Hugh Cairns: All about furnace filters

It’s not often that people think of their furnace filters, but they should. Furnace air filters serve a couple of main purposes. When your furnace blower is moving air around your home furnace filters can clean your indoor air of dust and allergens while simultaneously maintaining your HVAC system by keeping the mechanical components clean.

Changing your filters is easy to do and an effective way to maintain optimal furnace efficiency. If you have central air conditioning attached to your forced air system, furnace air filters are working for you during your cooling needs.

How often should I change my furnace filter?

Examining your filters at regular intervals is very important. Make it a point to check your filters once a month. Dirty filters restrict air flow through the furnace blower system causing it over work. Dirty filters can increase energy costs. Many people only think of changing their furnace filter in the winter months, but if you have central air conditioning operating during summer months, the filter needs to be looked at during those months as well.

You may need to change your filters more often if you have:

·         high traffic

·         a larger family

·         pets or pet hair

·         excessive dust

·         recent renovations or construction

·         your home is in a dry, or windy environment

Write the installation date on the filter for reference. Also, you can write the filter size on the ductwork next to the filter location as a reminder. Choose a handy day to remember like the first of the month to check your filter.

Optional air filter gauges are available to install between the blower and the filter and are easy-to-use. They measure particulate on an easy-to-read and suggest that the filter may require changing. Although inexpensive, they should be installed by an experienced service technician.

Best furnace filters – What type of filter is best?

Most furnaces are initially installed with an inexpensive fiberglass filter. More expensive filters can improve the air quality in your house by removing pollen, bacteria and mold spores from the air.

The best furnace air filter is the one that matches your specific needs. Protecting your equipment is secondary to your health needs. Should you have specific allergies or health concerns, matching the filter to your needs should be your primary concern. You can improve the air quality in your home immediately just by changing or upgrading your air filter. The best type of air filter you need depends on the presence of airborne particles in your home and the sensitivity of your home’s occupants to those contaminants.

Fiberglass or Polyester

·         Inexpensive

·         Capture large particles


·         Longer service life

·         Capture large particles


·         Larger surface area filters more efficiently


·         Can be reusable

·         Treated to capture small and large particles

Hugh Cairns furnace filter tip – Buy better filters, look at them monthly and plan to change them regularly. Good air quality matters to you, your family and your furnace.

Furnace filter ratings – What is a MERV rating?

Minimum Efficiency Reporting Value (MERV) is a filter rating system to standardize and simplify air filter efficiency ratings for the consumer to allow them to effectively compare one unit to another. The higher the MERV rating, the higher the efficiency of the air filter. This simply means that a MERV 13 filter will remove smaller particles from the air than a MERV 8 filter.

MERV 5 – Fiberglass filters offer equipment protection and large particulate filtration

MERV 8 – Pleated filter for better protection from dust and common allergens

·         offer MERVE 5 protection plus filters settling/average dust, dust mite debris, pollen and dander

MERV 13 – Pleated filters capture the highest percentage of suspended dust and/or if you have severe allergies

·         offer MERV 5 and MERV 8 protection plus filters suspended/fine dust, auto emissions , tobacco smoke particles, mild odours, bacteria, sub-pollen particles, viruses, pollution

Furnace filter location 

Furnace filters are best placed where they clean the air prior to entering your furnace. This is known as the “return” side of the air duct system. Once the air is conditioned (heated or cooled) the air is forced to the supply side of the air duct system. 

How can changing furnace filters save energy costs?

A significant purpose of a furnace filter is to remove airborne particles that might damage the furnace equipment. As the filter work, airborne particles build up on the filter restricting air flow making the furnace work harder to pull air through the intake (or return side), decreasing the efficiency of your furnace.

Clean furnace filters allow for better air flow, increase the efficiency of your furnace and reducing energy costs, which in turn saves you money.

Furnace filter installation – How do I change my furnace filter?

Make sure that the furnace blower is off. You can do this by turning the furnace off at the thermostat. Alternatively, you can locate the emergency power shut off switch in the furnace area and turn it off. If you don’t shut the power off, removing the filter while the blower is on will result in disturbing the collected particulate and it will be blown into the furnace equipment. Locate the filter – usually located on the lower front or side of the furnace.

Hugh Cairns furnace filter tip – On some systems, you will need to remove a metal strip that covers the end of the filter. In some cases the filter is located in the furnace case – open the panel door to have a look. Slide the filter out, inspect and replace as necessary.

Furnace filter sizes – What size of furnace filter do I need?

Furnace filters come in various dimensions. If this is your first venture into replacing your filter you should make sure that the existing filter is the correct size. The size of the filter should match the filter slot. The size of the existing filter should be written on its edge. If it is not, measure it with a tape measure. Next measure the filter slot and compare the results, they should match. If they don’t, rely on the slot size.

When shopping for filters it is important to realize that the size written on the packaging may not be the actual size of the filter. Filters can be sized in two ways—nominal and actual. Nominal sizing is the approximate size of the filter, rounded up to the nearest whole inch. This makes filter sizes easier to identify when shopping. Also, air filters can also vary by thickness. Commonly they are one inch thick, and in some cases they can be more than 4 inches thick.

Furnace air filters should fit snuggly in their slot. Loose filters let air flow around them reducing filtration.


About the House – Furnace filter fact

48% of BC homeowners have forced air furnaces, of those:

32% Changed or cleaned filter every three months

24% Changed or cleaned filter every six months

26% Changed or cleaned filter once in the past year

Source: Statistics Canada