How Many Filters Does an AC Unit Have? A Simple Breakdown for Homeowners

Most people think their air conditioner just blows cool air - but what’s actually happening inside the unit matters a lot. If you’ve ever noticed dust building up on your vents or your energy bill creeping up, the issue might not be the compressor. It’s probably the filters. And yes, the number of filters in your AC unit isn’t always one. It depends on your system, your home, and even how old your unit is.

One Filter? Two Filters? It Depends

The short answer: most standard central air conditioning systems have one main air filter. That’s the filter you see in the return air duct, usually near the furnace or air handler. But some setups have two - and that’s not a mistake.

If your home has a multi-stage HVAC system - like one with a separate air handler and a condenser unit - you might find a second filter right at the return grill inside your living space. This is common in homes built after 2010, especially in newer builds in Manchester and other UK cities where air quality standards are tighter.

Some systems even have a filter in the supply duct, right before the air leaves the unit. This isn’t standard, but it happens in homes with allergy-sensitive occupants or in areas with high pollen or construction dust. These extra filters aren’t always easy to spot. They’re often hidden behind panels or inside the air handler casing.

What Do These Filters Actually Do?

Think of filters as the first line of defence. They trap dust, pet hair, pollen, and even tiny particles of mould before they get sucked into your AC’s delicate parts. If those particles make it past the filter, they can clog the evaporator coil, slow down airflow, and force the motor to work harder. That’s how you end up with higher bills and a broken unit before its time.

A dirty filter doesn’t just make your AC inefficient. It can also make your indoor air worse. Studies from the UK’s Health and Safety Executive show that homes with clogged HVAC filters have 30% higher levels of airborne allergens. That’s not just about sneezing - it’s about long-term respiratory health.

Types of Filters You’ll Find

Not all filters are the same. The type you have tells you how many you might need and how often to replace them.

• fiberglass filters - These are the cheapest, usually $1-$5. They’re thin, white, and meant to catch big stuff like dust bunnies. They’re not great for allergens. Most homes with basic systems use one of these.
• pleated filters - These look like accordion folds. They’re more dense, catch smaller particles, and last longer. You’ll see these in homes with better air quality goals. They’re usually MERV 8-13. If you have pets or someone with asthma, this is the filter you want.
• electronic air cleaners - These aren’t replaceable filters. They use charged plates to pull particles out of the air. They’re common in high-end homes and need cleaning every month. If you have one, you still need a pre-filter - so that’s two layers.
• HEPA filters - These are rare in standard AC units. They’re too dense for most home systems to push air through. If you have a HEPA setup, it’s likely a standalone air purifier, not part of the central AC.

Why Two Filters Might Be Better (and Sometimes Worse)

Having two filters sounds like a good idea - more protection, right? But it’s not always better.

If both filters are too dense (like two MERV 13 filters), your blower motor can’t move enough air. That causes the system to overheat, freeze up, or cycle on and off too often. In fact, manufacturers like Carrier and Lennox warn against stacking filters unless the system is designed for it.

Here’s what usually works: one pleated filter at the return duct (the main one), and a simple fiberglass pre-filter at the grille. The pre-filter catches hair and lint before it hits the real filter. This setup extends the life of your main filter and keeps the system running smooth.

But if you’re not sure what you’ve got, don’t guess. Pull out the filter. If there’s another one behind it - great. If not, don’t add one. Over-filtering can cost you more in repairs than it saves in energy.

How Often Should You Change Them?

Most people think they only need to change filters once a year. That’s outdated advice.

• Fiberglass filters: Change every 30 days - they clog fast.
• Pleated filters (MERV 8-11): Every 90 days. If you have pets or live near a busy road, check them every 60.
• Pleated filters (MERV 12-13): Every 60-90 days. These are common in modern homes.
• Reusable filters: Wash monthly. Let them dry completely before putting them back.

Here’s a trick I’ve seen work in Manchester homes: write the install date on the filter with a marker. That way, when you walk past it, you’ll remember. No need for apps or reminders - just a Sharpie and a little discipline.

What Happens If You Ignore Filters?

Let’s say you don’t change your filter for a year. What’s the real cost?

• Your energy bill goes up 15-20% because the system works harder.
• The evaporator coil freezes over - and when it thaws, water leaks into your ceiling.
• Dust builds up inside the blower motor, and it starts making grinding noises.
• Eventually, the motor burns out. Replacing it costs £500-£800. A new filter? £10.

One homeowner in Salford replaced her AC motor after a year of ignoring filters. She didn’t know the unit had two filters - one was completely packed with cat hair. The second was so clogged, the technician had to pry it out with a screwdriver.

How to Find All Your Filters

Not sure where your filters are? Here’s how to check:

1. Look at your return air vent - usually a large grille on a wall or ceiling. Remove the cover. Is there a filter behind it?
2. Check your furnace or air handler. It’s often in a closet, basement, or attic. The filter slot is usually on the side or bottom.
3. If you have a ducted system, look for a rectangular panel near the blower. It might be held by screws or clips.
4. If you still can’t find it, turn off the AC, open the access panel on the air handler, and look for a frame with a mesh or cardboard filter inside.

Most homes have one. Some have two. Rarely more. If you find three, you’ve probably got a custom setup - and you should call a pro to make sure it’s designed right.

Final Tip: Don’t Assume Your System Is Standard

Every home is different. A 1980s terraced house in Manchester might have one thin filter. A new build in Altrincham might have two pleated filters and a smart sensor that alerts you when it’s time to change them.

The best way to know? Look. Open the panel. Check the size (it’s printed on the frame). Note the MERV rating. Then, set a reminder. Even if you think your system is fine, changing filters regularly is the cheapest maintenance you’ll ever do.

One filter. Maybe two. But never none. Your AC, your wallet, and your lungs will thank you.