The Home Air Filter: How to Choose the Right One

Choose a filter that fits your system's slot and airflow, usually MERV 8 to 11 for most homes. Higher ratings catch more but can strain a blower, so match the filter to the equipment, not just the label.

Why the filter matters more in our climate

For roughly six months of the year, Upstate New York and Vermont homes are sealed up tight with the furnace or air handler running for long stretches. Dust, pet dander, wood-stove ash and fine particles circulate again and again through the same ductwork.

The filter has two jobs: protect the equipment from dust and lint, and clean the air that passes through it. A clean filter helps with both. A clogged one does neither, and it forces the blower to work against restricted airflow.

MERV ratings in plain English

MERV measures how well a filter captures particles, on a scale of 1 to 16 for home use. Flat fiberglass filters sit at the bottom and mainly protect equipment. Pleated filters in the MERV 8 to 11 range are a good fit for most homes, catching dust, pollen and much of the pet dander.

Higher-MERV filters trap finer particles but are denser, which can restrict airflow. Many residential systems were not designed for the highest ratings. If you want to go above MERV 11, ask a technician whether your blower and ducts can handle it.

Get the size and the slot right

Filters are labeled with a nominal size, such as 16x25x1 or 20x20x4. Read the size printed on the edge of your current filter, and check the arrow that shows airflow direction. It should point toward the furnace or air handler, not toward the return grille.

Gaps around a filter let dirty air bypass it, so it should fit snugly. Some homes have a thicker media cabinet that holds a 4 or 5 inch filter, which lasts longer and restricts airflow less than a thin one.

If you are unsure, take a photo of the old filter and its label to the store. Washable filters exist but need to be completely dry before reinstalling, and they often have a lower rating than pleated ones.

Pets, wood stoves and construction dust

• Pets: choose a pleated filter and check it monthly, since hair and dander load it quickly.
• Wood or pellet stoves: ash and fine soot add to the load, so inspect more often.
• Renovation: sanding and demolition clog filters in days; swap it after the job.
• Allergies: a higher-quality filter and sealed ducts help; see our post on allergy tips.

If you burn wood in a stove or fireplace, expect to see the filter turn gray faster in winter. That is normal and a good reason to check it earlier than you would in a house without one.

How often to change it

The old rule is every one to three months, and that fits many homes. In practice, look at it. Hold it up to a light: if you cannot see light through the pleats, it is time. Thicker filters can last longer, as long as you check them regularly. In the heating and cooling seasons, when the system runs most, check monthly. Write the date on the frame so you do not have to remember.

If you have both a furnace and central AC sharing the same air handler, one filter serves both, so a dirty filter hurts heating in January and cooling in July. A filter that is just a little dirty is much better than a filter pushed in backwards, so take a moment to check the arrow whenever you change it.

Ductless systems and filters

Mitsubishi ductless indoor units have their own washable filters. Pull them out and rinse them according to the manual, typically every month or two. Dirty filters reduce airflow and efficiency just like a furnace filter does. Our maintenance programs include cleaning the coils and filters of your heat pump system.

Ducted heat pump air handlers use a filter much like a furnace does. A technician can show you where it sits, and what size to buy.

When a filter is not enough

If the air still feels stale, dusty or irritating, filtration alone may not be the answer. Leaky ducts, humidity problems and ventilation gaps all play a role. Our indoor air quality services include whole-home filtration and air cleaning, and a technician can recommend what makes sense for your house.

Frequently asked questions