I check the AC unit before I went inside. It was definitely running and inside you can hear the system working, coolish air is coming from the vents, and the thermostat is set exactly where it should be.

But the house still feels warmer than it should.

Before assuming the air conditioner is failing, there’s one simple thing worth checking—and many homeowners forget about it once winter ends.

I’m inspecting a newer home on a hot, sunny day.

The air conditioner is already running when we get there, but the house still feels surprisingly warm once we step inside. You can hear the system working, yet the indoor temperature isn’t dropping the way it should.

I check the air coming from the registers with my infrared camera. It’s cool, but not as cold as I’d expect, and the airflow feels noticeably weak.

That points me toward one of the simplest parts of the system.

So I head to the furnace room and when I pull out the furnace filter, the problem becomes obvious. It’s packed with dust, pet hair, and debris. Very little air can get through it.

As soon as I remove the dirty filter, the airflow increases noticeably.

Fortunately, there’s a new, clean filter sitting beside the furnace, so I install it rather than putting the old one back.

And all that buildup is a good reminder of just how much dust and debris is circulating through the average home.

Even though the furnace isn’t heating the house, its blower is still responsible for moving the cooled air through the ductwork.

In plain English: the AC is producing cool air, but the system can’t move enough of it through the house.

By the time we finish the inspection, the house is nice and cool.

A central air conditioner and a forced-air furnace usually share the same blower, filter, and ductwork.

During the summer, warm air from the house is pulled through the return-air ducts, passes through the filter, and moves across the air conditioner’s cooling coil. The blower then sends the cooled air back through the home.

When the filter becomes heavily clogged, it restricts that airflow. According to ENERGY STAR, a dirty filter can slow airflow and force the system to work harder to keep the house comfortable.

That can result in:

A severely restricted filter doesn’t just make the house uncomfortable. It can contribute to the cooling system shutting down or being damaged.

The furnace filter isn’t only a winter maintenance item.

If you have central air conditioning, it works all summer too.

If your air conditioner is running but the house isn’t cooling properly:

If the filter is clean but the house still isn’t cooling, there may be another problem, such as a dirty outdoor unit, a frozen cooling coil, a thermostat issue, low refrigerant, or an equipment failure.

That’s when it’s time to call a qualified HVAC technician.

Don’t continue running a system that has little airflow, visible ice, unusual noises, or water collecting around the furnace.

✅ Check the filter monthly during heavy heating and cooling seasons.

✅ Replace it when it’s visibly dirty rather than relying only on the date written on the package.

✅ Keep several replacement filters on hand. Stock up when they go on sale.

✅ Write the installation date on the filter frame.

✅ Make sure the airflow arrow points toward the furnace or blower.

✅ Don’t automatically choose the most restrictive filter available. Use a filter that’s suitable for your equipment and follow the manufacturer’s instructions.

✅ Keep return-air grilles clear so the system can pull enough air back to the furnace.

Homes with pets, renovations, wildfire smoke, or higher dust levels may need filter changes more frequently.

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Want to learn more about AC Maintenance?

This Old House — How to Maintain an Air Conditioner
Practical maintenance advice that can help keep a central air-conditioning system operating properly during hot weather.
Carrier — Why Is My AC Not Blowing Cold Air?
A homeowner-friendly breakdown of common cooling problems, including dirty filters, thermostat settings, blocked outdoor units, and frozen coils.
Trane — AC Not Blowing Cold Air?
Explains how restricted airflow can make an air conditioner work harder and may even contribute to a frozen evaporator coil.

How a small amount of movement caused $4,000 in damage—and the 30-second check that could’ve caught it. 🚽🔎