Key takeaways
Yes, it matters.
The filter protects the blower, heat exchanger and AC coil, not just your lungs. A clogged filter is one of the most common causes of no-heat and frozen-coil calls.
1-inch filters
Check monthly, change every 1–3 months.
4–5 inch media filters
Change every 6–12 months.
Best MERV for most homes
MERV 8 to 11. Go to MERV 13 only if your system is set up for it.
Same filter, both seasons.
Your furnace blower also moves air for central AC, so the filter works all year.
Why changing your furnace filter is important
Every bit of air your furnace heats, and your air conditioner cools, passes through the filter first. When it clogs, the blower has to pull harder, airflow drops and the system runs longer to reach the thermostat setting.
In winter, low airflow lets the heat exchanger run hot. The high-limit switch then shuts the burners off, and you get short bursts of heat or none at all. Over time that heat stress can contribute to cracks in the heat exchanger, which is a safety issue.
In summer, a dirty filter starves the indoor AC coil of warm air. The coil drops below freezing, ices over and stops cooling. Dust that slips past a dirty or poorly fitted filter also coats the coil and blower wheel, which cuts efficiency until they are professionally cleaned.
A fresh filter costs little. A blower motor replacement typically runs $600 – $2,000.
MERV ratings explained
MERV (Minimum Efficiency Reporting Value) rates how well a filter captures particles, on a scale of 1 to 16 for home and commercial filters. Higher numbers catch smaller particles, but they also resist airflow more, especially in a thin 1-inch frame.
| MERV | Typical filter | What it captures | Good for |
|---|---|---|---|
| 1–4 | Flat fibreglass | Large dust, lint | Basic equipment protection only |
| 5–8 | Pleated | Dust, pollen, mould spores, carpet fibres | Most homes; the everyday standard |
| 9–12 | Better pleated | Fine dust, pet dander, some smoke particles | Pets, mild allergies, busy households |
| 13–16 | High-efficiency media | Very fine particles, smoke, many bacteria | Asthma, allergies, wildfire smoke; best in a 4–5 inch cabinet |
Retail brands sometimes use their own scales (such as MPR or FPR). Look for the MERV number on the package to compare fairly.
Can a filter be too good?
Yes. A MERV 13 filter jammed into a 1-inch slot on an older furnace can restrict airflow as much as a dirty filter. Signs include a whistling return grille, weaker airflow at the vents, a furnace that shuts off early, or an AC coil that freezes.
If you want hospital-grade filtration, the better answer is a 4- or 5-inch media filter cabinet. The deeper pleats give much more surface area, so you get MERV 11–13 capture with less resistance and a filter that lasts 6–12 months. A technician can measure the static pressure of your system to confirm it can handle the upgrade.
How often to change your furnace filter in Ottawa
| Situation | 1-inch pleated | 4–5 inch media |
|---|---|---|
| Average home, no pets | Every 2–3 months | Every 12 months |
| Peak heating (December–February) or cooling (July–August) | Every month | Check at 6 months |
| Dogs or cats | Every 1–2 months | Every 6–9 months |
| Allergies or asthma at home | Every month | Every 6 months |
| Renovation, drywall sanding or new build | Every 2–4 weeks during work | Check monthly during work |
| Wildfire smoke days | Check weekly; change when grey | Check monthly |
New homes in Findlay Creek, Riverside South and other growing neighbourhoods often carry construction dust for the first year, so expect to change filters more often. A simple habit: check the filter on the first of every month, and change it at least every season.
How to change a furnace filter
- 1
Turn the furnace off
Set the thermostat to Off or flip the furnace service switch so the blower does not pull dust in while the slot is open.
- 2
Find the filter slot
It is usually between the return air duct and the blower, on the side or bottom of the furnace, sometimes behind a small cover.
- 3
Note the size and direction
The size is printed on the frame, such as 16x25x1. The airflow arrow must point toward the furnace blower.
- 4
Slide in the new filter
Make sure it sits snugly with no gaps around the edges, then close any cover so air cannot bypass the filter.
- 5
Turn it back on and write the date
Restore power, then write the date on the frame or set a phone reminder so you know when it is due again.
What a furnace filter can and cannot fix
A good filter cleans the air your system recirculates. It does a solid job on dust, pollen and pet dander, and a MERV 13 media filter helps a lot on wildfire smoke days, when Ottawa air quality warnings tell people to keep windows shut.
What a filter cannot do is bring in fresh air or remove moisture and odours. Tight, newer Ottawa homes can feel stuffy in winter, with condensation on windows and lingering cooking smells. That is a ventilation problem, and the fix is a heat or energy recovery ventilator, not a thicker filter. Our HRV vs ERV guide explains the difference.
For smoke, run the furnace fan on continuous during bad air days so the air passes through the filter more often, then switch back to Auto once the air clears.
Common filter mistakes
Running without a filter
, even for a day. Dust goes straight to the blower and AC coil.
Arrow pointing the wrong way.
The filter works less well and can collapse into the blower.
Wrong size with gaps.
Air takes the easy path around the filter.
Forgetting in summer.
The same filter serves your central AC.
Forgetting other filters.
Humidifier pads, HRV filters and electronic air cleaner cells all need their own schedule.
Need filters? Call (613) 866-1869 to ask about MERV 8 filter packs; Oria has offered them with doorstep drop-off. A yearly tune-up also includes a filter check, a blower inspection and a combustion safety test. See our furnace maintenance checklist and, if your house is dry or dusty in winter, our guide to ideal indoor humidity in Ottawa. Book furnace maintenance or explore a maintenance plan to have it handled for you.
Frequently Asked Questions
Is it really important to change my furnace filter?
Yes. A clogged filter cuts airflow, which makes the furnace overheat and shut off, freezes the AC coil in summer and drives up energy use. It also lets dust coat the blower and coil. Changing a 1-inch filter every 1 to 3 months is one of the cheapest ways to avoid breakdowns and keep your system efficient.
What MERV rating is best for a home furnace?
For most homes, MERV 8 to MERV 11 is the best balance of filtration and airflow. MERV 8 captures dust, pollen and mould spores, while MERV 11 adds pet dander and finer dust. MERV 13 offers better capture for allergies or smoke, but works best in a 4- or 5-inch media cabinet rather than a thin 1-inch slot.
Can a dirty furnace filter stop my furnace from working?
Yes. When the filter clogs, airflow drops and the heat exchanger overheats. The high-limit safety switch then shuts the burners off, so the furnace may run for a few minutes and stop, or blow cool air. Replacing the filter often restores normal heating. If the problem continues with a clean filter, call a technician.
Which way does the arrow go on a furnace filter?
The arrow on the filter frame shows the direction of airflow, so it should point toward the furnace blower and away from the return air duct. On most basement furnaces, that means the arrow points into the furnace cabinet. Installing it backwards reduces filtration and can cause the filter to bow or collapse.
Do I need to change my furnace filter in summer?
Yes. If you have central air, the furnace blower moves the cooled air through the same filter. A dirty filter in summer can freeze the AC coil, reduce cooling and raise your hydro bill. Check it monthly during July and August heat waves, when the system runs the most, and change it whenever it looks grey.
Are washable furnace filters a good idea?
Washable filters save buying replacements, but most capture less than a good pleated MERV 8 to 11 filter. They must be fully dry before going back in, or they can grow mould. For most Ottawa homes, a disposable pleated filter changed on schedule, or a 4- to 5-inch media filter, gives better protection for the furnace and air quality.
Related Reading
Need help with your heating or cooling?
Family-owned, licensed HVAC contractor serving Ottawa and Gatineau since 2014. 5.0 stars from 578 Google reviews.