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By Ruth MacEachern
Product Manager
We have created an informative guide on everything you need to know about MVHR and MEV before buying or starting a sustainable ventilation system project.
MVHR stands for Mechanical Ventilation with Heat Recovery. MVHR systems offer a completely balanced system, providing both extract ventilation and a supply of fresh tempered air. They provide optimum ventilation for a property with a minimum loss of energy by extracting moisture-laden air from the wet rooms (bathrooms, WCs, kitchen and shower rooms) and at the same time supplying fresh air into the living areas (living rooms, bedrooms, dining room). The energy from the extracted air is collected in a high efficiency heat exchange cell before being transferred to the new fresh air and resupplied into the property, creating an all year round, ideal indoor environment.
MEV stands for Continuous Mechanical Extract Ventilation. MEV systems can be either a whole house centralised unit, or decentralised fans. The centralised MEV is also a whole house ventilation system, typically located in a loft space or hallway cupboard. Multiple ducts run from the unit to the kitchen, bathroom, ensuites and other wet rooms of a property to simultaneously and continuously extract moisture-laden air from these rooms to control humidity levels. They do not feature a heat recovery element, they just provide extract ventilation.
Building designers are recognising that MVHR helps them to cost effectively contribute towards the improvements in CO2 emissions required by Building Regulations. With MVHR, the incoming air is filtered, improving internal air quality and it also negates the need for window trickle vents. The benefit of MVHR is that it is much more controllable, it’s a fully balanced system and has a larger impact on the energy efficiency of the dwelling. With an MEV system, window trickle vents are required to meet the whole building ventilation rate.
Recovering over 90 per cent of the heat that would normally be lost to the outside via trickle vents or extract fans, and feeding it back into the house as warm fresh air, MVHR provides a sustainable ventilation system and ensures that developers are able to reduce the overall energy requirement of the building. There are obvious benefits to homeowners looking to reduce their energy bills.
Where housebuilders are looking to build to greater levels of air tightness (of 3m3h/m2 or below) MVHR is often specified to achieve a larger percentage reduction between the DER and TER (Target Emission Rate). A high performing MVHR system through SAP (Standard Assessment Procedure) may lower the DER by 7-8 per cent.
Another reason for the increasing adoption of MVHR by the new build sector is the ErP Directive, which rates ventilation systems according to their energy efficiency. This is encouraging a move to a more systems-based approach to ventilation, as it mainly impacts MEV and MVHR systems and requires all ventilation units, except dual use versions to be equipped with a multi-speed or variable speed drive. In addition, all bi-directional units are required to have a thermal by-pass facility as an essential part of the design criteria, which prevents warm air being recovered in warmer weather. Due to the high thermal efficiency of MVHR systems, these typically achieve an ErP rating of A or A+, whereas MEV systems typically achieve a maximum rating of B.
However, it is worth noting that both MEV and MVHR systems are most suitable for new properties rather than retrofit, but MVHR systems do require a more airtight property where virtually all of the airflow can pass through the heat exchanger, if they are to perform efficiently.
Whilst advances have been made over the past ten years, good installation is an essential part of ensuring the effectiveness and efficiency of any ventilation system, particularly MVHR and MEV systems, and an important part of this is the ducting. Choosing the right ducting can improve and prolong the efficiency of the whole system. On the other hand, 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.
It is therefore essential that a ventilation system is correctly sized – if it is undersized, it could lead to under performance of the unit and noise issues. This is the reason why we work closely with specifiers from the design stage to ensure that new homes can meet the requirements for both ventilation and air tightness.
Architects and developers these days need to ensure they have a good understanding of regulations and ventilation requirements or to seek expert advice on this. There is, undoubtedly, an increase in specification of more energy efficient and sustainable ventilation systems, such as MVHR, however, systems are often incorrectly specified, which can lead to issues. Single room extract fans are still a popular solution for certain situations, but as dwellings are built with an increased airtightness, MVHR becomes a more logical choice.
