Top 10 Reasons MVHR Systems Fail Testing

Top 10 Reasons MVHR Systems Fail Testing | MVHR Flow Rate Testing Guide

MVHR systems are designed to provide balanced, energy-efficient ventilation throughout a dwelling, but achieving compliance with Part F Building Regulations depends on far more than simply installing the equipment. Many dwellings fail their MVHR flow rate testing because the airflow rates do not match the MVHR design specifications, resulting in insufficient ventilation to key rooms or inadequate extract rates from wet areas – so what are the Top 10 Reasons MVHR Systems Fail Testing. Common causes include poorly designed duct-work, excessive resistance within the system, incorrectly adjusted valves, air leakage from duct connections, dirty filters, commissioning errors, or changes made during construction that were never accounted for in the original design.

Without proper testing and balancing by independent MVHR Flow test engineer, these issues can remain hidden, leading to failed inspections, delayed Building Control sign-off, reduced indoor air quality, and costly remedial works. Understanding why MVHR systems fail flow rate testing is the first step towards ensuring compliance, improving occupant comfort, and maximising system performance.

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Top 10 Reasons MVHR Systems Fail Flow Rate Testing (And How to Avoid Them

It’s a strong commercial keyword that targets developers, M&E contractors and house-builders searching for commissioning and compliance advice.

When undertaking MVHR (Mechanical Ventilation with Heat Recovery) testing and commissioning, the most common failures are typically related to poor installation, incorrect commissioning, or design issues rather than the MVHR unit itself.

1. Airflow Rates Not Achieved

The most common failure occurs when the system cannot achieve the minimum supply or extract rates required under Approved Document F.

Common causes of MVHR test failures:

• Undersized duct-work
• Excessive duct lengths
• Too many bends in the duct-work
• Excessive system resistance

2. Poor Duct-work Installation

Duct-work should be installed as straight and smooth as possible.

Typical problems:

• Crushed flexible ducting
• Sharp bends
• Kinked duct runs
• Poorly supported duct-work

These issues dramatically increase pressure losses and reduce airflow. in many cases clients install good quality systems, only to fail due to poor duct-work.

3. Incorrect MVHR Unit Settings

Many systems fail simply because they have not been properly commissioned.

Examples include:

• Fan speeds set too low
• Incorrect commissioning settings
• Factory settings left unchanged
• Boost mode not operating correctly
• Caps left on MVHR units

4. Dirty Filters

Even new-build properties can suffer from clogged filters during construction.

Construction dust can:

• Restrict airflow
• Increase fan resistance
• Cause rooms to fail airflow requirements

5. Poor Air Valve Adjustment

Supply and extract valves need balancing during commissioning.

Common issues:

• Valves fully open or closed
• Incorrect balancing
• Air-flows favouring certain rooms
• incorrect sequencing during balancing

This can result in some rooms passing while others fail.

6. Leaking Duct-work

Air leakage within the duct system can have a major impact on performance.

Typical causes:

• Poorly sealed duct joints
• Damaged duct-work
• Missing tapes or seals
• Loose connections

Air lost into roof voids or service risers is airflow unavailable at the terminal.

7. Incorrect Duct-work Design

Even when installed correctly, poor design can prevent compliance.

Examples:

• Insufficient duct diameter
• Excessive equivalent lengths
• Lack of design calculations
• Incorrect branch arrangements

8. Wrong Terminal Locations

Supply and extract valves installed in the wrong locations can affect both performance and testing.

Examples:

• Supply valves too close to extract valves
• Poor airflow distribution
• Incorrect room allocation

9. Building Envelope Changes

MVHR systems are designed around anticipated dwelling air-tightness.

If the property is:

• More airtight than expected
• Less airtight than expected
• Missing undercut doors
• Missing transfer air paths

The airflow characteristics of the dwelling can be compromised.

10. Failure to Commission Before MVHR Flow Rate Testing

A surprising number of systems are tested before proper commissioning has been completed.

Common examples:

• Valves not balanced
• Fan speeds not adjusted
• System not fully operational
• Heating engineers not completed commissioning

This almost guarantees poor results during validation testing.

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Warning Signs to Look Our For Prior to MVHR Flow Rate Testing

✅ Excessive noise from valves

✅ Weak airflow at terminals

✅ Dirty construction environment

✅ Flexible ducting used extensively

✅ Missing commissioning records

✅ Unbalanced room air flows

✅ Complaints about draughts

✅ Condensation in bathrooms

✅ Strong cooking smells remaining after use

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How to Maximise the Chances of Passing a MVHR Flow Rate Test

1. Complete installation before testing.
2. Replace or clean filters.
3. Verify all duct-work joints are sealed.
4. Check boost controls operate correctly.
5. Ensure internal doors and undercuts are complete.
6. Undertake full commissioning before validation testing.
7. Use an independent UKAS-accredited testing company for verification.