How often should you change your HVAC filter?

What moves the number

Four things, roughly in order of how much they matter.

• How thick the filter is. A one inch filter has very little surface area, so it loads up fast. A four or five inch filter has several times the area in the same airflow, and lasts proportionally longer. The trade is that the thick ones only fit cabinets built for them.
• Pets, and how many. Hair and dander are the fastest way to fill a filter, and this is the single biggest variable between two otherwise identical houses.
• How hard the system runs. A filter loads up according to how many hours the blower has actually run, not how many days have passed, so a filter that lasts a whole mild spring can be finished in six weeks of continuous January heating.
• What is in the air. Construction nearby, an unpaved road, wildfire smoke, or a lot of open windows all shorten the life of a filter considerably.

Thicker and denser aren't the same thing

Filters carry a MERV rating, which describes how fine the particles are that they catch. It's the number printed on the packaging. Finer sounds better, and up to a point it is, but a very fine filter is also more resistant to airflow, and a residential blower is only built to pull so hard against it. A filter that's too restrictive for the system can starve it of air, which shows up as poor cooling, longer run times and, in the worst case, a frozen coil.

The size printed on the side of the old filter is the size to buy. Anything beyond that, and particularly a move to a much finer rating, is worth asking the person who services the system about, because the answer depends on the blower rather than on the filter.

The signs one is past it

What a clogged filter costs

A filter isn't there to clean your air first and foremost. It's there to keep the equipment clean, and it does that by standing between the return air and the coil.

When it clogs, airflow drops. The system works harder for the same result, so it runs longer and costs more. In cooling weather, low airflow across the coil can drop it below freezing, and a coil in a jacket of ice moves no heat at all until it thaws. The blower motor wears faster too. None of it announces itself. It just turns up spread across a power bill.

What to leave alone

Changing a filter is a filter job: the slot is designed to be reached without tools, and the arrow on the frame points the way air travels. Anything past that slot is a different category of work.

Look monthly, change when it's gray

Filters are the easiest maintenance task in a house to describe and the easiest to forget, because nothing bad happens on the day you miss one. The cost arrives months later, spread across a power bill, and never announces what caused it.

Three months is the interval Patina starts you on. If yours turns out to be a one month house, change it once and the app counts in ones from then on, forward from the day you actually swapped the filter.