Getting advice from a dedicated ventilation system design team is key to specifying the correct ventilation system for a project, where duct runs can be clearly shown, airflow rates provided and technical data sheets. Specifying ventilation systems at the early stages of a building’s design is vitally important to ensure that a home can achieve the correct airflow requirements. Failure to do this can impact on the health of occupants and lead to damage to the fabric of a building.
Domestic ventilation is notifiable work, requiring ventilation provision to be installed by a competent and qualified person. It is vital that the installation of any domestic ventilation system is correct and meets the minimum requirements set out in regulations and standards. Over 90% of faults with ventilation systems are down to poor installations. BSRIA recently carried out inspections on dwellings, 100% of continuous systems failed upon first inspection.
Housebuilders are therefore encouraging their installers to be NICEIC trained and certified, which means they can work with the latest types of domestic ventilation products safely and efficiently, receiving technical information and advice plus practical support. Training via an NICEIC training centre, like the EnviroVent NICEIC approved centre in Harrogate, also ensures that installers are qualified to inspect and test ventilation systems, as well as to commission them effectively.
With inner cities obviously having higher pollution levels, MVHR systems are seen as more suitable because of the filter element, but in apartment blocks it’s often the case that only the first four floors are fitted with MVHR and the remaining with MEV as polluted air is heavier and therefore stays closer to the ground.
Integral to an MVHR system is the high efficiency heat exchange cell. This retains the energy from the extracted air and transfers it to the incoming supply air. These typically achieve efficiencies of between 87%-92%.
As mentioned above, the key to both MEV and MVHR systems is the ductwork. EnviroVent offer a complete thermal and semi-rigid ducting range called Fast Track. It is very easy and quick to install and requires no tape or sealant, resulting in minimal energy consumption and noise.
The myenvirovent app for the energiSava 200 and 250 range of MVHR systems is extremely user-friendly and flexible, allowing the installer to set up the ventilation rates with visibility of the unit's current status at the touch of a button. Through its use, homeowners can also access user guides, technical documents and can provide feedback and future product requests to the EnviroVent R&D centre. Other practical benefits for the homeowner include notifications when filter changes are due and user access to EnviroVent’s technical support team with any questions or queries. The user is provided with a secure connection from the unit to the app through the homeowner's router.
The app offers installers an intelligent commissioning feature, which saves considerable time on-site. It allows them to connect directly to the unit through the app without the need for a wireless internet connection and to access the dedicated installer mode to commission the airflow rates and select additional options in the unit’s ventilation settings page.
Remote boost switches and/or remote controls are typically available for both MVHR systems and MEV units.
In addition, some MEV and MVHR systems, such as the energiSava MVHR range and Fast Track MEV range from EnviroVent come as standard with Intellitrac® humidity tracking controls. This allows the unit to increase the fan speed automatically and in direct proportion when it senses a rise in humidity levels (showering, cooking, ironing etc). The fan speed then tracks back to normal once the humidity level falls. This controls condensation quietly and efficiently.
Cost saving is the main reason why extract fans are chosen over “whole house” systems in new builds as MVHR is naturally more expensive and complex to implement. However, with most modern MVHR systems, they are much more effective means of ventilating the property and provide a long-term sustainable solution. MEV systems can be a good alternative to avoiding multiple installations of extractor fans in the home. From a maintenance and servicing point of view, annual maintenance should take place to ensure that any filters are cleaned and replaced.
In striving to achieve Net Zero and reduce carbon emissions in homes, we will no doubt see a shift in ventilation strategies for new homes moving towards a ‘systems’ approach to ventilation in ensuring that adequate airflow rates are achieved in more airtight dwellings in the most energy efficient way.
When considering MVHR and MEV, both have their pros and cons. Ultimately, it will be down to the design, budget, application and requirements of the specifier, including how airtight the properties are being built to, with the assistance of a good ventilation designer, as to whether MVHR or MEV is the best choice for the project.
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