This blog post is contributed to by Alex Tonge, Director of Ecotest and Jim Gililand, Partner of C&G Design Consultancy LLP – our Trusted Design Partner.

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Alex Tonge, Director

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Jim Gililand, Partner,

C&G design consultancy LLP

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When building a new-build residential building in the United Kingdom (UK), we believe that there are really only two options available to you, even though Part F of the Building Regulations (the governing document for ventilation within the UK) formally recognises three.

Why do we believe there are only two suitable options? May you ask…

Well, it is because Part F only recommends Natural Ventilation (Background Ventilation with Intermittent Extract Fans) in less air tight dwellings – which it defines as those designed at > 5m³/h/m² at 50Pa or tested at > 3m³/h/m² at 50Pa. Most New-build dwellings are now being built and designed with air tightness in mind, and why wouldn’t they? Buildings with increased levels of air tightness require less heating and/or cooling to maintain a comfortable temperature, reducing energy costs and our carbon emissions.

There are also other more overlooked downsides to Natural Ventilation, such as the increased requirement for background ventilation and the higher extract fan flow rates required at each fan vs. the other two continuous systems. FYI background ventilation in dwellings is commonly provided via a window slot vent (aka trickle vents) and the difference between Natural Ventilation’s requirements for background ventilation against the other continuous systems is staggering.

Table 1.7 of Approved Document F of the Building Regulations 2021 edition

Additionally, from an energy assessment perspective, it is also possible to accurately account for both continuous systems within your dwelling’s SAP assessment vs. a nominal input made for Natural Ventilation.

So without further delay let’s delve into the options we believe are available to you.

Option one:

System TypeContinuous mechanical extract ventilation:


Background ventilation requirements
  • 4000mm² equivalent area per habitable room
  • Providing a minimum total number of ventilators that is the same number of bedrooms plus two e.g. a one bedroom dwelling should have at least three background ventilators.
  • Centralised
  • Decentralised
  • Rigid Plastic
  • Semi-rigid (less common in CMEV, but useful in certain circumstances)
  • Lower cost equipment compared with MVHR
  • Lower background ventilation requirements than Natural Ventilation
  • The system provides whole dwelling ventilation
  • Good quality systems contain an integral humidistat
  • Suitable for all levels of air tightness (exc. Scotland if the air tightness < 3m³/h/m², the ventilation system must be MVHR)
  • Energy loss in non-summer months as continuous mechanical extract ventilation systems are extract only
  • There is still a requirement to make up ‘fresh air’ via background ventilation
  • They have a negative effect on SAP assessment and consequently your EPC
A Decentralised Mechanical Extract Unit
A Centralised Mechanical Extract Ventilation unit

There are two types of continuous mechanical extract ventilation system, the first is the decentralised mechanical extract ventilation system (DMEV) and this is where you would have an individual fan in every wet room, for example the Kitchen, Utility, Bathroom, EnSuite, WC and each fan would be ducted externally to the outside of the dwelling to remove the stale air from the dwelling (similarly to Intermittent extract fans in Natural Ventilation).

So five wet rooms with five fans would result in five terminations to the outside.

The only concern with this solution is that there are limitations to it, with regards to the distance between the fan and its outside termination and also in terms of the flow capacity and the allowable noise levels at each fan transferring to the habitable rooms (e.g. when using an EnSuite at night). As a result, DMEV solutions are well suited to small – medium sized dwellings.

For larger dwellings where there are larger rooms and greater distances to the outside, the second type of continuous mechanical extract ventilation system is well suited. This is known as centralised mechanical extract ventilation (CMEV), which is usually located in a store cupboard or in the loft space and is ducted to each wet room, where there is a ceiling/wall valve (with no fan in it). Ductwork from each wet room valve is linked to the central fan unit in the store cupboard/loft and from which it is then ducted to the outside, to remove the stale air from the dwelling.

Using our previous example, five wet rooms would have five ceiling/wall valves and one termination to the outside.

