Furnace Filters

Find your filter size

The size is printed on the edge of your current filter as width x height x depth, for example 16x25x1. This is the nominal size: the real dimensions are usually a little smaller, so order the number on the label rather than your tape measure reading. All three numbers must match, including depth.

Filter depth and where it fits

For 4 to 6 inch media filters, the brand part number on your old filter is the surest match, because filters with the same nominal size can be built for different cabinets.

MERV ratings at a glance

• MERV 8: basic protection that captures dust, lint and larger particles.
• MERV 11: a popular everyday choice that also captures pollen, pet dander and mould spores.
• MERV 13: finer filtration, including smaller particles such as some smoke. A good step up for allergies.
• MERV 14 to 16: high-efficiency media, mostly in deep 4 to 6 inch cabinets.

If you are unsure, replace like for like with the rating your system uses now.

Furnace filter FAQs

What is the difference between MERV 8, MERV 11 and MERV 13 furnace filters, and which should I choose?

MERV 8, 11 and 13 furnace filters differ in how much fine dust they catch. In the ASHRAE 52.2 test, MERV 8 must capture at least 70% of 3 to 10 micron particles, while MERV 11 must also catch at least 20% of 0.3 to 1 micron particles and MERV 13 at least 50%. MERV 8 suits basic dust; Health Canada recommends MERV 13 or higher for wildfire smoke.

What does the MERV rating on a furnace filter mean?

MERV, short for Minimum Efficiency Reporting Value, is a 1-to-16 rating from the ASHRAE 52.2 lab test that shows how well a filter captures particles from 0.3 to 10 microns. A higher MERV catches more and smaller particles, such as fine dust and smoke, but also adds more resistance to airflow. Home furnace filters at PureFilters range from MERV 7 to MERV 16.

How do FPR and MPR ratings compare to MERV?

FPR and MPR are rating scales created by individual filter brands, not industry test standards, so there is no official conversion to MERV. MERV comes from the ASHRAE 52.2 lab test and means the same thing for every brand that tests to it. To compare filters fairly, look for the MERV rating on the package or the maker's spec sheet; PureFilters shows the MERV rating on its furnace filters.

What size furnace filter do I need, and how do I measure it?

The furnace filter size you need is printed on the edge of your current filter, such as 16x25x1. If no size is printed, measure it and round each number up to the nominal size: PureFilters 1-inch pleated filters measure 1/2 inch less in width and height and are 3/4 inch thick, so 15-1/2 x 24-1/2 x 3/4 is a 16x25x1. For deeper media filters, match the part number instead.

Why does my furnace filter measure smaller than the size on the box?

Furnace filters are sold by nominal size, and the actual size is a little smaller so the filter slides into its slot. Depth varies most by brand: many 5-inch media filters are about 4-3/8 inches deep, and two filters sold as 16x25x4 can differ by more than half an inch. Match the part number on your old filter, or see our furnace filter size guide.

How often should I change my furnace filter?

Check a 1-inch furnace filter every month and change it at least every 3 months, or sooner if it looks dirty, as ENERGY STAR advises; check more often during peak heating and cooling months. Deeper 4- and 5-inch media filters last longer: Lennox rates its Healthy Climate media filters for up to 12 months. Always follow your filter maker's interval.

Will a MERV 13 furnace filter restrict airflow or damage my furnace?

Most home furnaces can use a MERV 13 filter without equipment problems if it is replaced on schedule, according to the U.S. EPA. Higher-MERV filters do add airflow resistance, and resistance rises as a filter loads with dust. Deeper 4- to 6-inch pleated media spreads air over more surface and limits that resistance. If your furnace is older or you are unsure, ask a licensed HVAC technician.

How do I choose a furnace filter for my home?

Choose a furnace filter in three steps: match the size or part number of your current filter, keep the depth your furnace or media cabinet was built for, then pick a MERV rating. MERV 8 covers everyday dust, while MERV 11 and MERV 13 capture more fine particles. PureFilters pleated filters come in MERV 8, 11 and 13, and media filters follow your cabinet maker's options.

How do I change a furnace filter, and which way does the airflow arrow point?

To change a furnace filter, turn the system off at the thermostat or the furnace power switch, note the arrow on the old filter and slide it out. Slide the new filter in with its airflow arrow pointing the way air moves: away from the return-air duct and toward the furnace blower. Close the filter door, then turn the system back on.

Do activated carbon furnace filters remove odours?

Activated carbon furnace filters can reduce some household odours and gases, because activated carbon is the most common material HVAC filters use to adsorb gases. The U.S. EPA notes carbon is weak against some gases, such as formaldehyde, and thin carbon layers can saturate quickly. Lennox Carbon Clean filters, such as the X6672 (16x25x5), combine activated carbon with MERV 16 media.

Where can I buy furnace filters in Canada?

You can buy furnace filters online from PureFilters, a Toronto-based Canadian store that ships across Canada. PureFilters carries pleated filters in MERV 8, 11 and 13, media filters from brands such as Lennox, Honeywell, Carrier, Aprilaire and GeneralAire, and custom sizes made to order. Search by size or by the part number on your old filter.