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Installing a new furnace filter

Installing a Furnace Filter

Sean S.Verified

HVAC Owner/Technician

Residential HVAC

292 positive ratings
Air Filter Efficiencies

Air filters are given efficiency ratings based on a guideline: the Minimum Efficiency Reporting Value (MERV), with 1 being the lowest and 16 being the highest.

Many people believe that installing a furnace filter is easy, and it is! But, did you know there is a proper way to install a filter? To maximize the clean air quality in your home, we will walk you through how to choose the best furnace air filter as well as proper installation.

How to choose the best air filter

A schematic of the inside of a furnace filter

The best air quality begins with selecting an air filter best suited for your heating needs.  

There are three basic types of air filters:

Mechanical air filters. This type removes dirt and other pollutants by capturing them onto the filter medium.  This kind of filter can come as a flat filter, which is the cheaper option, or a pleated air filter. Both options are available in one-inch fiberglass, but the pleated filter has more area to filter contaminants. Therefore, it costs a little more. Fiberglass filters are often the most popular choice because they are affordable and disposable.

Electronic air filters. This type of filter works by having multiple layers of vented metal through which the air passes.  As the air passes through the first layer of filtration, the air molecules are positively charged by the friction between the air and the filter.  The now positively charged air molecules attach themselves to the next few layers as they pass through the rest of the screen.

Electrostatic air filters. These are washable air filters that theoretically never need to be replaced.  Instead of replacing your electrostatic air filter at regular intervals like you would a conventional air filter, you take them out back and wash them off with a hose about once a month.

People with severe allergies might consider going with an ultraviolet or electrostatic high-efficiency particulate arrestance (HEPA) filter that conforms to standards set by the United States Department of Energy (DOE).  

The effectiveness of your air filter

Each HVAC approved air filter has a rating located on the package. The MERV rating tells you how the filter measures up when it comes to trapping dirt and debris. The scale goes from 1 being least efficient, to 16 being the best.

Flat disposable filters rated 1-4: Are not considered truly effective at removing dirty air.

Pleated filters rated 5-13: Have up to 4 times more surface area than the flat disposable filters, which allows them to trap more dirt and dander.

Electrostatic furnace air filter rated 7-16: Have the highest rating because they act as an electromagnet. They can trap microscopic pollutants as tiny as 10 microns.

A breakdown of how filters with different MERV ratings block different pollutants

While everyone would think that going with the higher rating would be best, many filters with a score of 8 or greater are made of a thicker fiber. Some home heating units, therefore, may have trouble working with these filters.

When to replace furnace air filters

A clean air filter is the key to indoor air quality and improved energy efficiency. So, it is important to check your filter regularly. When the filter gets clogged, your home heating unit can strain to produce quality airflow. This can also dramatically increase your utility bill. HVAC professionals often recommend checking the filter every month or so. If you have a dirty filter, replace it.

The right (and wrong) way to install a furnace filter

There is a right and wrong way to install an air filter. If the filter is put in the wrong way it can not only hamper your furnace’s airflow which can cause it to strain, but may also cause it to malfunction.

Air filters are all designed to clean the air by filtering in a direction through what is called the primary filter medium. The filter medium is the part that prevents dirt from accumulating in the motor and blowers of your heating and air conditioning unit.

Many air filters have an arrow printed on each side; this acts as an indicator. These arrows tell you how the airflow is directed. The arrow must always point toward the furnace so that the airflow is directed outward.

How to replace furnace filters

Locate the furnace air filter box

Stand in front of your furnace and look for the air ducts. The warm air ducts should come out near or at the top of the furnace, while the cool air return air ducts should be attached near the bottom. The air filter box is located within the cool air return ducts.  

Find the air filter

Air filters are found inside the return vents in your home. These returns might be on the ceiling, wall, or in the system itself.

In most furnaces, the air filter is located right next to the furnace itself. Some air filter boxes will have a metal door with a long hinge; others may only leave the filter exposed. If there is a door, you will need to open it for the filter to be visible. If there is no door, the filter may be slide into grooves meant to keep it in place.

Most homes have 2 or 3 returns inside the house, or, two 5” filters that go within the HVAC system itself. Typically, the inside returns are more accessible, which makes filter changes easier in these locations.

Remove the dirty filter and replace it with a clean one

Pull out the dirty filter and throw it away. The filter may be quite messy, so you may want to have a trash sack handy. Unwrap the new filter and locate the arrows printed along the sides.

Now, slide the filter (arrow side in) into the designated slot. Don’t forget to shut the door if your furnace has one. You have correctly installed your furnace filter.

If you have problems locating your furnace air filter, contact an HVAC professional to assist you.

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