If you run a mechanical ventilation with heat recovery (MVHR) system, the filters are the one part you will replace again and again — and the grade printed on them (G4, M5, F7…) decides how well your system cleans the air coming into your home and how long the unit lasts. The catch is that the grading system changed a few years ago, so you will see two different sets of ratings on filters and in manuals, and they do not line up neatly. This guide clears it up: what the grades mean, how the old EN 779 ratings map to the current ISO 16890 standard, and exactly which grade belongs on the supply and extract sides of your unit.
Why filter grade matters
An MVHR unit draws fresh air in from outside (the supply side) and pushes stale air out from inside (the extract side), passing both through a heat exchanger so the outgoing air warms the incoming air. A filter sits on each airflow, and they do two different jobs:
- The supply filter protects your indoor air quality. It is the finer of the two, catching pollen, traffic pollution and fine particulate before it reaches your rooms.
- The extract filter mainly protects the unit. It is coarser, stopping household dust from reaching and clogging the heat exchanger and fans.
Get the grade wrong, or leave old filters in too long, and you pay for it twice: dirtier air, and a unit that has to work harder. A blocked filter throttles airflow, drags down heat-recovery efficiency and pushes up running costs — we cover that side of things in MVHR Efficiency Explained and MVHR System Running Costs.
The standard changed: EN 779 to ISO 16890
Most of the confusion comes from the fact that two grading systems are still in circulation.
- EN 779:2012 — the older European standard, using grades like G3, G4, M5, M6, F7, F8 and F9.
- ISO 16890 — the current international standard, in general use across the UK and Europe since 2018. Instead of one lab test, it grades filters by the particle sizes they actually capture: Coarse, ePM10, ePM2.5 and ePM1, each with a percentage efficiency.
ISO 16890 is the more useful system because those particle sizes — PM1, PM2.5 and PM10 — are the ones that matter for health and for what your filter is really being asked to do. During the changeover, many genuine filters are labelled with both ratings, which is why you will often see something like “F7 / ePM1 60%” on the same filter.
Approximate grade equivalents
The two standards test differently, so there is no exact one-to-one conversion — anyone offering a precise swap is oversimplifying. These are the widely used approximate equivalents, which are good enough to translate an old spec into a new one:
| EN 779 | ISO 16890 (approx.) | Typically used for |
|---|---|---|
| G3 / G4 | Coarse | Extract filters; pre-filtration; protecting the heat exchanger |
| M5 | ePM10 ~50–65% | Extract, or basic supply filtration |
| M6 | ePM2.5 ~50–65% | Improved supply filtration |
| F7 | ePM1 ~50–65% | Standard supply filter in most domestic MVHR units |
| F8 | ePM1 ~70–80% | Upgraded supply filter for pollen, allergies and urban pollution |
| F9 | ePM1 ~80%+ | High-grade supply filtration |
Use these as a guide, not gospel. If in doubt, go by your unit's manual or the genuine part number rather than the grade alone.
Which grade do I need — supply vs extract?
For the large majority of domestic MVHR units, the standard pairing is:
- Supply (incoming air): F7 / ePM1. Fine enough to capture pollen and most traffic pollution before it reaches your rooms.
- Extract (outgoing air): G3–G4 / Coarse, or M5 / ePM10. Coarser, because its job is simply to stop household dust reaching the heat exchanger.
That is exactly how a genuine “standard” filter kit is specified: a finer supply filter paired with a coarser extract filter, not two of the same. If you suffer badly from hay fever, or live on a busy road, an upgraded F8 supply filter — where your unit offers one — captures more of the finest particles. Systemair, for example, lists higher-grade “optional” kits alongside the standard kit for several of its SAVE units.
How to read your filter and find the right part
Three things tell you what you need, in order of reliability:
- The genuine part number. The most reliable of all — it specifies the exact filter, so you cannot get the grade or dimensions wrong. It is usually printed on the old filter's cardboard frame or listed in the manual.
- The unit model. If you know your exact unit (for example a Systemair SAVE VTR 300, or a Topvex FC15), the correct filter follows from that.
- The grade and size. A useful cross-check, but on its own the easiest to get wrong, because two filters of the same grade can be different sizes.
If all you have is the grade printed on a grubby old filter, note the dimensions too and check them against the listing before ordering.
How often should MVHR filters be changed?
As a rule of thumb, replace MVHR filters every 6 to 12 months. Change them sooner if you are in a high-pollen area, near heavy traffic or a building site, or while renovation work is going on. A quick visual check every three months is good practice — if the supply filter is visibly grey and loaded, it is due. Filters are a maintenance item, not fit-and-forget; see our ventilation system maintenance guide for where they sit in the wider service routine.
Genuine vs “compatible” filters
Genuine filters are made to the exact dimensions, media and grade the manufacturer specifies, so they seat properly and hit the rated efficiency. “Compatible” alternatives can shave a little off the price but vary in fit and media quality — and a filter that does not seal correctly lets unfiltered air bypass it altogether. At eFans we supply genuine Systemair, Vent-Axia and other manufacturer filters, sourced from the same supply chain as the manufacturers themselves — the real part, at trade-friendly prices.
Find the right filter for your unit
The surest way to order correctly is by your unit model and the genuine part number. Browse our ranges:
- MVHR & ventilation replacement filters
- Systemair Topvex filters
- Systemair Geniox filters
- Air curtain filters
Not sure which you need? Send us your unit model or the old part number and we will confirm the correct filter before you order.
Related guides
- MVHR Efficiency Explained
- Ventilation System Maintenance: Best Practices for Healthy Air
- MVHR System Running Costs: 2026 Guide
- What Is MVHR? Pros & Cons Explained
Frequently asked questions
Is F7 better than M5?
For supply air, yes — F7 (ISO ePM1) captures finer particles than M5 (ePM10). That is why F7 is the usual supply grade while M5 or Coarse is used on the extract side.
What does ePM1 mean?
It is the ISO 16890 measure of how effectively a filter captures particles up to 1 micron — the finest, most health-relevant fraction, including much traffic pollution. A higher ePM1 percentage means finer filtration.
Can I vacuum and reuse my filters?
No. MVHR filters are designed to be replaced, not cleaned. Vacuuming damages the media and destroys the rated efficiency.
Do I need to match the original grade?
Match or improve the supply grade (for example F7 to F8), but do not drop below the manufacturer's specification. Keep the extract filter at the specified coarser grade to protect the heat exchanger.
Are higher-grade filters always better?
Not always. A higher supply grade captures finer particles but also adds airflow resistance, so only go above your unit's specification if you need to and the unit is rated for it — and never downgrade the extract filter to compensate.