Choosing a Filter for Your Furnace

Having the right filter in your furnace is important for not only the quality of air in the home, but it is also a key component for maintaining the proper function of the machine. While having the right filter is important, it is also necessary to keep a clean filter in the furnace. One problem is that many people don’t know which type of filter they should buy for their furnace.


Many homeowners work under the misconception that the primary job of the furnace filter is to maintain the air quality of the home. While this may be one of the functions that it will perform, the primary job of the air filter is to protect the heating system from dust and debris. A standard air filter is not designed to improve indoor air quality; it is simply there to protect the heater. When the air filter gets old it will reduce the quality of the air that is in the home, but a new filter is not made to remove a significant level of contaminants from the air. However, the consumer does have some options when it comes to furnace filters and some are designed to improve air quality.


Most furnace filters are rated by what is known as the minimum efficiency reporting value (MERV) scale. The MERV rating scale goes from 1-16 and it is basically a numerical representation of the effectiveness of a given filter. The lower the MERV rating of a filter, the less effective it is at removing material from the air. While going up the scale does mean that the filter will be more effective at removing contaminants from the air, it also means that it will be more restrictive on the airflow of the system. This can cause the furnace to work harder, which will raise the cost of home heating and it can also cause the components of the heater to breakdown prematurely.

The MERV 1-4 filters are the lowest quality available and they will provide the least amount of filtration. It will get the basic stuff like dirt, dust, carpet fibers and pollen and it is really only protecting the machine. These are the filters that are commonly found in window air conditioning units and they come in both disposable and washable.

With MERV 5-8, you find the filters that are most commonly used in home and workplace furnaces. They will catch all of the particles that a MERV 1-4 filter does, but they will also filter finer particles like mold spores, pet dander and some types of chemical contaminants. The recommended MERV rating for most home heaters sits within this range because it offers a good balance between filtration and efficiency.
When you are looking at MERV 9-12, you are getting the high-end of residential and commercial furnace filtration. In addition to filtering the same types of contaminants that lesser quality filters would, these filters can catch things like lead dust, automotive exhaust and even finer particles of chemical contamination.

When you get beyond MERV 12, you are talking about air filters that are not even an option in most residential furnaces. These are the types of filters that they might use in a hospital or a laboratory. They can effectively remove contaminants like cigarette smoke, bacteria and viruses.

When you are looking to buy a filter for your heater, you want to basically consider three things. These are airflow, air quality and the price. You obviously want to have a filter that will effectively filter a reasonable amount of contamination, but you don’t want to buy a filter that will compromise the operation of the furnace. Most heaters will have a manufacturer recommendation for the appropriate MERV rating in the owner’s manual. If you are unsure as to which type of filter you should get, it is recommended that you consult with an HVAC professional.