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How to Change a Furnace Filter | HVAC Tips

on November 3, 2017

change a furnace filter

Temperatures have started to drop here in Idaho and it’s time to fire up the furnace. Your furnace has had a nice summer rest so, before you turn it on, be sure to change your furnace filter. This will help to ensure that your furnace is working optimally and efficiently during the winter months when you’ll need it the most. While it may seem like a daunting task, it’s actually quite simple to change a furnace filter. All you will need is the new  filter and access to your furnace.

How do you know when it’s time to change your filter?

Check your filter monthly. Depending on the type of filter used, it may need to be changed every month or every 2-3 months. A filter needs to be replaced when you can no longer see light through it.

In the winter months, when your heating system fan is running most of the time, you may need to change your furnace filter more frequently.

What kind of filter do you need?

Every furnace requires a specific size of filter. The size your furnace needs can usually be found on the filter air box or on the furnace itself.

To find the filter air box, identify the two ducts on your furnace. These are the heating and cold air return duct. The air duct near the bottom of the furnace is the cold air return duct. It holds the filter air box.

Once you’ve determined the size of filter for your furnace, it is time to choose the type of filter best for your home. Basic air filters prevent dust and other air borne particulates from entering the HVAC unit and damaging it. All filters are rated on the MERV scale (Minimum Efficiency Reporting Value). The higher the MERV score, the more efficiently the filter will remove particles and pollutants from the air.

For allergy and asthma sufferers we recommend investing in a HEPA or an electrostatic air filter. These are the best filters to remove pollutants, allergens and bacteria from the air inside your home. Indoor air quality is very important during the winter months when more time is spent inside. A HEPA or electrostatic air filter can greatly improve the indoor air quality and help prevent allergies, cold and flu.

Once you have chosen the best filter for your furnace, it’s time to change it!

How to Change the Furnace Filter

Step 1: Turn off the furnace.

Step 2: Locate the filter air box.

Step 3: Locate the filter. The filter should be right inside the filter air box.

Step 4: Discard the old filter. Air filters contain a lot of allergens and bacteria. Have a large plastic bag on hand when changing your filter. Once you’ve removed the old filter, place it inside the plastic bag immediately. Tie it shut and take it outside to the trash as soon as possible.

Step 5: Insert the new filter. Slide the furnace filter into the filter air box. The arrows alongside the filter indicate the direction of airflow. The arrows should always point towards the furnace.

Step 6: Turn the furnace back on.

Remember, it’s always important to change your filter before turning your furnace on for the winter. This prolongs the life of your furnace, makes it more efficient and improves the indoor air quality in your home. If you change your furnace filter and notice anything unusual give us a call at Mainstream.


Winter is the worst time of year to have your furnace stop working. If you suddenly find yourself without heat call Mainstream Electric, Heating, Cooling & Plumbing at any time, day or night. We have a dedicated service team that’s available 24/7!