Envirovent advises self builders when choosing an MVHR system

Dominic Gilhooly, Customer Services Team Manager at EnviroVent Ltd, looks at why MVHR (Mechanical Ventilation with Heat Recovery) is becoming more important in new and renovated homes and advises self-builders on choosing an MVHR system.

“The updated Building Regulations in 2022 relating to energy performance introduced increased requirements for air tightness and ventilation in new and renovated homes.  Self builders are therefore seeking effective mechanical ventilation systems that improve air quality, prevent condensation and mould forming, as well as providing low carbon and low running costs.

Many self-builders are choosing to build to “near PassivHaus” or PassivHaus standards, which means creating an airtight dwelling, which maximises energy efficiency whilst ensuring a high level of indoor air quality.  For many new build homes, a mechanical ventilation heat recovery (MVHR) system can be the most effective option, as they are highly effective in ensuring heat can be recovered and therefore energy costs are minimised.

MVHR systems recover the heat from the extracted stale, moisture-laden air within a property’s kitchen, bathroom and ensuites and transfers it through a high efficiency heat exchange cell into fresh filtered air which is then re-supplied to the living areas of the home. 

A typical MVHR system comprises a heat recovery unit containing a high efficiency heat cell. This is usually installed in a utility room, cupboard or loft space and is connected to the habitable rooms in the property via a network of ducts, which open into the rooms via extract or supply valves in the ceiling.  

What are the benefits of MVHR? 

For many new properties, MVHR is the most energy efficient form of ventilation currently available. The right system, correctly installed, will take care of any condensation and mould problems.   

By maintaining an optimal indoor climate and removing condensation, MHVR not only saves energy but can also reduce wear and tear on paint, wallpaper and furnishings, reducing maintenance costs. 

Having good indoor air quality also helps to reduce symptoms of asthma and other respiratory illnesses, which can be triggered or worsen in humid, moist conditions where there are dust mites and mould.

Ventilation is so important that it needs to be given equal consideration and priority at the start of the build project to other services, such as heating and electricity.   Carefully designing, planning and commissioning ventilation systems can ensure years of trouble-free, low maintenance operation.

Use high quality ducting

There are a number of recommendations we make to self-builders and homeowners when planning the installation of MVHR systems.  The first is to use high quality ducting, as a MVHR system is only as good as the ductwork installed.

We recommend semi rigid ducting with MVHR systems and, in normal circumstances, a 90mm radial system. It is the positioning and size of the joists in a build that dictates the size of the ducting – so slightly small diameter ducting is available, if required.  

Most semi-rigid systems are self-seal, which means they are an airtight ducted system.  It can be false economy to specify the most expensive type of MVHR unit and install it with sub-standard ducting. When ducting is selected and installed correctly, it improves and prolongs the efficiency of the whole system, leading to long term low maintenance. 

On the other hand, a ventilation system that has issues with the ducting, which can range from ‘slump’ of flexible types, through to inadequate jointing mechanisms, is always going to underperform. Poorly installed duct work can potentially damage the ventilation unit and the fabric of the building. 

When ducting is installed in cold spaces, such as the loft of a house, it is important to ensure it has adequate insulation.  This helps to avoid condensation build up, which could damage the unit and restrict airflow.

Installation is key

The performance of an MVHR unit can be affected significantly by its installation.  This means that it is essential to find a ventilation installer who is fully trained and has achieved the necessary accreditation and who carefully follows the manufacturer’s system design. 

BSRIA states that 95% of ventilation problems are caused by installation issues. Installers should be NICEIC Ventilation- Approved to ensure they are trained in system design, installation and commissioning of MVHR systems. If required, our engineers can be contracted to make site visits to advise and assist at the start, midway or at the commissioning stage of a project.

Correct commissioning of the system is also crucial to ensure that the right airflow is delivered and that the system is properly balanced. 

Prevent overheating with a Summer bypass function

Look out for MVHR systems that have a summer bypass function and preheater as standard, as this ensures that the home doesn’t become too hot, due to recirculation of internal air. The summer bypass function helps to reduce the air temperature coming into the dwelling during the warmer months. It operates automatically by diverting air around the heat recovery cell when the supply air temperature to the property is above 25°C, by measuring indoor and outdoor temperatures.

Frost protection

Protecting a ventilation unit from frost is important, so it is worth looking out for one with intelligent frost protection.  These are fitted with a preheater, which guarantees high efficiency, even at extremely low temperatures.  Often these systems help to enhance energy efficiency and generate significant energy savings over the year.  The preheater provides protection to the heat cell and ensures good heat recovery from the unit even in cold weather.  

Consider high grade filtration

If your home is in an urban area or one with air quality issues, then choosing an MVHR unit with high grade filtration is an important consideration.  This is because MVHR draws in air from outside the building, transferring heat from the outgoing stale air to the new fresh filtered supply air. Obviously, if the air from the outside is polluted with particulate matter then this could be brought back into the house. 

MVHR units come with filters that may be capable of filtering very fine particles, for example ISO Epm2.5 50% plus carbon filter.  This is used to prevent odour and pollution transfer from outdoors to indoors.

Filters help to maintain the efficiency of the heat exchange unit, as well as ensuring consistently clean air inside the property. Over time, filters can clog with dirt and other substances. A clogged filter can no longer effectively clean the air, and, just as importantly, the MVHR system will be unable to run at maximum efficiency.  Filters need to be monitored and replaced periodically in high pollution areas.

Remote control and monitoring

Another factor to consider is how you want to control your ventilation system, as some MVHR units, like energiSava®, are available with smart apps, which make it easier to set up and operate the ventilation system via an ioS or Android phone. Modern MVHR units also have humidity sensors, CO2 sensors or manual boost switches which can make them operate more effectively. 

EnviroVent offers a completely free design service, providing a unique specification and layout for each dwelling.  The company has its own in-house technical team and provides on-site support to help self builders and home renovators on their journey towards great indoor air quality.