Selecting the ideal furnace filter and changing it when it becomes dirty is as important to your HVAC system as changing the oil is to your car. Each plays a crucial function in keeping its system running safely, efficiently and for a long time.

A clogged furnace filter loses its effectiveness, permitting potentially harmful particles to move through your home. It also restricts airflow, which can damage your furnace and shorten its life span.

Making certain your furnace uses a clean filter that is appropriate for your needs is not only about keeping your furnace working efficiently. It’s also about providing excellent indoor air quality for your residence.

Your health is important to the heating professionals at Contra Costa Heating & AC. We've long focused on improving indoor air quality in The Bay Area. Here, we’ve answered frequent questions about HVAC filters, including that especially tricky question of what direction do you point a filter in your furnace or air conditioner?

How Often to Replace the Air Filter in a Furnace

It's critical to replace dirty air filters in a furnace or air conditioner periodically. Soiled filters cause the system to worker harder than it should because it takes more energy to move air through the plugged-up filter.

Officials advise checking your furnace filter every month and replacing it if it’s dirty. You’ll know if the filter needs to be changed because it will coated with dirt or dust. People who have pets will likely have to replace their furnace air filter more often, because a good air filter will trap pet hair circulating in a home.

Where Is the Air Filter in My Furnace?

In general, a furnace air filter is usually found in the return air duct or blower compartment before the return air gets to the furnace. This makes sure air being pulled into the system is filtered before it goes through the furnace components and is heated.

Depending on the furnace brand, the filter may be found on the right, left, bottom or in some cases, inside the furnace. It's generally housed within a slot, frame or cabinet for easy access and replacement. Always refer to your furnace's owner manual for details regarding filter location of the furnace in your home.

Is a Furnace Filter the Same as an Air Filter?

The easy answer is, yes. In HVAC, a furnace filter and an air filter or AC filter are effectively the same. While they might be called different things based on the current season— warm or chilly months—they are all filters that clean the air in your HVAC system.

They each eliminate dust, allergens, bacteria and other particulates from the air that is drawn into the furnace and air conditioning system, making sure the air circulating throughout your home is clean and safe.

What Is a MERV Rating and What MERV Rating Do I Need?

Once you track down your old furnace filter and determine when it should be replaced, it’s time to choose a replacement. That means picking the level of filtration that you need. One approach to this is by picking an appropriate MERV rating for your needs.

MERV is an abbreviation for Minimum Efficiency Reporting Values. The MERV rating measures the effectiveness of air filters at trapping airborne molecules. The rating scale ranges from 1 to 20, with bigger numbers indicating the power to filter tinier particles.

Experts say a filter with a MERV rating between 8 and 13 offers an ideal balance between having healthy indoor air quality without needlessly restricting airflow. However, people with certain health conditions could need to use a filter with a higher MERV rating.

Which Way to Put the Air Filter in a Furnace or Air Conditioner

Installing an air filter in a furnace or air conditioner properly is necessary for the efficient operation of the heating or cooling system. Air filters have a certain direction, indicated by an arrow printed on the side of the filter frame. The filter should be put in with this arrow pointing in the direction of the furnace or air conditioning unit, which is the direction of the airflow. If you're doubtful about the airflow direction, remember that air always moves from the return duct to the heat or cooling source. Therefore, make certain the arrow points at the furnace or air conditioner.

Many people are confused by which direction to install their system's air filter. To help remember, consider snapping a quick photo with your cell phone after the filter has been accurately installed by a professional. Or, you also could ask a technician to use a marker to write on the outside of your furnace which direction the filter should be installed. A perfect time to inquire about this is during a routine furnace maintenance visit.

How to Change a Furnace Air Filter

Changing the filter on your furnace or air conditioning system is a simple process. Here is a step-by-step rundown of how to take out a dirty air filter and replace it with a new one:

  1. 1. Turn off your furnace: Make sure to turn off your furnace before starting up the process.
  2. Look for the furnace filter: Typically, the filter is located within the furnace or in the air return vent. Make note of which direction the arrow points on the filter, because you’ll want the arrow on the replacement filter to point similarly.
  3. Remove the old filter: Be careful not to knock out any dust or particles.
  4. Note the date: Write down the date you replaced the filter on the new filter's frame. This will help you keep track of when it's time for you to change it again.
  5. Put in new filter: Put in the new filter with the arrow pointing toward the furnace, which is the direction of airflow and should be the same direction the arrow pointed on the old filter you are replacing.
  6. Secure the filter: Make sure the new filter fits securely and close any latches or clips that hold it in the compartment.
  7. Turn on your furnace: Once the new filter is completely installed, you can turn your furnace back on.

Will a Dirty Air Filter Cause a Furnace Not to Work?

The short answer is, yes, a dirty air filter can cause a furnace to cease working or reduce its lifespan. Changing your furnace or air conditioning filter is one of the simplest things you can do to keep your system running efficiently.