However, there is also the possibility of creating a dual/hybrid system between DMEVs and CMEVs and this can be very useful when there are limitations. Examples being when the DMEV’s don’t have the ability to generate the required flow rates, the CMEV can step in to do the heavy lifting, or where the accessibility to run ducting to certain areas of dwelling is not possible, as a result of structural obstacles (e.g. steels, joists, etc.), a DMEV can then be used to support the CMEV in those hard to reach areas.

How does a continuous mechanical extract ventilation system work?

In day-to-day operation when the humidity levels within a dwelling rise above a pre-programmed level (40%-60% relative humidity range – considered the sweet spot for human occupation) the system will enter boost mode to reach at least the minimum flow rates as defined by Part F. It does this to remove the moisture and take the humidity below the pre-programed level. For example, if an occupant enters a wet room such as a bathroom and takes a shower, the system via its integral humidistat will boost to clear the increased moisture within the bathroom. There are also further options available for configuration, you could integrate PIR sensors with your system to detect when someone enters a wet room or a CO2 sensor that also can assist with detecting human occupation.

Commonly the MEV systems are installed using rigid ducting. However, in some scenarios it may be more practical to utilise semi-rigid ducting. Semi-rigid ducting provides the ability to create more sweeping bends (benefits to airflow resistance vs. traditional 45° and 90° rigid bends) and is easier to install, by reducing the number of connections and chances of air loss, whilst saving time.

It is important to remember that with MEV systems there is still a requirement to make up ‘fresh air’ via background ventilation (although the requirements are much smaller than Natural Ventilation). Each habitable room will require a minimum of 4000mm² equivalent area of background ventilation (there is no background ventilation in the wet rooms) and in total the dwelling will need to provide a minimum number of ventilators. The minimum number is the number of bedrooms plus two e.g. a one bedroom dwelling should have at least three background ventilators.

Option two:

System TypeMechanical Ventilation with Heat Recovery:


Background ventilation requirements
  • No background ventilation required
  • Centralised
  • Rigid
  • Semi-rigid
  • Energy recovery vs. an extract only system resulting in reduced heating costs
  • Fresh filtered air is supplied to dwelling
  • No background ventilation requirements
  • The system provides whole dwelling ventilation
  • Opportunity for integration with other technologies such as Air/Ground Source Heat Pumps
  • Integral humidistat
  • Positive effect on SAP assessment and consequently your EPC
  • Suitable for all levels of air tightness
  • Higher initial equipment costs
Graphic of a house with a cross section displaying a mechanical ventilation with heat recovery unit (mvhr) ventilation system
Zehnder CSY Ventilation Unit ComfoAir

Mechanical Ventilation with Heat Recovery (MVHR) is the most energy efficient domestic ventilation system and comprises of a centralised system with one intake and exhaust terminal to the outside of the dwelling per unit.

MVHR supplies fresh filtered air from the outside to the habitable rooms and extracts stale air from the wet rooms in the dwelling. The system is balanced which means if the MVHR extracts 50 l/s of air then it will also supply 50 l/s of air.

MVHR therefore does not require background ventilators like Natural Ventilation and MEV systems. Furthermore, it utilises a heat exchanger to recover heat from the stale extracted air and transfers it to the incoming fresh filtered air.

Sound good? What does this all look like in practice?

If for example you had an MVHR with a 92% efficient heat exchanger, then the dwelling would only be losing 8% of the heat from the extracted stale air. This contrasts greatly with Natural Ventilation and MEV systems where 100% of the heat in the extracted stale air would be lost to the atmosphere.

MVHR also improves the distribution of heat throughout the dwelling, and this is much more efficient in terms of supporting your dwelling’s heating. Think of a traditional vs. fan assisted oven and how a fan assisted oven will distribute the heat around the oven more evenly and efficiently.

Dependent on several factors, dwellings with MVHR can expect a 20% to 30% on average improvement on their heating bills.

It’s clear to see that there are real benefits of MVHR covering the occupant’s health and living environment, the building’s health and the wider environment by reducing carbon emissions from heating demands.

