HVAC filters are your first line of defense against dust, pollens and other allergens. The following touches on how dirty filters can impact your HVAC system, how the MERV rating system works and the basic types of filters used in residential settings.
The Perils of Dirty Filters
HVAC systems are designed to cycle on and off. A clogged air filter restricts airflow, eventually forcing the HVAC system to run almost continuously. Depending on the age and noisiness of your system, you may not even notice the difference until your utility bill arrives. While the sticker shock of a high bill isn't pleasant, that blocked filter can shorten the lifespan of your HVAC system, creating a more expensive problem.
The MERV Ratings
Residential HVAC filters are rated on a scale of 1 to 16 on the MERV scale, which stands for Minimum Efficiency Reporting Value. The scale indicates a filter's effectiveness at removing dust and other contaminants that measure from 0.3 to 10 microns in size. That covers items such as dust, pollen, pet dander, mold spores and tobacco smoke.
The higher the MERV rating, the more efficient a filter is at removing contaminants. In a residential setting, the most effective filters are rated between 7 and 12. The higher the filter, the smaller the pores in that filter and the faster it clogs up. That means more frequent changes to keep your HVAC system running efficiently.
Ideally, you want the highest MERV rated filter in your system that still allows maximum airflow. Consulting an owner's manual or asking questions before buying filters is your best bet.
Types of Filters
Your HVAC system's owners manual should indicate the type of filter needed and where they are located in the machine. These are the types typically used in home HVAC systems.
These are usually the least expensive and carry a MERV rating of between 1 and 4. Some have cardboard frames; others are cut-to-fit and are set inside a special frame inside the HVAC system. They are good for catching large airborne particles.
Pleated Polyester Fibers
These filters are also disposable and often come in cardboard frames. The polyester has a tighter weave and the pleated design provides more surface area for catching particles. Most have MERV ratings of between 5 and 13. Pleated filters with higher ratings are available, but are usually used in industrial or surgical settings.
Washable Air Filters
Washable air filters are another option, but with a MERV rating of 1 to 4, are not very efficient at catching particulates such as pet dander or cigar smoke. They usually cost more than a disposable filter, and while they are reusable, the washing process brings its own challenges. The waste water is considered hazardous, and must be disposed of properly. If the filter is reinstalled while still damp, mold may form within the fibers. When the HVAC system is turned on, mold spores can travel throughout the home.
HEPA filters are the most efficient, having a MERV rating of between 17 and 20. Most of the time, these filters are found in industrial, scientific and surgical settings. The highest rated filters can pick up airborne particles as small as 0.3 microns and have a 99.97 percent efficiency rating.
In some situations, such as when a homeowner is overly sensitive to allergens, HEPA filters are used in home systems. In most cases, the HVAC system will need to be professionally retrofitted to accommodate the HEPA filter's shape and size and to accommodate the higher resistance to airflow. The system refit will greatly improve the air quality inside the home.
You can discover more here about how to choose the right HVAC filter for your home.