MVHR Filter FAQs: Choosing, Measuring and Replacing Ventilation Filters

Choosing the correct MVHR filter is important for maintaining airflow, supporting indoor air quality and helping your ventilation system operate efficiently. However, filter sizes, efficiency ratings and replacement intervals can sometimes be difficult to understand.

In this guide, we answer common questions about MVHR filters, HVAC filters and other ventilation filters, including the difference between original and replacement products, how to measure a filter correctly and what ratings such as ePM1, ePM2.5 and ePM10 mean.

What is the difference between OEM and replacement filters?

OEM stands for Original Equipment Manufacturer. An OEM filter is supplied by, or produced specifically for, the manufacturer of the ventilation or HVAC system.

For example, an official Systemair filter is designed for use with the relevant Systemair ventilation unit and is supplied according to the manufacturer’s intended dimensions, filtration specification and system requirements.

The term replacement filter has a broader meaning. It can refer to any filter purchased to replace an existing filter, including:

• An official OEM filter
• A manufacturer-approved replacement filter
• A third-party compatible filter

This means an OEM filter can also be described as a replacement filter, but not every replacement filter is an OEM product.

When choosing an MVHR replacement filter, check more than the overall dimensions. The filter should also have the correct construction, efficiency rating, airflow characteristics and sealing arrangement for the ventilation unit.

At NuFilter, we supply official filters from Systemair brands, helping customers select products designed for their compatible ventilation and air-handling systems.

How do I measure an air filter correctly?

Accurate measurements are essential when selecting an MVHR filter or ventilation filter. A filter that is too large may not fit into the unit, while one that is too small could leave gaps that allow air to bypass the filter media.

To measure a filter, remove it carefully from the ventilation unit and measure:

1. Length – Measure the longest side of the filter frame.
2. Width – Measure the shorter side of the filter frame.
3. Depth or thickness – Measure from the front of the filter to the back.

Measurements are commonly written as:

Length × Width × Depth

For example:

400 × 200 × 48 mm

Measure the actual filter frame rather than estimating the size of the opening. It is also worth checking the product label, filter reference and ventilation-unit model before ordering.

For bag filters, measure the external frame dimensions and check the number and length of the filter pockets. For filters with unusual shapes, seals or stepped frames, identifying the unit model and original product number is often more reliable than using dimensions alone.

If your existing filter has become distorted, damaged or compressed, refer to the ventilation-system manual or search using the unit model rather than relying entirely on its current measurements.

How often should HVAC filters be changed?

There is no single replacement interval that applies to every HVAC or ventilation filter. Filter life depends on factors including:

• Operating hours
• Outdoor air quality
• Local dust and pollen levels
• Nearby traffic or construction
• Filter size and dust-holding capacity
• Airflow rate
• The type of ventilation system
• The manufacturer’s maintenance guidance

Systemair recommends changing residential ventilation filters regularly and indicates that replacement one or two times per year may be appropriate for many systems. Dirty or clogged filters can restrict airflow, affect ventilation performance and increase system energy use.

However, filters may need to be inspected or changed more frequently in areas with high levels of dust, pollen, pollution or other airborne particles.

General HVAC guidance may recommend shorter replacement intervals for some domestic heating and cooling filters. The correct schedule therefore depends on the filter type and system rather than a universal timetable.

For an MVHR system, always follow the instructions supplied by the ventilation-unit manufacturer. Check the filters regularly and do not rely solely on a calendar reminder. If a filter is visibly dirty, damaged or heavily loaded, it may require replacement sooner.

Are replacement filters as good as original filters?

Some replacement filters can perform well, but quality varies between manufacturers and products.

A third-party filter may have similar dimensions to an original filter without necessarily having the same:

• Filter efficiency
• Airflow resistance
• Media area
• Dust-holding capacity
• Frame strength
• Seal design
• Manufacturing quality

An original filter has the advantage of being specified for the intended system. This can make it easier to confirm compatibility and avoid selecting a product based only on approximate dimensions.

When comparing an alternative replacement filter with an OEM product, check for independently tested performance data and confirm that its dimensions, efficiency rating and airflow characteristics are appropriate for the system.

