Filter Frequently Asked Questions

Comfortmasters constantly gets questions about filter use and replacement. Below are some of the most commonly asked questions and some interesting notes about HVAC filters. Keep in mind that we stock and sell the proper sized filter for your unit, no matter what equipment you use, and we can get non-standard sizes custom fit for your unit. See our filter sales page for more information.

There are three basic types of filtration methods in use today:

Passive filters use a fibrous filtering material. They can range from very inefficient to highly efficient HEPA filters. Generally, the cheaper they are and the less air resistance they have the less effective they are. The higher the air resistance, the more energy is required to overcome the airflow resistance, contributing to higher energy costs to operate. If a passive filter is too restrictive, it can cause damage to mechanical equipment. Electronic, corona discharge filters electrically charged particles passing through them, then attract the particles to a series of grounded collector plates where the dirt accumulates. The collector is then removed for cleaning. Unlike passive filters that become more efficient as they load, corona discharge devices tend to become less efficient as they load, especially if the cleaning regimen is not maintained properly. When they become dirty, arcing may occur which can cause the production of ozone, which the EPA has identified as being harmful to health. Active electronic, polarized media filters, represent the third major group of air filtration devices. This newer technology uses an electrical charge to establish a polarized field. Airborne particles pass through the field and adhere to the media. Like a passive filter, polarized media filters increase in efficiency as they load. Designed for use primarily in recirculation systems, polarized media filters can provide very high efficiency with a static pressure drop much lower than passive filters claiming a similar efficiency. This may lead to a decrease in energy consumption.

Here are answers to some common questions.

1. How do you change an air filter?

First find the location. Modern central heating and air conditioning systems generally have the filter located as close to the blower unit as possible in the cold air return duct. Remove the grill or box cover holding your filter in place.
Remove the dirty filter. Dispose of the dirty filter in a garbage bag to contain the dirt.

Install the new or cleaned and dried air filter with the air flow arrow pointing toward the blower. Record the date and wash or change the filter within the recommended period. If you experience higher dust levels in your home due to changes in outside air, construction or dry weather, you may need to wash your filter more frequently than the recommended period. For questions or concerns about the location or installation of the filter contact the HVAC manufacturer or Comfortmasters.

2. How long does my filter last?

The life of your filter depends on your environment and the type of filter you have installed. If there are a lot of airborne contaminants in your home because of a remodeling project, use of candles or incense, or increased activity levels, you will need to check your filter more frequently. If it is loaded with dust and dirt, it should be changed. Different types of filters have different life expectancies. Fiberglass filters last up to 30 days. Pleated filters last up to 90 days. Washable filters should be washed every 30 days and last up to 5 years. Mini-pleated filters should be vacuumed every 60 days and last up to one year. There is no standard for an "average home."

3. Why do pleated filters last longer than fiberglass filters?

Pleating the filter media allows more media contained within the filter frame. More media means more surface area to capture and hold particles. This capacity to hold more dust makes the pleated filters last longer than fiberglass filters.

4. What does MERV mean?

MERV is an acronym for Minimum Efficiency Reporting Value. This is now an international industry standard set up by the committee that oversees the filter industry. This standard is used for determining the filters ability to capture and hold dirt and dust in specific size ranges. Some filter manufacturers use their own rating system which is not approved as an industry standard. Always ask for the standard MERV number. MERV is a number from 1 to 16 that is relative to air filter efficiency. The higher the MERV value the more efficient the air filter is at removing particles. At the lower end of the efficiency spectrum a fiberglass panel filter may achieve MERV 4 or 5. At the higher end, filters that achieve MERV 13 or 14 are typically used in hospitals and clean rooms. High MERV filters are capable of removing higher quantities of extremely small contaminant (particles as small as 1/300 the diameter of a human hair). Higher MERV ratings can create problems with greater airflow resistance. This is also referred to as static pressure. With most filters the filter media becomes denser and static pressure increases as the filter efficiency increases. Increased static pressure forces the system fan motor to work harder as it pushes air through the filter. This also increases energy consumption. So for optimum performance, consumers should select the highest efficiency filter with the lowest static pressure.

5. What’s a micron?

Particle size is usually measured in microns, a metric unit of measure. A micron is 1/25,000 of an inch. There are over 20 million particles in the average cubic foot of indoor air.

Approximately 98% of all particles (by count) are in the size range of 5 microns or less. Visible particles represent only a small fraction of particles found in indoor air.

For example:

Eye of a needle	500 to 2000 microns
Grain of Sand	100 to 2000 microns
Human Hair	40 to 300 microns
Pollen	10 to 50 microns
Mold Spores	10 to 30 microns
Bacteria	.07 to 10 microns
Typical Dust	.01 to 10 microns
Paint Particles	.05 to 3 microns
Tobacco Smoke	.01 to 3 microns
Viruses	.001 to .01 microns

6. What does "Pressure Drop" and "Resistance" mean?

The terms pressure drop and resistance are the same with respect to HVAC units and the elements, including filters that comprise them. Airflow is a very important factor in an HVAC system. It helps in determining the efficiency of the unit and an excess amount can even spell problems that could result in expensive repairs or replacement of a system. The more pressure drop or resistance experienced by an HVAC system the less airflow that system has. The way a system is designed and installed as well as the filter used determines how much resistance to the airflow there is. This includes the type of duct work used, the amount and size of the room outlets and return air openings, the length and amount of turns in the system duct work, the filter size and other factors. It is important to try and keep the filter changed or washed as often as needed. As the filter gets dirty the resistance increases, thus increasing the total resistance of the HVAC system and lowering the amount of airflow. There is no easy way for a homeowner to tell how much resistance there is in the HVAC system. Check you filter often and if it is dirty, it is time to change your filter or wash it.

7. What is the difference between nominal and actual filter sizes?

The nominal filter size is the approximate size up of the filter rounded up to the nearest "whole" inch. This makes the filter easier to identify for the consumer and allows for filters to comfortably fit into specified space in a residential furnace without air bypass. The tolerances allowed for with this nominal size are +/- 1/8" for both the width and length. The actual size is just as it sounds; it is literally the exact dimensions of the filter. For example, the nominal size of a Dust Shield filter could be 20 x 20 x 1 however its actual size is 19^5/8"x 19^5/8" x 13/16". This may not be true for all manufacturers. Be sure to note the actual size required for you unit before purchasing and changing your filter.