Furnace Filter Guide for Ottawa Homes: Types, MERV Ratings and How Often to Change

This furnace filter guide is for Ottawa homeowners who want to get one of the simplest, most important maintenance tasks right. Your furnace filter is a small, inexpensive part, yet it affects your comfort, your energy bills, your indoor air and how long your furnace lasts.

With windows closed for months during our long winters, often from November right through March, your furnace recirculates the same indoor air for much of the year. A good filter, changed on time, makes a real difference.

What Does a Furnace Filter Actually Do?

The main job of a furnace filter is to protect the furnace. It traps dust, lint, hair and debris before they reach the blower motor, heat exchanger, burners and other internal components. A clean system runs more efficiently and breaks down less often.

Its second job is improving indoor air quality by capturing particles like dust, pollen and pet dander. Higher-grade filters capture smaller particles, but as you'll see below, there's a balance to strike.

Furnace Filter Guide to Types and Materials

Fibreglass filters

Thin, inexpensive and usually blue or green. They catch larger debris and protect the furnace, but do little for air quality. They need frequent replacement.

Pleated filters

The most common choice for Ottawa homes. The folded material provides more surface area, capturing finer particles while maintaining reasonable airflow. They come in a range of ratings.

Washable or reusable filters

These can be rinsed and reused, but they must be completely dry before reinstalling, and their filtration is often modest. If not cleaned properly, they can hold onto dust and moisture.

Media filters (thicker cabinet filters)

Some systems have a filter cabinet that holds a thicker filter, often several inches deep. The extra depth gives more surface area, so they capture finer particles with less airflow restriction and typically last longer between changes.

Electronic and electrostatic air cleaners

These use an electrical charge to capture particles. Some require regular washing of collector cells. If you have one, follow the manufacturer's cleaning schedule.

Understanding MERV Ratings

MERV (Minimum Efficiency Reporting Value) rates how well a filter captures particles of different sizes. Higher numbers capture smaller particles.

Some retailers use their own rating scales instead of MERV. Check the packaging for the MERV equivalent so you can compare.

Is a higher MERV always better?

No. A very restrictive filter in a standard 1-inch slot can reduce airflow, especially as it loads with dust. Reduced airflow can cause the furnace to overheat and shut off on its high-limit safety, put extra stress on the heat exchanger, raise Hydro Ottawa bills from the blower working harder, and shorten equipment life. The right filter is the highest rating your system can handle without restricting airflow. A technician can recommend the best option for your setup.

How Often Should You Change Your Furnace Filter?

A good general rule:

• 1-inch filters: check monthly, replace every one to three months
• Thicker media filters: check regularly, replace every several months or as the manufacturer recommends

Change more often if you have:

• Pets, especially ones that shed
• Allergies or asthma in the household
• Renovations or construction dust
• A large household or a home that's frequently closed up during winter
• Wildfire smoke drifting into the region in summer

The best test is visual: pull the filter and hold it up to light. If it looks grey and you can't see light through it easily, replace it.

Ottawa seasonal tip

Set a reminder to check your filter at the start of heating season in the fall and again in mid-winter, after the holidays. Those are the times your furnace is under its heaviest use.

Furnace Filter Guide to Sizing and Fit

A filter only works if it fits. Gaps around the edges let dirty air bypass the filter completely and go straight into the blower and heat exchanger.

• Use the exact nominal size printed on your old filter or on the filter slot. Don't round up or down.
• Check the depth. A 1-inch slot needs a 1-inch filter; a media cabinet needs the thicker filter it was designed for.
• Look for a snug fit with no visible gaps. If your filter slot has no cover, ask a technician about sealing it, since an open slot can draw in unfiltered basement air.
• Buy a few at a time so you always have a spare on hand when it's time to change.

How to Change a Furnace Filter

1. Turn the furnace off at the thermostat or the service switch.
2. Find the filter slot. It's usually between the return duct and the furnace, or in a filter cabinet beside the unit.
3. Note the size printed on the frame of the old filter, such as 16x25x1. Sizes must match exactly.
4. Check the airflow arrow. Install the new filter with the arrow pointing toward the furnace, in the direction air travels.
5. Close any access door and make sure the blower door is properly seated, then turn the furnace back on.
6. Write the date on the filter frame so you know when it was changed.

Never run the furnace without a filter. Even a few days can let dust coat the blower and other internal parts.

Signs Your Filter Needs Attention

• Weak airflow from vents
• More dust on furniture than usual
• The furnace runs longer or shuts off unexpectedly
• Rooms aren't heating evenly
• Higher energy bills without explanation
• A whistling sound from the return or filter slot

If changing the filter doesn't fix these issues, there may be a deeper problem. Book a furnace repair visit, or call KAD Mechanical & HVAC at (613) 790-2675 if you are without heat.

Filters Are Just One Part of Maintenance

Changing your filter is the most important thing you can do between service visits, but it doesn't replace a professional tune-up. Annual furnace maintenance includes checking combustion, safety controls, the flame sensor, condensate drain and heat exchanger, which are areas homeowners shouldn't open themselves. Any work on gas components must be done by a TSSA-licensed technician.

If you ever smell gas or your carbon monoxide alarm sounds, leave the home and call the Enbridge Gas emergency line or 911 from outside.

KAD Mechanical & HVAC serves homeowners in Ottawa, Kanata, Nepean, Barrhaven and Orleans with certified technicians and upfront pricing. Not sure which filter is right for your system? Contact us or call (613) 790-2675.

Frequently Asked Questions

Written by the KAD Mechanical & HVAC team

Certified, TSSA-licensed heating, water heater and gas technicians serving Ottawa, Kanata, Nepean, Barrhaven and Orleans for 10+ years.

Next article: Carbon Monoxide Safety and Your Furnace: A Practical Guide for Ottawa Homes