Typical replacement intervals
There is no single correct interval — it depends on the filter type, how many hours a day you run the machine, and how dirty your air is. The table below gives realistic ranges rather than a false precise figure; treat them as a starting point and adjust to what you actually see.
| Filter type | Replace every | Notes |
|---|---|---|
| MERV-13 media (16×25 / 20×25) | 6–12 months | Sooner after smoke or building work |
| True HEPA (H11–H13) | 6–12 months | Cannot be washed |
| Activated carbon | 3–6 months | For odours/VOCs; saturates faster |
| Pre-filter / mesh guard | Vacuum monthly | Extends the life of the main filter |
What actually shortens filter life
- Wildfire smoke & bonfires — a heavy smoke episode can load a filter in days.
- Pets — hair and dander clog media faster; a pre-filter or mesh guard helps.
- Running 24/7 — more air cleaned means more particles trapped (a good thing) but a shorter filter life.
- Building & decorating dust — fine plaster and sanding dust is brutal on filters.
Carbon is the exception to the “particles clog it” rule: it saturates by adsorbing gases, so a home with strong cooking or smoke odours will exhaust the carbon well before the particle media looks dirty. That is why activated-carbon filters need their own, shorter schedule.
How to tell it’s time (without guessing)
- Airflow has dropped noticeably at the same fan speed.
- The media is grey and evenly loaded, or smells musty.
- A dust or pollen flare-up isn’t clearing like it used to.
- It’s simply been 6–12 months — put a reminder in your calendar when you fit a new one.
Don’t rely only on the indicator light
Most purifiers estimate filter life from runtime, not from the actual state of the filter, so the countdown can be optimistic in a dirty environment and pessimistic in a clean one. Use it as a prompt, then check the media yourself. And resist the temptation to wash a tired filter to squeeze out more life — washing wrecks HEPA and MERV media, as we explain in can you wash air-purifier filters. A washed filter that looks clean can barely work.
Genuine vs compatible, and grade
How long a filter lasts also depends on its quality. A thin generic may clog or sag sooner than genuine media of the stated grade. When you rebuy, check you are getting a real MERV number or true HEPA (H13) rather than unrated “HEPA-type” material — our HEPA H13 guide explains the difference.
The cheapest way to stay on schedule
Because a MERV-13 pair is only around £58–£75 and lasts up to a year, staying current costs pennies a day. Buy a spare pair with your replacement so you are never running a clogged filter while waiting on delivery — and because standard sizes never go out of stock the way proprietary cartridges do, that spare will always fit. Our replacement filter guide sets out the annual maths.
Seasonal timing in the UK
It helps to line changes up with the UK year. Fitting a fresh filter before the spring and summer pollen seasons, and again before autumn brings closed windows and more indoor time, keeps the machine at its best when you most need it. If you have been through a bonfire-night spell, a heatwave with wildfire haze, or any decorating, treat those as reset points regardless of the calendar — a heavy load can use up months of filter life in a matter of days, so check the media afterwards rather than waiting for the reminder.