HVAC Air Filters in UK Homes: Ratings, Sizes and When to Change

Most UK homes never think about air filters until something whistles, smells musty or stops cooling. But if you have air conditioning, an air-to-air heat pump, mechanical ventilation with heat recovery (MVHR) or an older warm-air heating system, a filter is quietly doing a lot of work. This guide explains how filter ratings compare, how to read the size printed on the frame, how often to change or clean filters, and when a washable filter makes sense.

Which systems in a UK home use air filters?

• Split air conditioners and air-to-air heat pumps: the indoor wall unit has fine mesh filters behind the front panel that catch dust before it reaches the coil.
• MVHR: common in new-build and energy-efficient homes. The unit (often in a loft or utility cupboard) has a filter on the fresh-air intake and another on the stale-air extract, and some kitchen extract valves hide a small filter cone of their own.
• Warm-air heating: still found in some older UK houses and flats, with a filter in the air-handling cabinet.
• US-style furnaces and central air: these use standard-size framed filters sold by nominal inch sizes. Many of the framed filters in our furnace filters range are this type.

If you have an air-to-water heat pump that feeds radiators, there is usually no household air filter to change; follow your installer's service schedule instead.

Filter ratings: MERV vs ISO 16890 (ePM)

You will see two main rating systems, depending on where a filter was made to be sold.

MERV (North American)

According to the US Environmental Protection Agency, MERV (Minimum Efficiency Reporting Value) comes from an ASHRAE test method and reports a filter's ability to capture particles between 0.3 and 10 microns, on a scale from 1 to 16. The higher the number, the better the filter is at trapping particles of those sizes. Some US brands also use their own scales alongside MERV, so compare like with like.

ISO 16890 (UK and Europe)

ISO 16890 replaced the older EN 779 grades (G4, M5, F7 and so on) that you may still see on MVHR filters. It groups filters by how much of each airborne particle fraction they remove:

• ISO ePM1: particles of 1 micron and smaller (the finest group).
• ISO ePM2.5: particles up to 2.5 microns.
• ISO ePM10: particles up to 10 microns.
• ISO Coarse: filters that capture less than 50% of the ePM10 fraction.

To be placed in an ePM group, a filter has to capture at least 50% of that fraction, and the result is reported in 5% steps, for example "ISO ePM1 55%". The European heating and ventilation federation REHVA points out that there is no direct conversion between the old EN 779 classes and ISO 16890, because the test methods are too different. The same caution applies to MERV: treat any MERV-to-ePM chart as a rough guide only.

Higher is not always better

A denser filter traps more fine dust but also resists airflow more. The EPA suggests choosing as high a rating as your system's fan and filter slot can accommodate, and asking an HVAC technician if you are unsure. For MVHR, stick to the grades your unit's manual specifies.

How to read and measure filter sizes

Framed filters are usually labelled with a nominal size in inches, such as 16 x 25 x 1 (width x height x depth). This is a rounded name, not the exact measurement. The actual size of the frame is typically a little smaller so it slides into the slot easily; a "1-inch" filter is often closer to three-quarters of an inch deep.

1. Take out the old filter and read the size printed on the frame. Note both the nominal and actual sizes if both are shown.
2. If nothing is printed, measure the frame with a tape measure in millimetres: width, height, then depth.
3. To convert, divide millimetres by 25.4 to get inches. A nominal 16 x 25 x 1 filter is about 406 x 635 x 25 mm as named, with the real frame slightly smaller.
4. Match the actual dimensions on the product listing to your old filter, not just the headline size. Brands do not always round the same way.
5. Check the airflow arrow on the frame and fit it the same way round as the old one.

For MVHR and warm-air units, filters are often specific to the make and model. The safest approach is to match the model number in your manual, or the part number on the old filter, rather than going by size alone.

How often to change or clean filters

• Framed pleated furnace filters: filter makers commonly suggest replacing 1-inch filters about every three months or sooner, and deeper 4-inch filters about once a year or sooner. Pets, building work and heavy use shorten this.
• MVHR: the Passivhaus Trust's MVHR maintenance good practice guide says filters should typically be checked, cleaned or changed about every 6 months, and replaced with new ones at least once a year. When the filter material is fully covered in dust, it is time to act. Many units show a filter timer message; reset it after fitting new filters.
• Split air conditioners: Daikin's operation manual recommends cleaning the indoor unit's air filters every two weeks during use, with water or a vacuum cleaner, then drying them in the shade without wringing them out. Some specialist filters in the same units must not be washed, so read your own manual.

Signs a filter is overdue include weaker airflow, a louder fan, dust building up around vents, musty smells and, with MVHR, higher running costs. The Passivhaus Trust notes that clogged filters make constant-airflow MVHR fans speed up, and that poor-quality or non-genuine filters can raise resistance, energy use and noise.

Pleated vs washable filters

Pleated disposable filters

Pleated filters pack a large surface of filter material into a thin frame, which helps them capture finer particles for longer. They are simple to swap and are the usual choice for furnaces and MVHR units. Pleated filters are directional, so fit them the right way round.

Washable and reusable filters

The mesh filters in split air conditioners are designed to be rinsed and reused. Washable framed filters for furnaces also exist, but they are usually rated lower than pleated ones and must be completely dry before refitting, or you risk damp and mould in the system. If your manual specifies a disposable filter, do not substitute a washable one.

Plug-in extras: purifiers and portable units

If you are adding a portable air conditioner, dehumidifier or air purifier for a single room, check the rating plate says 100–240V (or 230V) and that it comes with, or is safe to use with, a UK plug. Never plug a 110–120V-only appliance into a UK socket, even with an adapter. Our air purifier buying guide covers room sizes and HEPA filters in more detail, and replacement cartridges are in air purifier filters.

Quick checklist

• Identify your system: split AC, MVHR, warm-air or US-style furnace.
• Read the old filter's nominal and actual size, or measure it in millimetres.
• Match the rating to what the manual specifies; do not assume higher is better.
• Note the airflow arrow before removing the old filter.
• Set a reminder: every 2 weeks for split AC mesh filters in use, about 6 to 12 months for MVHR, about every 3 months for 1-inch pleated filters.
• Browse the full HVAC range, including Filtrete framed filters, and book a professional service for anything beyond a filter swap.

Frequently asked questions

Is MERV the same as ISO 16890?

No. MERV is a North American rating from an ASHRAE test, while ISO 16890 groups filters into ePM1, ePM2.5, ePM10 and Coarse. The test methods differ, so there is no exact one-to-one conversion.

What does ISO ePM1 55% mean?

It means the filter captured 55% of the airborne particle fraction of 1 micron and smaller in testing. Results are reported in 5% steps, and a filter must reach at least 50% to be placed in an ePM group.

Why is my 16 x 25 x 1 filter not exactly that size?

That is the nominal size, a rounded name. The actual frame is usually slightly smaller so it slides in easily, and a 1-inch filter is often closer to three-quarters of an inch deep, so match the actual dimensions.

How often should MVHR filters be changed?

The Passivhaus Trust's good practice guide says to check, clean or change them typically every 6 months and to fit new filters at least once a year, or sooner if the filter material is covered in dust.

Can I wash a pleated furnace filter?

Disposable pleated filters are not designed to be washed. Only wash filters your manual says are washable, such as split air conditioner mesh filters, and let them dry completely before refitting.