This is supported by the Passivhaus Institute (recognised as one of the highest standards of energy efficient building). The Passivhaus Institute insist that you must have an MVHR system that is Passivhaus Certified to have your building recognised as a Passivhaus Build. The Passivhaus Institute recognise that it is important to calculate the input of heat vs. the output of heat to make sure it is a thermally efficient building and to do that you must have a high level of air tightness. At Passivhaus levels of air tightness (< 0.6 ACH) you must have mechanical ventilation to supply fresh air and extract stale air pollutants from the dwelling. Passivhaus’ are designed to be at 20°C year-round, so in these scenarios MVHR can be used continually year-round, recovering heat and maintaining a comfortable indoor environment.

For non-Passivhaus builds MVHR also has a feature known as a summer bypass. The summer bypass works by detecting the temperature of the air inside vs. outside and when the unit detects it is cooler outside, the summer bypass kicks in and prevents the recovery of heat from the extracted air. It then supplies just the cooler fresh, filtered air to the dwelling to assist with cooling and reducing overheating risks. Overheating risks can also be mitigated by MVHR with comfort cooling (this is air tempering and not to be confused with air conditioning), where the system is integrated with an integral heat pump or a reversible Air/Ground Source Heat Pump via a water jacket on the supply ducting, to utilise cold water to cool the incoming air into the property. On the other hand, in winter, you can also use hot water from the heat pump to warm the air coming into the property.

Consequently, MVHR has developed and evolved into a technology that could be useful in combatting the results of global warming in the UK. With an average life expectancy that is ever extending and if you’re a self-builder building your forever home (which you could see yourself living in for the next 20 to 30 years) MVHR really is a no-brainer. The same applies for conscientious developers and architects, alike.

Worryingly though, in the UK, an opinion has developed that high levels of air tightness < 3m³/h/m² at 50Pa are required for MVHR to be useful. This is a stigma that has clouded MVHR and its wider adoption. To combat this opinion the Passivhaus Trust created ‘The Case for MVHR’. They remind us that, ‘an MVHR system’s primary function is ventilation and, in this capacity, it is far more effective at providing a good quality indoor air environment – regardless of external conditions – than natural ventilation’. The Passivhaus Trust finally concludes that, ‘This analysis has shown that MVHR systems result in improved ventilation and lower carbon emissions for all levels of airtightness. There is a compelling case for MVHR systems to be fitted in all new dwellings and to be strongly encouraged in retrofits where significant reductions in energy demand are being targeted.’ You can find and read their analysis here:

Furthermore, in a post Covid-19 world where we are more concerned about pollutants, viral contamination and mould build up, one of the great things about MVHR is that because of the integral humidistat, we can manage the level of humidity within a dwelling which prevents bacterial and fungal growth.

Moisture can also have more of an unseen affect. The common household dust mite lives in humid conditions and a fungus grows on its faeces. The fungus is a prime cause of asthma. Removing the moisture therefore reduces the presence of dust mites and therefore makes a building a hostile environment for the fungus to grow.

Other pollutants can include:

  • Carbon monoxide and carbon dioxide
  • Volatile Organic Compounds
  • Allergens
  • Odours
  • Radon

MVHR can also be beneficial for occupants with allergens to pollutants such as pollen. The fresh filtered air will remove any pollen and if you’re a hay fever sufferer living in a dwelling with MVHR you’re going to be breathing easy!

In terms of maintenance, end-users of MVHR need to remember to check, clean, and change the filters as over time they will become blocked. Following commissioning of your system we recommend that the filters are checked after 3, 6 and 12 months so that a maintenance profile can be developed across the seasons.

So what would we have?

Hands down MVHR!

But please remember it is essential whether you go for a MEV system or an MVHR system you need to ensure that the system is designed, supplied, installed, and commissioned by a competent people.

Thank you for taking the time to read this blog and we would welcome the opportunity to support you by providing the best ventilation solution for your project.

Published On: May 25th, 2023Comments Off on Need to Vent about ventilation?Categories: Ventilation Solutions