaners and filtration systems are one alternative available to the homeowner to positively affect their homelife environment to reduce:
- Particulates (air pollutants that have mass) like dust, dander, pollen, and cigarette smoke
- Volatile Organic Compounds (V.O.C.s) which are gases from chemicals that are released into the air by various household materials and liquids such as housecleaning chemicals, detergents and building materials.
- Bio-Burden is the amount of biological material in the air, living or dead. Dead material can cause allergic reactions, living material can cause allergic reactions some can cause disease.
About the author: Gary Cerantola is a professional engineer and chemist having worked in the areas of water treatment and air filtration.