How Often to Change Your Furnace / HVAC Filter

If we could get every homeowner in Bartholomew County to do one thing for their house, it might be this: change your filter on time. It's a five-dollar part and a two-minute job, and skipping it is behind a shocking number of the expensive "my furnace died" and "my A/C can't keep up" calls we see. A neglected filter is the slow leak that sinks the whole ship.

Let's make sure you never get caught by it — and that means understanding what the filter is actually for, because most people get that part wrong.

What your filter actually does (it's not what you think)

Almost everyone assumes the furnace filter is there to clean the air you breathe. It helps a little — but that's not its main job. The filter's real job is to protect the equipment. It sits where your system pulls air in, and it catches the dust, hair, and grit that would otherwise coat the inside of your furnace and air conditioner — the blower, the sensitive coils, all of it.

Here's the chain of events when a filter gets clogged, because this is the part that costs people money:

• A dirty filter chokes off airflow. Your system is trying to breathe through a clogged mask.
• In winter, that trapped heat can make the furnace overheat and shut itself down on a safety switch — usually on the coldest night, because that's when it's running hardest.
• In summer, the restricted airflow can make the A/C coil freeze into a block of ice, so it stops cooling entirely and can damage the compressor — the single most expensive part in the system.
• Year-round, the system works harder and runs longer to do the same job, which means a higher energy bill and a shorter equipment life.

So how often, really?

You'll see "every 90 days" on the package, but that's a starting point, not a rule. The real answer depends on your filter and your house. Here's how we'd guide you:

Filter type	Change about every…	Good for
1-inch fiberglass (the cheap blue ones)	30 days	Protects equipment only; minimal air cleaning
1-inch pleated	60–90 days	The everyday sweet spot for most homes
4–5 inch media filter (in a big cabinet)	6–12 months	High-efficiency systems; less frequent swaps

Then adjust shorter if any of these apply to your home:

• Pets — fur and dander load a filter up fast. Check monthly.
• Allergies or asthma in the house — change more often and consider a better-rated filter.
• Smokers, or a recent remodel kicking up dust.
• Peak season — during the hardest months of heating (deep winter) and cooling (humid summer), your system runs nonstop, so the filter loads faster. Check it monthly in those stretches even if you'd normally wait.

How to change it — the right way

• Find the filter. It lives in a slot on the return-air side of your furnace/air handler, or behind a return-air grille on a wall or ceiling. It slides into a thin slot — you usually don't need any tools.
• Read the size off the old one before you toss it. It's printed right on the cardboard frame — something like 16x25x1. Buy that exact size. (Snap a photo so you've always got it at the store.)
• Note the airflow arrow. Every filter has an arrow printed on the frame. It must point toward the furnace/blower — the direction the air is moving. Putting it in backwards hurts performance. Match the new arrow to how the old one came out.
• Slide the new one in, arrow first, and make sure it seats flat with no gaps around the edges. Air will always take the path of least resistance, so a gap means dust sneaks around the filter instead of through it.

That's the whole job. Do it standing up, takes longer to read this than to do it.

A quick word on MERV (which filter to buy)

That "MERV" number on the box rates how fine the filter is — higher catches smaller particles. It's tempting to grab the highest number thinking more is better, but there's a catch worth knowing: a too-restrictive filter can choke airflow on a system that wasn't designed for it, causing the very problems we're trying to avoid. For most homes, a MERV 8–11 pleated filter is the sweet spot — real air cleaning without strangling the system. If someone in the house has serious allergies and you're eyeing a high-MERV filter, it's worth a quick check with an HVAC tech that your system can handle it.

Know when to call a pro

Changing the filter is pure DIY. But the filter is also an early-warning gauge — if you're keeping up with it and still having trouble, the system is telling you something a filter can't fix. Call a licensed HVAC pro when:

• You change the filter and the system still can't keep up, blows weak, or short-cycles (kicks on and off constantly).
• The A/C coil keeps freezing even with a clean filter — that points to low refrigerant or a deeper airflow issue.
• You smell burning or anything gas-like, or hear new grinding/squealing.
• It's simply been more than a year since a professional tune-up. Heating and cooling equipment, like a vehicle, lasts far longer with regular service — and a tech checks safety items (like the heat exchanger for cracks) that you can't and shouldn't.

Filters are your job; refrigerant, gas, and electrical inside the unit are ours and the HVAC trade's. If your system's acting up after a fresh filter, don't keep wrestling it — let us get the right person on it before a small issue becomes a new furnace.