Ask the expert: Design an MVHR system

Ask the expert: Design an MVHR system

There is now a growing demand for whole house ventilation systems with Mechanical Ventilation with Heat Recovery (MVHR). Christian Rehn, National Design & Project Manager at National Ventilation, answers some key questions around designing a system.

A customer wants me to install an MVHR system where do I start?
When installing a whole house ventilation system good design is vital. This ensures the system provides the correct levels of ventilation and will physically work within the structure. When it comes to designing the system, you need to be familiar with the range of technologies available such as MVHR, as well as specialist knowledge of the equipment or software programmes to correctly calculate the pressure in the system over a given length of ducting. This can be quite daunting if you’re unfamiliar with ventilation system design. Luckily, there are design services available, such as the free service National Ventilation offers, which can help electricians by designing the ventilation system for their project. The free design services include a full design, kit list and recommended duct routes – whilst ensuring Building Regulations compliance.

How do I choose the correct whole house ventilation unit for the project?
We recommend mechanical ventilation with heat recovery (MVHR) as the best solution for new builds as it is a controlled way of ventilating a whole property, and it saves energy by recovering heat that would otherwise have been lost. You can control humidity, CO2 and indoor air quality. MVHR systems need to be properly designed to ensure you buy a correctly sized unit. If an MVHR unit is oversized it won’t provide efficient ventilation for the home. If it is undersized there won’t be adequate levels of ventilation, which will lead to poor indoor air quality and problems with condensation and mould. The ventilation designer will use architect drawings and property details to enter this data into an airflow calculator. This ensures that the ventilation specified can meet the airflow demands of each room. Once the total airflow figure is calculated an MVHR unit can be specified to meet these requirements.

What type of ducting should I use with MVHR?
Larger projects generally use traditional rigid ductwork which requires experience and time to install correctly with a number of connections needed. For smaller developments we recommend semi-rigid radial ductwork which is a cost-effective and simple alternative. Simple installation means fewer mistakes with even a novice able to install it to a high standard, resulting in an airtight installation and improved system performance. Semi-rigid radial ducting is both low profile and flexible meaning it is perfect for installing in places where space is restricted. Semi-rigid ducting is available in 50m rolls which can then be cut to the appropriate length meaning far fewer connections, easy routing and supporting of the duct, when compared with traditional rigid ductwork.  With a plethora of plenums and accessories available, this system can overcome over 99% of issues that may occur on-site. Radial ducting can also be used in Passivhaus projects.

What controls should I use with an MVHR Unit?
Traditionally MVHR is boosted on bathroom lighting but not every bathroom visit is a shower or a bath and so systems can boost unnecessarily. As an alternative, an in-duct humidistat can be fitted in the ducting, meaning the boost only triggers when humidity rises, avoiding nuisance running. It’s also quicker and more cost-effective to install, as electricians only need to wire the humidistat back to the fan. Meanwhile, the latest MVHR units, such as the Monsoon Energysaver IntelliSystem Heat Recovery Unit, feature wireless commissionable controls via an App, encouraging best practice and high performance through simplified commissioning, saving installers time on-site.

My customer is worried MVHR may be noisy?
As long as the MVHR system has been designed, installed and commissioned correctly it won’t be noisy. Using the relevant acoustic test data, manufacturers are able to model the operating conditions of the MVHR unit and understand the acoustic properties. When on trickle, therefore, the MVHR is so quiet that it should not be heard.

To access more details about National Ventilation’s design service for customers, click here.

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