A higher filtration rating does not automatically make a filter a better choice. A filter must work effectively within the airflow and pressure capabilities of the ventilation unit. ASHRAE advises checking whether a system can accommodate a filter upgrade and considering both fit and pressure drop.

For customers who want the filter specified for their equipment, an official manufacturer filter provides a clear and straightforward choice.

What filter efficiency rating do I need?

The correct efficiency rating depends on the ventilation system, the filter position and the level of filtration required.

For many MVHR systems, the supply-air filter is intended to reduce the amount of outdoor particulate matter entering the building. The extract-air filter may have a different rating because its role can include protecting internal components from indoor dust.

Always begin with the filter class recommended by the equipment manufacturer. Do not assume that the highest available rating is automatically the most suitable.

More efficient filtration can capture a greater proportion of smaller particles, but the filter must remain compatible with the system. Filter design can affect resistance to airflow, and an unsuitable filter may reduce airflow or place additional demand on the fan.

When selecting a ventilation filter, consider:

• The manufacturer’s specified filter rating
• The ventilation-unit model
• The required air quality
• Local outdoor conditions
• The filter’s airflow resistance
• The system’s available fan capacity

If you are uncertain, use the original filter reference or consult the equipment manufacturer or an experienced ventilation specialist.

Can I replace my filter with a different brand?

It may be possible to use a filter from a different brand, but compatibility should not be assumed simply because the external dimensions appear similar.

Before fitting another brand, check that the replacement has:

• The correct length, width and depth
• The required filter efficiency
• Appropriate airflow characteristics
• A frame and seal that fit correctly
• Suitable construction for the application

Small differences in dimensions or frame design may result in gaps around the filter. Air can take the path of least resistance through these gaps rather than passing through the filter media.

Changing to a substantially different filter class may also affect system resistance. Any proposed upgrade should be checked against the ventilation unit’s specifications.

For Systemair equipment, choosing the official Systemair filter removes much of the uncertainty around product identification and intended compatibility.

What do ePM1, ePM2.5 and ePM10 mean?

ePM ratings are used under the ISO 16890 standard to describe the efficiency of filters used for general ventilation.

ISO 16890 groups filters according to their performance in relation to different particle-size ranges. The principal classifications include:

• ISO ePM1
• ISO ePM2.5
• ISO ePM10
• ISO Coarse

The number refers to particle size in micrometres, often written as µm.

ePM10

An ePM10-rated filter is assessed for its efficiency in relation to particles up to approximately 10 micrometres in size.

This particle-size range can include larger airborne material such as:

• Coarse dust
• Pollen
• Larger airborne particles

ePM2.5

An ePM2.5-rated filter is assessed in relation to finer particles up to approximately 2.5 micrometres.

These smaller particles may include:

• Fine dust
• Combustion-related particles
• Some smoke particles

ePM1

An ePM1-rated filter is assessed in relation to very fine particles up to approximately 1 micrometre.

The percentage shown after the classification indicates the filter’s measured efficiency for that particle-size category.

For example:

ISO ePM1 60%

indicates a tested ePM1 efficiency of 60% under the ISO 16890 classification process.

The correct rating depends on the application. Avoid choosing a filter based only on the highest percentage. Compatibility with the ventilation system, airflow requirements and manufacturer specification remain important.

How can I reduce HVAC maintenance costs?

Regular filter maintenance can help reduce avoidable ventilation and HVAC servicing costs.

A heavily loaded filter can increase resistance to airflow. This may reduce ventilation performance and make the system work harder to maintain the intended airflow. Systemair notes that clogged or dirty filters can impair ventilation and increase energy consumption.

The following steps can help control maintenance costs:

1. Inspect filters regularly

Do not wait until airflow has noticeably reduced. Regular checks make it easier to identify dirty or damaged filters before they contribute to system problems.

2. Use the correct filter

A correctly specified MVHR filter can help maintain the intended balance between filtration and airflow. Avoid selecting filters solely because they are cheaper or have a higher efficiency rating.

3. Keep a replacement record

Record the installation date whenever a new filter is fitted. This can help identify how quickly filters become loaded in a particular building.

4. Replace filters when required

Leaving a filter in service after it is heavily loaded can increase airflow resistance. In larger commercial systems, filter condition is often assessed using pressure-drop measurements rather than time alone.

