Why Airflow Rates Matter More Than Ever in UK Homes
As building regulations evolve and more homes are designed with airtight envelopes to meet energy-saving targets, the importance of ventilation performance has risen sharply. One of the most overlooked—but crucial—elements of a Mechanical Ventilation with Heat Recovery (MVHR) system is its airflow rate. Without the correct airflow, even the best MVHR unit won’t comply with UK Building Regulations—and worse, it won’t deliver the fresh air and comfort your home needs.
In 2022, updates to Approved Document F introduced new airflow standards for domestic properties, along with clear requirements for commissioning and performance testing. Whether you're building a new home or retrofitting MVHR into an existing property, getting the airflow rate right is not optional—it’s a legal and functional necessity.
In this blog, we break down exactly what airflow rates your MVHR system needs to meet, how they’re measured, and what to consider when designing or installing a system that complies with the latest UK rules.
Why the Right Airflow Is Essential
A properly functioning MVHR system does more than just move air. It ensures:
- Stale air is extracted from wet and polluted areas
- Fresh, filtered air is supplied to bedrooms and living areas
- Humidity is controlled to prevent mould and condensation
- CO₂ levels are reduced to promote better sleep and concentration
- Your system meets legal compliance under Building Regulations
Getting the balance wrong can lead to:
- Uncomfortable air pressure differences
- Poor indoor air quality
- Failures at building control sign-off
- Reduced energy savings from ineffective heat recovery
Every MVHR system must be balanced to match the supply and extract airflow rates based on the size and usage of your home.
Understanding Airflow Requirements Under Part F
Approved Document F (2022) outlines two key requirements for MVHR systems:
- Whole Dwelling Ventilation Rate (background ventilation)
- Extract Ventilation Rate (intermittent or continuous extraction)
Let’s explore both.
1. Whole Dwelling Ventilation Rate
This refers to the total volume of air that must be supplied and extracted across the property, measured in litres per second (l/s).
The formula depends on:
- Number of bedrooms
- Total floor area
- Number of occupants (assumed or actual)
Here’s a summary based on the number of bedrooms:
- 1 bedroom: 13 l/s
- 2 bedrooms: 17 l/s
- 3 bedrooms: 21 l/s
- 4 bedrooms: 25 l/s
- 5+ bedrooms: 29 l/s
This rate must be delivered continuously—MVHR systems are required to provide this level of background ventilation 24/7.
2. Extract Ventilation Rate (Wet Rooms)
Certain rooms generate more moisture and pollutants. These need local extract ventilation, either via continuous extraction or boosted rates.
Minimum extract rates under Part F:
- Kitchen (with a hob): 13 l/s (continuous) or 30 l/s (intermittent)
- Utility room: 8 l/s
- Bathroom: 8 l/s
- En-suite shower room: 6 l/s
- WC (with no window): 6 l/s
MVHR systems must meet or exceed these rates and balance the extracted air with a matching supply elsewhere in the home.
Case Study: 4-Bed New Build in Hampshire
A family building a 4-bedroom home in Fleet, Hampshire, contacted ClimateWorks to design and install an MVHR system that met new Building Regulations.
System design goals:
- Comply with Part F for both whole dwelling and extract ventilation
- Distribute fresh air to bedrooms and lounge
- Extract from kitchen, bathroom, and utility
- Ensure quiet operation with correct duct sizing
System setup:
- Total airflow rate: 25 l/s (supply and extract)
- Kitchen extract: 13 l/s
- Bathroom extract: 8 l/s
- Commissioned and balanced on-site with documentation for Building Control
Outcome:
- Pass on first inspection
- Air quality sensors showed CO₂ levels consistently below 800 ppm
- Family noted fresher rooms and no signs of condensation during winter
How Airflow Is Measured and Verified
At commissioning, installers must:
- Use an anemometer or hood to measure airflow at each terminal
- Confirm supply and extract match the calculated design rate
- Ensure no individual room is under- or over-ventilated
- Provide a report to be kept with the Building Regulations file
This process must be carried out by a competent person trained in system commissioning and familiar with Approved Document F.
What Affects Airflow in Practice?
Even well-designed systems can underperform if certain factors are overlooked.
Common airflow barriers:
- Undersized ductwork
- Sharp bends in ducts
- Dirty or blocked filters
- Grilles placed in awkward locations
- Poor sealing during installation
To maximise airflow and ensure regulation compliance, systems should be designed and installed by qualified ventilation specialists.
System Design Tips for Meeting Airflow Standards
When planning an MVHR system for regulatory compliance, consider the following:
- Start with a full room-by-room airflow calculation
- Use rigid or semi-rigid ducting for lower pressure loss
- Minimise duct length and bends
- Select an MVHR unit with suitable fan power and heat recovery efficiency
- Ensure easy access to filters for maintenance
- Allow space for duct silencers to control noise without airflow compromise
ClimateWorks handles all design, installation, and commissioning, ensuring systems not only perform but are easy to maintain long term.
The Impact of Airtightness on Airflow
Approved Document F assumes an air permeability of ≤5 m³/h·m² @ 50Pa for new homes. Homes with lower airtightness may require additional airflow or pressure adjustments.
If your home is extremely airtight (e.g., Passivhaus standard), the system must be designed to avoid pressure build-up and overventilation. Similarly, homes that are too leaky may lose warm air faster than the system can recover it.
A pressure test or blower door test before or after installation can help verify airtightness and guide final adjustments.
How MVHR Airflow Affects Energy Use
Balanced airflow isn’t just about legal compliance—it’s also essential for energy savings. If supply and extract aren’t equal:
- Heat recovery will suffer
- Indoor pressure will fluctuate
- Energy use may rise due to fan overcompensation
- Room comfort levels may drop
In some cases, an imbalance as small as 10% can result in a 20% drop in system effectiveness.
Looking to Install MVHR That Meets All Airflow Standards?
ClimateWorks designs every MVHR system around the exact layout, occupancy, and usage of your home. We ensure:
- Full compliance with Part F ventilation rates
- Detailed commissioning reports
- Smooth building control approval
- Long-term energy savings through optimal airflow balance
For expert advice, system design, and installation that meets UK regulations, start by booking a home assessment at https://www.climateworks.co.uk
Author Bio
Dr. Julian Carter is a highly experienced thermal systems expert with over 15 years in the field, holding a PhD in thermal systems. His career spans academic research, consulting, and teaching, focusing on air conditioning and refrigeration systems. Dr. Carter bridges the gap between theoretical advancements and practical applications, providing expert insights to organisations like ClimateWorks, where his guidance informs decision-making and industry best practices. Notably, he has worked on international projects with organisations such as Daikin Industries, the International Institute of Refrigeration (IIR), and the United Nations Environment Programme (UNEP). Currently a lecturer at Edinburgh University, Dr. Carter combines his expertise with a passion for educating the next generation of engineers and advancing climate control technologies.
References
[1] UK Government. (2022). Approved Document F – Ventilation. Retrieved from https://www.gov.uk/government/publications/ventilation-approved-document-f
[2] CIBSE TM60. (2020). Good Practice for MVHR Design and Commissioning. Retrieved from https://www.cibse.org
[3] Energy Saving Trust. (2024). Heat Recovery Ventilation Explained. Retrieved from https://energysavingtrust.org.uk
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