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Understand the Role of My Furnace Filter

Posted by Ramphal Bridgemohan on

Although you may not be aware with the detailed workings of your furnace as a homeowner, it is beneficial to have a basic understanding of some key components, such as the furnace filter. Understanding the significance of your furnace filter in your home heating system may help you save money on maintenance and heating bills while also improving the quality of your indoor air.

What is the purpose of a Furnace Filter?

The main purpose of a furnace is to protect the components inside the furnace such as the blower/fan motor, the secondary heat exchanger and the air condition evaporator coil.  Airborne particles such as dirt, dust, and dander will be drawn to your heating/cooling system fan and eventually circulate in your home's air if your furnace does not have a filter or poor-quality filter (e.g., a fiberglass filter). This can harm equipment and drastically reduce the life and efficiency of the furnace. If your filter is unclean or clogged, the same negative effects occur. A clogged or loaded filter restricts airflow to your furnace, requiring it to work more than required to keep your house warm in the winter or cool in the summer.

Do I need to buy a furnace filter that is the same brand as my furnace?

The answer is usually NO, but you may double-check the User Manual that came with your unit or ask your trusted HVAC professional. There are multiple replacements in the market which are interchangeable, generally brand name filters cost more and aftermarket/ generic filters are less expensive, but both remove particle size based on MERV ratings. If your system air cleaner uses a standard size 1", 2", 3" or 4" filter then the only option you will need to consider is the MERV rating, but if you use a Non-Standard 4" or 5" filter then you must do more research to find the exact /actual replacement size, part number or model number to ensure a snug fit.

What is a MERV rating?

The MERV (Minimum Efficiency Reporting Value) rating system determines the size of particles (microns) a filter will capture, it is the most used rating system developed by ASHRAE for furnace filters and is widely used by Furnace Equipment Manufacturers to rate their brand of filters. Known ratings vary from MERV 3 to MERV 16, with MERV 11 being the most common. A higher rating will capture smaller the particle size for example a MERV 11 filter will capture 65% to 80% of particle size 1 to 3 Microns and 85% to 100% of particle size 3 to 10 microns. There are many variable factors to consider before choosing which MERV rating is best for

Your needs as every home has different needs and there are advantages and disadvantages to choosing a MERV rating, including a higher rated filter will cost more. A filter with a high MERV rating will capture more particles but it will also restrict the air flow, a filter with low MERV rating will allow more air flow but will not filter the air as well. It can be difficult to decide which is the best rated filter for you needs, if you cannot make a choice consider a balance of both air flow and level of filtration which a MERV 11 filter provides. Here are some other things to consider before choosing the MERV rating, if there are options available for your filter.

  1. The age of your furnace and whether the filter/system was maintained properly from new. 
  2. If you have pets or not, it is recommended to go with MERV 11 or higher if you have pets.
  3. Does your home have a poorly designed duct system - some rooms not heated or cooled evenly.
  4. Does someone in your home have allergies or breathing problems.
  5. Is your system equipped with a DC / ECM / Variable Fan/Blower Motor.  

Most furnace manufacturers recommend a High Efficiency Air Cleaner to pair with their high efficiency furnace or Air Condition System and they also provide these air cleaners to the furnace installers/dealers   but not all installers follow these recommendations, a high efficiency air cleaner should be installed to acquire the level of efficiency stated by the furnace manufacturer (e.g. 96% AFUE - Annual Fuel Utilization Efficiency).

How often do I need to replace the filter?

Every household is different in too many ways, there are recommendations written on most filters, but these are just not practical because there are just too many variables when it comes to the life of a filter, we simply cannot know the exact time when to change a furnace filter, but after some time most homeowners get to know the average time their particular filter will last, here are a few factors to consider.

  • Is your furnace fan setting on the thermostat set to run on Auto Mode or Continuous Mode or Intermittent Mode? - filter will get clogged sooner if fan is set on Continuous or Intermittent Mode. If fan is set on Auto mode, check filter more frequently during peak heating and cooling seasons. During shoulder seasons (off peak heating and cooling season) filter will not need to be changed as frequently. 
  • Do you have pets in your house? - household with pets should check and change filter more often, especially if your pet sheds.
  • MERV rating of filter you're using, as well as the furnace manufacturer's advice – There are filters which can last for 8-12 months - some filters have many pleats and so have plenty of surface area, these filters generally last longer than filters with regular pleats.
  • Whether you or a member or someone in your household has allergies - check the condition of your filter if your symptoms become worse.
  • Thickness of your filter (when thicker the filter longer the replacement time) – If you are using a 1" filter you should check every 1-2 months where 5" filter can last for 4-12 months depending on variable factors.
  • Size/Square footage of your home - If you have a larger home you will need to check the filter more often especially if you use a 1" filter.
  • Number of people living in the house - 2 people living in a house with a 1" filter can last 3 months and the same filter with 8 people living in the same house will only last 1 month, most dust generated in a home is from fabrics/lint.

We always recommend checking the condition of your filter when switching from heating to cooling and cooling to heating and remember to close damper to humidifier during cooling season and open damper heating season.

For more information or any concerns regarding your furnace filter, please reach out to us on our communication channels on or

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