5. Keep air intakes clear

Inspect external grilles and air intakes for leaves, dirt or other obstructions. Restricted intake airflow may affect system operation.

6. Follow the manufacturer’s maintenance schedule

Filter replacement is only one part of ventilation maintenance. Follow the unit manufacturer’s recommendations for cleaning, inspection and servicing.

7. Order the correct product first time

Check the ventilation-unit model, filter reference and dimensions before ordering. Correct identification can reduce returns, delays and unnecessary maintenance visits.

What is the lifespan of a panel filter, bag filter or HEPA filter?

Filter lifespan varies significantly according to the system, environment and operating conditions. A fixed replacement interval should therefore be treated only as a guide.

Panel filter lifespan

Panel filters are compact filters commonly used in ventilation systems and air-handling equipment.

Their service life depends on:

• Filter depth
• Media area
• Airborne dust levels
• Operating hours
• Airflow rate
• Filter efficiency

Some panel filters may require replacement within a few months, while others can remain suitable for longer under cleaner operating conditions.

The correct replacement point should be based on the manufacturer’s guidance, visual condition and, where applicable, the final recommended pressure drop.

Bag filter lifespan

Bag filters generally provide a larger filter-media area than shallow panel filters. This can provide greater dust-holding capacity in suitable applications.

However, service life still varies according to operating conditions. High dust levels or continuous operation may shorten the interval between replacements.

Bag filters should be inspected for:

• Excessive dirt loading
• Damage to the pockets
• Loose seams
• Moisture
• An increase in pressure drop

Systemair offers bag filters designed for applications where low airflow resistance and high dust-holding capacity are beneficial.

HEPA filter lifespan

HEPA filters are used in applications requiring very high levels of particulate filtration, including some healthcare, laboratory, cleanroom and specialist industrial environments.

Their lifespan cannot be determined by appearance alone. Replacement may depend on:

• Pressure drop
• Integrity testing
• Contamination risk
• Operating hours
• Environmental conditions
• Site procedures
• Regulatory or quality requirements

A HEPA filter should not be changed according to a generic domestic-filter schedule. Maintenance and replacement should follow the requirements of the system, application and facility.

Choosing the right MVHR filter

The best MVHR filter is not necessarily the filter with the highest efficiency rating or the lowest price. It is the filter that provides the correct combination of:

• Compatibility
• Filtration performance
• Fit
• Airflow characteristics
• Quality
• Suitability for the ventilation system

Before ordering, identify the make and model of your MVHR unit and check the original filter reference wherever possible.

NuFilter supplies official filters from Systemair brands for a wide range of residential, commercial and specialist ventilation applications. Using the correct replacement filter helps maintain the intended filtration and airflow performance of your system.

Frequently asked questions about MVHR filters

How often should an MVHR filter be replaced?

Many residential ventilation filters should be changed regularly, with one or two changes per year being appropriate for many systems. The actual interval depends on local air quality, system use and manufacturer guidance. Check filters regularly and replace them sooner if they become heavily loaded.

Can I clean and reuse an MVHR filter?

Only clean or reuse a filter when the manufacturer specifically states that it is washable or reusable. Many ventilation filters are designed to be replaced rather than washed. Washing an unsuitable filter may damage its media and affect its performance.

How do I know which MVHR filter I need?

Find the manufacturer and model number on the ventilation unit, then search for the matching filter set. You can also check the product number on the existing filter or refer to the system manual.

Does a higher ePM rating always mean a better filter?

Not necessarily. A higher efficiency may provide improved capture of smaller particles, but the filter must be suitable for the system’s airflow and pressure capabilities. Follow the ventilation-unit manufacturer’s specification.

Why is my MVHR filter getting dirty quickly?

Filters may become dirty more quickly due to high pollen levels, nearby traffic, construction work, agricultural activity, smoke, dust or extended system operating hours. Rapid loading does not necessarily indicate a problem with the filter; it may reflect the amount of particulate matter being captured.

Find the correct replacement filter with NuFilter

Choosing the correct ventilation filter helps support system airflow, filtration performance and reliable operation.

Browse NuFilter’s range of official filters from Systemair brands or search using your ventilation-unit model to find the appropriate replacement filter for your system.