MERV, and choosing a filter.

MERV stands for Minimum Efficiency Reporting Value. It is a scale, roughly 1 to 16 for residential filters, describing how small a particle the filter reliably catches. Higher numbers catch more.

That sounds like a simple case of buying the highest number available. It is not.

Why the highest number can hurt

A filter that catches more also resists airflow more. Residential systems are designed around a fairly narrow range of static pressure, and a very dense filter in a system that was not designed for it starves the blower.

Starved airflow across the evaporator coil is one of the two things that freezes a coil, and a frozen coil risks the compressor. It also makes the system work harder for less cooling, all season.

So the filter that protects your air the most can be the one that damages your equipment.

A reasonable default

For most Valley houses, something in the middle of the range is the sensible trade: enough filtration to protect the coil and the blower wheel, not so much that it chokes them. If somebody in the house has genuine respiratory needs, the right answer is usually a thicker media filter in a properly sized cabinet rather than a dense one inch filter jammed into an undersized slot.

What matters more than the number: the filter being the right size, seated properly with no air bypassing it, and actually being changed.

How often, here

More often than the box says. Those intervals assume a system that runs part of the year. A Phoenix system runs close to flat out for five months, and a dust storm will load a filter in a day.

Check it monthly through summer. If you cannot see light through it, it has been restricting the system for a while already.