Air purifier filter replacement: how often, and the signs it is due

Air purifier filter replacement is the one job that decides whether a purifier is cleaning your air or just moving it around. The machine keeps humming and the light stays blue long after the filter has done its work, so the date slips by without anyone noticing.

October is the right month to deal with it. The heating comes on, the windows stay shut, and whatever is in the air indoors stays there. Ragweed pollen in many regions lasts until the first frost, fallen leaves release mould spores, and a purifier that ran hard through a summer of smoke or pollen is often already at the end of its filter.

The three filters inside most purifiers

Most purifiers stack several filters, and each wears out on its own clock.

• Pre-filter. A coarse mesh on the outside that catches hair, lint and dust. It protects everything behind it and is usually meant to be cleaned, not replaced.
• True HEPA filter. A pleated fibre mat that traps fine particles: to meet the US Department of Energy standard it has to capture 99.97 percent of particles at 0.3 microns. It cannot be washed, and it fills up gradually.
• Activated carbon filter. A layer of carbon granules or a carbon coated sheet that absorbs odours, smoke smell and some gases. It does not clog visibly, it saturates, and once it is full it simply stops working.

Many purifiers combine HEPA and carbon in one cartridge. In that case the cartridge is due when the first of the two is spent, which is usually the carbon.

How often to change an air purifier filter

Normal use means a few hours a day in a home without pets or smokers. Heavy use means running around the clock, pets, a smoker in the home, a busy road outside, or wildfire smoke. Your manual has the figure for your model; when in doubt, follow it.

When air purifier filter replacement is due: the signs

The calendar gets you close. These tell you the filter is finished whatever the date says.

• The intake side has turned grey. A new HEPA filter is white. When the pleats facing the room are uniformly grey or brown, it is loaded.
• Less air comes out. Hold a hand over the outlet on the same fan speed you always use. A clogged filter means noticeably weaker flow.
• The fan sounds louder. The motor works against more resistance, and in auto mode the purifier stays on its higher speeds for longer.
• Smells come back. Cooking smells or pet odour lingering again is the carbon layer telling you it is saturated.
• The air quality reading stays high. If the built in sensor shows poor air for hours in a room that used to clear in minutes, the filter is the first suspect.

A purifier with a spent filter still makes all the right noises. It just stops making the air any cleaner.

How to clean an air purifier filter, and when you should not

Clean the pre-filter every few weeks: vacuum it with a brush attachment, or rinse it under the tap if the manual allows, and let it dry completely before it goes back in. A damp pre-filter grows mould, which is the opposite of what you bought the purifier for.

Do not wash or vacuum a True HEPA filter unless the manufacturer explicitly calls it washable or permanent. The suction or the water deforms the fibre mat and opens gaps that particles pass straight through. It looks cleaner and filters worse. Carbon cannot be cleaned either, because what makes it work is absorbed into the carbon itself.

Replacing the filter in five minutes

1. Switch the purifier off and unplug it.
2. Open the cover and lift the old filter out, noting which way the pull tab or the airflow arrow faces.
3. Vacuum the dust out of the housing and wipe the cover.
4. Take the new filter out of its plastic bag. A filter installed still in its wrapping is a common mistake, and the purifier will run without complaint.
5. Fit it the same way round as the old one, close the cover and reset the filter indicator.

Buy a filter made for your exact model, genuine or a compatible one that fits and seals properly. A gap around the edge lets air bypass the filter entirely. If a compatible filter is sold as HEPA type or HEPA style rather than True HEPA, it is not held to the 99.97 percent standard.

Keeping the routine

The hard part is not the swap, it is remembering it six or twelve months later. Write the date on the filter frame with a marker when you install it, keep a spare in the cupboard, and put the next date somewhere that will tell you.

Freshy can do the remembering. Add the purifier as its own item under the HVAC filter category, name it after the room, and set a custom cycle of 365 days for the HEPA filter, or 180 days if it runs around the clock (custom cycles are part of Freshy Plus). Track a separate carbon filter as a second item on the default 90 day cycle. You get a reminder with your chosen lead time, early enough to order the filter before the old one is spent. The central heating filter is covered in the HVAC filter guide, and everything else in the complete schedule.

Questions

How often should you change an air purifier filter?

Most True HEPA filters need replacing every 6 to 12 months and carbon filters every 3 to 6 months. Plan the air purifier filter replacement sooner if the purifier runs around the clock, you have pets, or it ran through wildfire smoke.

Can you clean an air purifier filter instead of replacing it?

Only the pre-filter, and any filter the manufacturer labels as washable or permanent. Vacuuming or rinsing a True HEPA filter damages the fibre mat, so it filters less while looking cleaner.

What happens if you never replace the filter?

Airflow falls, so the purifier cleans far less air per hour while the fan gets louder and uses more power. A saturated carbon layer also stops removing odours.

Does the filter light mean the filter is dirty?

On many purifiers the light is a timer that counts running hours, not a sensor. Treat it as a reminder to check the filter, and reset it after every air purifier filter replacement.