Ensuring long term ventilation efficiency

David Frimpong, Head of Specification – New Build at EnviroVent, looks at the importance of effective system design, specification and installation when it comes to long-term ventilation success. 

“Updates to Building Regulations’ Approved Document L and F introduced in 2022 mean that new dwellings require increased levels of air tightness and enhanced ventilation rates.   

Approved Document F introduced a significant increase in the whole dwelling ventilation rate, which has seen the minimum ventilation rate for a two-bedroom property shift from 17l/s to 25l/s.  The updated regulations have increased the compulsory use of background trickle vents when utilising Continuous MEV (Mechanical Extract Ventilation) to 4,000mm2 in each habitable room from the previous requirement of 2,500mm2.

The updates to Building Regulations mean C-MEV and MVHR are becoming the norm, as the industry moves further towards 2050 and the Net Zero target, providing sustainable methods of ventilating both new and existing dwellings.

Increased energy efficiency requirements

The new Regulations also mean that all new build homes need to be future-proofed with low carbon heating and world-leading levels of energy efficiency.  They set out the ambitious target of a 30% reduction in carbon emissions for new dwellings as an interim target, up to the introduction of the Future Homes Standard in 2025. 

There is now a minimum performance tolerance for elements of a dwelling such as walls, doors and windows which will increase energy efficiency and prevent poor standards of construction.  This is as a result of the updates to Approved Document L: the conservation of fuel and power, which needs to be taken into consideration as ventilation systems consume electricity and remove heat from the building (and in the case of MVHR systems recover heat into the home). 

The main ventilation-relevant change to Document L is that heat recovery systems are now required to be 73% efficient – an increase of 3% on the previous specification. As our heat recovery systems provide an efficiency well in excess of 85%, this change raises no cause for concern.

In addition, the new Approved Document O Regulations provide guidance for mitigating overheating. The document concentrates on two main methodologies, the Simplified Method and Dynamic Thermal Modelling which look at many variables to determine whether further action is required to reduce the risk of overheating. The key point from Approved Document O is that any potential overheating issues should be identified at the design and pre-construction stage, rather than via mechanical ventilation and cooling focused options once the property is built.

A further change is the addition of Section 2 and Appendix B to Approved Document F which relate to indoor air quality and volatile organic compounds (VOCs).  This includes a list of harmful contaminants, exposure limits, time and advice on action to be taken should the property exceed safe levels.

Because of the wide variety of changes affecting ventilation, many within the new build sector are therefore increasingly looking to specify whole house ventilation systems, even for smaller properties, as they can be a more efficient way to meet the regulations. 

Correct sizing is essential

Ensuring the correct sizing of the ventilation unit is essential to its long-term efficiency.

Sizing the unit is calculated based on a minimum high rate for the wet areas and a minimum low rate, over the number of bedrooms or with a minimum trickle ventilation rate which should not be less than 0.3l/s per m2 of internal floor area.  All floors need to be taken into account.  Correct sizing is based on Approved Document F calculations and system ductwork resistance, with the resistance calculated using our BIM software.   

Housebuilders and developers should refer to Approved Document F of Building Regulations for more details. If the airflow rate calculations are incorrect, the fan will not be optimized to push and pull air through the property, resulting in inefficient ventilation.

If the unit is undersized, it could lead to the under performance of the system, with an air flow rate that is too low, which can result in high humidity levels in dwellings. It can also mean a unit has to work extra hard in an attempt to achieve airflow rates, resulting in excessive noise and increased energy usage. This is the reason why ventilation manufacturers work closely with specifiers from the design stage to ensure that new homes can meet the requirements for both ventilation and air tightness.

Often the reason for the under sizing is due to space restrictions in the utility cupboard or lofts of new properties, but this can cause serious problems over the long term. Careful consideration should be given to the location of the MVHR unit as it should be accessible for maintenance. 

In a study conducted by multidisciplinary building performance experts BPE Specialists, they found that only 16% of systems were commissioned correctly with respect to air flow and balancing. 


Good installation is an essential part of ensuring the effectiveness of this type of ventilation system. A ventilation system that has issues with the ducting, which can range from ‘slump’ of flexible types in the loft/roof space, through to inadequate jointing mechanisms, is always going to underperform.  Poorly installed ductwork can potentially damage the ventilation unit and the fabric of the building. 

To prevent slump, contractors should ensure that any flexible ducting used is less than 300mm long. However, some flexible ducting is needed to run from the MVHR unit to isolate ducting and prevent vibration from the unit.  Acoustic flex is often recommended to be used here.

It is essential for installers to follow manufacturer guidelines on installation.  Correct installation will ensure that there is minimal noise and vibration from a unit.  For example, anti-vibrational mounts may be required to help reduce noise transfer from wall or floor mounted units, often in multi-occupancy dwellings.

Poor installation can affect a ventilation unit’s efficiency, power consumption and noise levels. The result could be increased running costs and a negative impact on indoor air quality if the unit is not installed correctly. 


Whole house heat recovery units, like our energiSava® range, offer a cost-effective way of meeting Building Regulations Part F requirements, as well as providing low running costs for homeowners.  The new apps available on MVHR units, like the myenvirovent app on the energiSava MVHR units provide greater control to the user and ease of commissioning for the installer.

enviroSupply and extract airflows should be balanced in accordance with the design requirements and individual room flow rates of the property.  Many developers choose to work with the ventilation manufacturer from the early stage of design to agree the correct system design, specification and installation as part of ensuring the long-term efficiency of ventilation systems.”

For a summary of all the key changes to ventilation Regulations, take a look at EnviroVent’s series of webinars ‘Building Regs Bitesize’ for quick and easy to understand content on all the changes to the Future Homes and Building Standard legislation. Find these here…. https://www.envirovent.com/help-and-advice/building-regulations/