How often should I change my furnace filter?

How often should I change my furnace filter?

The rule of thumb is that you should change your furnace filter at least every three months at a minimum. Think of changing it like the changing of the seasons. However, owning pets, smoking, having allergies, and the number of people living in a home can all affect how quickly your filter can become plugged up meaning you may have to change it more frequently. Additionally, filters with higher MERV ratings (see below) may need to be changed more often as they can reduce airflow which can lead to inefficient performance or even damage your furnace.

What does the filter do exactly?

Most people think that all furnace filters do is catch the dust, dander and debris that makes its way into the ductwork but there are actually a few other important roles that the filter plays in protecting your furnace and home. Filters perform three very important functions:

  1. Catch dirt, pollutants and allergens
  2. Protect internal parts from being overworked or damaged
  3. Provide improved air quality throughout the house


What kind of furnace filter do I need?

There are a lot of options out there when it comes to furnace filters. There are inexpensive options that will simply keep dirt and dust out of the blower compartment, but do little else. Then there are more expensive options that can also improve air quality by filtering out bacteria, pollen and mold. But which option is right for you?

There are a few factors that come into play when deciding what kind of filter you need. Filter cabinet dimensions, MERV rating and the specific needs of your home and those living in it can all affect the outcome. When looking for the right filter, it’s always a good idea to consult the manufacturer’s recommendations for the make and model of your appliance. You can usually find this information in the furnace’s user manual.

Help! I don’t have my furnace manual!

Don’t worry! You can likely find the manual you need by searching for it online. Just start a new Google search and type in your furnace’s manufacturer name (e.g. York, Lennox, Trane, etc.), model number (usually found on the outside of the furnace or inside the front panel and the word “manual”. For instance: “Lennox SLP98V manual” will return several copies of the manual you can browse through. Tip: you can do this for virtually any product in your home. Who keeps the manual??

Types of furnace filters

There are several types of filters that you can buy online or at major home improvement stores like Canadian Tire and Home Depot:

Pleated paper – this are the most common disposable furnace filter and often made of paper and polyester.

Fiberglass – also disposable, this furnace filter typically look like a sheet of spiderwebs and often blue in colour. It’s typically lower quality than a pleated paper filter and needs to be replaced more often.

Permanent reusable – also sometimes referred to as “washable”, this type of furnace filter are usually plastic or aluminum and is the most efficient filter on the market. With regular cleaning every three months, a reusable filter should last up to five years.

Note: electrostatic filters are available in disposable or permanent varieties. These filters self-charge and actively attract particles. You should check your furnace’s manual to confirm that you can use electrostatic filters.

What are furnace filter MERV ratings?

Every furnace filter has a standardized Minimum Efficiency Reporting Value (MERV) rating between 1-16 that indicates its efficiency. The higher the rating, the better the fiber and the better it is at attracting the smallest particles but it also comes with a higher price and also reduces the flow of air that may cause your furnace to work harder. The inverse is true for lower MERV-rated filters. The average furnace uses a filter with a MERV rating of 8 to 11. But buyer beware, because bigger isn’t always better. It might be natural to assume that a higher MERV rating is what you want but a higher rating means a greater restriction of air flow which could have an adverse impact on your furnace’s efficiency.

Filter sizes

There are three dimensions to consider: length, width and thickness. The length and width depends on your particular furnace. Pull out the existing filter or consult your manual to determine the appropriate size but the most common is 16×25. There are filters of different thickness but 1” is typical for the average furnace and up to 4” for larger models. A filter with 1″ thickness (with a MERV rating of no more than 8) is the minimum required by the manufacturer. It’s designed to catch micro-particles before they enter the furnace but do not provide any improved air quality. Thicker filters will provide better air quality but as we mentioned, bigger isn’t always better. There comes a point where thickness is impeding your furnace from doing its job and actually causing harm to the internal parts as they work harder to circulate air. Your furnace’s manual will recommend the best filter for your model.

Why does my filter get clogged up more frequently than every three months?

Clogged filters reduce air flow making the furnace work harder. This costs you money and reduces air quality. Your lifestyle and other external factors will have an effect on how long your filter lasts. Here are some common issues that may force you to replace your filter more frequently:

Pets – the number and type of animals (i.e. long-hair) in the house will generate pet dander that will get caught in the filter.

Smoking – even a single smoker can take a month off the life of an average filter.

Fresh air – as crazy as it sounds, the amount of time your doors and windows are open will have an impact. Fresh air brings dust, pollen and other particles from outside into your home.

Construction & renovations – as with fresh air, any recent or nearby construction will create a large amount of dust that will end up in the filter. The same goes for any woodworking hobbies in the home.

Changing your furnace filter should be part of your regular home upkeep throughout the year. When to change the filter will depend on your particular lifestyle as described above. If you’re still not sure how often you should change it, try replacing your filter soon and keep checking it monthly to get an idea of how long it takes to see significant build-up. That will give you an idea of what a typical change cycle will be for your filters.

Can I run my furnace without a filter?

The short answer is no. You should NEVER run your furnace without a filter. Without a filter your furnace and evaporator coil will be exposed to dust and debris that could cause harm to its inner workings. Think of your filter as the last line of defense for your HVAC system.