How Do I Know If My AC Needs a Filter?

If your air conditioning isn’t cooling like it used to, or if you’ve noticed dust building up around vents, your AC filter might be the culprit. Most people don’t think about their AC filter until something goes wrong-then they wonder if it’s time to replace it. The truth is, you don’t need to wait for a breakdown. There are clear signs your filter is clogged and needs changing, and catching them early can save you money, energy, and stress.

Sign #1: Your Airflow Feels Weak

Turn on your AC and stand near a vent. If the air feels like a slow breeze instead of a steady flow, your filter is probably choked with dust, pet hair, or pollen. A clean filter lets air move freely. A dirty one acts like a wall. In homes with pets or in areas with high pollen, this can happen in as little as 30 days. In cleaner environments, maybe every 90 days. But if airflow drops noticeably, it’s time to check.

Sign #2: Your Energy Bills Are Rising

Your AC works harder when the filter is clogged. It has to push air through a blocked path, which means the motor runs longer and uses more electricity. If your bills have gone up over the last few months without changes in usage or weather, a dirty filter is likely why. In fact, the U.S. Department of Energy says a clogged filter can increase your cooling costs by up to 15%. That’s not small change over a British summer.

Sign #3: Dust Is Accumulating Faster

Notice more dust settling on your furniture, shelves, or TV screen than usual? That’s not just bad luck. A dirty filter can’t trap particles anymore. Instead, it lets them blow right back into your rooms. You might even see a greyish film around vents or on window sills. If you’re wiping down surfaces more often than normal, your filter is probably failing at its job.

Sign #4: You Smell Something Off

When filters get soaked with dirt, moisture, or mold, they start to smell. You might notice a musty, stale, or even slightly burnt odor when the AC kicks on. This isn’t just unpleasant-it’s a health risk. Mold spores and trapped allergens circulate through your home, which can trigger coughing, sneezing, or worsen asthma. If you smell something odd coming from your vents, don’t ignore it. Replace the filter immediately and clean the vents if you can.

Sign #5: The AC Freezes Up or Shuts Off

This one’s serious. If your AC unit starts icing over or shuts off before reaching the temperature you set, it’s likely because airflow is so restricted that the evaporator coil gets too cold. When that happens, moisture freezes on the coil, and the system can’t cool properly. In extreme cases, it may overheat and shut down completely. This isn’t just inconvenient-it can damage the compressor, which costs hundreds to repair. A simple filter change could have prevented it.

How Often Should You Really Change It?

There’s no one-size-fits-all answer. Manufacturers often say every 90 days, but that’s a baseline. If you live in a dusty area, have pets, smoke indoors, or have allergies, you should check your filter every 30 to 45 days. In Bristol, with its damp climate and frequent rain bringing in pollen and mold spores, many homeowners swap filters every two months. Keep a note on your fridge or set a phone reminder. Some filters even have a date printed on them-use it.

What Happens If You Don’t Change It?

Ignoring a dirty filter doesn’t just make your home stuffier. It strains the whole system. The motor wears out faster. The ductwork gets coated with grime. Indoor air quality plummets. Over time, you’re looking at premature AC failure. Replacing a filter costs £5 to £20. Replacing a compressor? £800 or more. It’s not even a close call.

How to Check Your Filter

Locating your filter is easier than you think. It’s usually in the return air duct near the furnace or air handler, or inside the blower compartment. Some homes have filters in the wall or ceiling vents. Take it out. Hold it up to the light. If you can’t see through it, it’s dirty. If it’s grey, brown, or covered in clumps of hair and debris, replace it. If it’s still mostly white or light grey, you’re good for now.

Types of Filters and What to Buy

Not all filters are the same. The most common types are:

• Disposable fiberglass - cheap, basic, good for dust. Replace every 30 days.
• Pleated polyester - better at trapping small particles. Lasts 60-90 days. Recommended for allergy sufferers.
• Electrostatic - self-charging, reusable. Washable, but needs careful cleaning. Good for long-term use.
• HEPA - medical-grade. Only works with high-end systems. Can restrict airflow if your unit isn’t designed for it.

Check your old filter for the size printed on the frame. Common sizes are 20x25x1 or 16x25x1. Buy the same size and type unless you have specific health needs.

Pro Tip: Keep a Log

Write down the date you change your filter. Use a notebook, a sticky note on the unit, or a note in your phone. After a few months, you’ll start to see patterns. If you change it every 45 days and it’s still clean, you can stretch it. If it’s dirty in 30, don’t wait. Your system-and your lungs-will thank you.

What About Washable Filters?

Washable filters sound great-save money, eco-friendly. But they’re only as good as how often you clean them. If you don’t dry them completely before putting them back, you risk mold growth. And if you wait too long between washes, they become just as clogged as disposable ones. For most households, disposable pleated filters are simpler and more effective.

Final Thought: It’s Not a Luxury-It’s Maintenance

Your AC filter is one of the easiest, cheapest things to maintain. Yet it’s one of the most ignored. A clean filter means better air, lower bills, fewer repairs, and a longer-lasting system. You don’t need to be a technician to do this. Just check it once a month. If it looks dirty, swap it out. That’s it.