There's a reason I check furnace filters during home inspections.

It's not the most exciting part of an inspection. There's no sophisticated equipment involved, and it takes only a few seconds to slide the filter out and see what condition it's in.

Most of the time, there's nothing particularly interesting to report.

But every once in a while, I pull out a filter that makes me wonder how the furnace has been managing.

This was one of those times.

During a home inspection, I pulled out the furnace filter and found this.

Take a close look.

The pleats are buried beneath a thick layer of grey dust and debris. There's so much buildup that the filter looks more like an old piece of carpet than something that's supposed to allow air to pass through it.

Now imagine trying to breathe through that.

Not very appealing, is it?

But that's essentially what the furnace is dealing with. Every time the blower runs, it needs to draw air through the filter before circulating heated air throughout the house.

And when a filter gets this dirty, moving enough air can become a problem.

Here's what surprises some homeowners.

The furnace doesn't necessarily stop working when the filter gets clogged.

It may continue heating the house, giving everyone the impression that everything is fine.

Meanwhile, restricted airflow can cause temperatures inside the furnace to climb higher than they should.

Eventually, that can trigger a safety shutdown, potentially leaving the homeowner without heat.

It's a good example of how something as ordinary as forgetting to change a furnace filter can turn into a much bigger headache.

And it's why I recommend checking yours regularly, especially before another Alberta winter arrives.

What Happens When a Furnace Can't Get Enough Air?

Your furnace relies on a steady flow of air to operate properly.

As cooler air from the house returns to the furnace, it passes through the filter, which traps dust, pet hair and other airborne particles.

Over time, those particles accumulate. If the filter isn't replaced, the buildup can restrict airflow.

And that's where things can get interesting.

The heat exchanger inside your furnace transfers heat from the combustion process to the air circulating through your home.

When there isn't enough air moving across it, the furnace can get hotter than it's designed to operate.

A severely clogged filter can actually cause your furnace to overheat.

Modern furnaces have safety controls to help prevent dangerous overheating. If temperatures climb too high, a high-temperature limit switch can shut off the burners while the blower continues running to cool the furnace.

Once things cool down, the burners may restart. But if the airflow problem remains, the cycle can repeat.

Depending on the furnace, repeated overheating can eventually cause a safety lockout, leaving the homeowner without heat until the problem is addressed.

Not every furnace shutdown is caused by a dirty filter. But when a problem is this easy to help prevent, it's worth taking a few minutes to check.

Want to learn more about Furnace Filters?

This Old House — How Often to Change a Furnace Filter

Angi — Why You Should Change Your Furnace Filter Regularly

The Spruce —What Happens If You Forget to Change Your HVAC Filters? We Asked an HVAC Pro

Take Two Minutes Today

If you can't remember when you last checked your furnace filter, now's a good time.

Turn the furnace off and locate the filter. In most homes, you'll find it in the return-air duct beside the furnace or inside a filter cabinet.

Carefully slide it out and inspect it.

If it's heavily loaded with dust and debris, replace it with the correct size and type for your system.

Before removing the old filter, note the airflow arrow printed on its frame. Install the replacement with the arrow pointing in the direction of airflow, normally toward the furnace blower.

One other thing worth mentioning: don't automatically assume the most expensive filter is the best choice.

Higher-MERV filters can capture smaller airborne particles and improve indoor air quality. However, not every filter is suitable for every heating system.

Choose a filter that provides good filtration without creating more airflow resistance than your furnace and ductwork can accommodate. Check the manufacturer's recommendations or ask your HVAC technician.

If your furnace is repeatedly shutting down, overheating or struggling to heat your home, have it assessed by a qualified HVAC technician. Changing the filter alone may not resolve an existing problem.

You don't need to make furnace filter maintenance complicated.

Here's one simple trick I recommend.

Set a recurring reminder on your phone for the first day of every month: "Check furnace filter."

You won't necessarily need to replace it each time, but you'll know what condition it's in.

And with another Alberta winter approaching, that's one less thing to worry about.

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Is Your Home Ready for Another Cold Winter?

With winter approaching, there's one place in your home that's worth checking before the temperatures drop.

Next week, I'll show you how to tell whether you have enough attic insulation — and why what's above your ceiling can make a big difference to your comfort and heating bills. ❄️🏠