Popular MVHR systems set to tackle efficiency targets !

Mechanical ventilation and heat recovery systems are on course to dominate new sustainable homes by 2016.

The transition towards airtight homes in the UK means that purpose-provided ventilation to help eliminate condensation and pollutants is more necessary than ever before. Mechanical ventilation and heat recovery (MVHR) in particular is starting to gain a significant foothold in new housing. In the year 2010-11, over 18,000 MVHR units were sold. If growth continues at this pace MVHR systems will become the most common sort of ventilation in new homes in the UK by 2016.

MVHR units generally comprise a small box, usually hidden in a cupboard or in the loft, which contains two electric-powered fans, one to supply air and one to remove air from the home. Warm moist air is extracted from kitchens and bathrooms and ducted back to the unit, while fresh outside air is ducted from the unit to other habitable rooms. The inclusion of a heat recovery unit to transfer energy between the supply and extract ducts can help reduce the heating load.

To realise the benefits of reductions in building energy and improved indoor air quality, MVHR systems must be properly designed and installed. In practice this does not always happen, with many MVHR installations failing to meet their potential – as a recently-published interim report by the Zero Carbon Hub and NHBC Foundation/BRE Trust makes clear.

The report, Mechanical Ventilation with Heat Recovery in New Homes, says failures in design, installation and commissioning are “all too common”. In particular, the report identifies problems with the design and provision of controls to enable the system to be operated correctly and with the location of MVHR units, particularly in roof spaces, where access for user-maintenance is restricted.

The study runs in parallel with the Good Homes Alliance’s report Ventilation and Good Indoor Air Quality in Low Energy Homes. This looks at the lessons of various domestic ventilation strategies including MVHR, based on data collected from a variety of sites, using a variety of different systems. It reached similar conclusions for the installation of MVHR systems.

What will make MVHR systems more effective?

Recommendations contained in the Zero Carbon Hub’s report include:

• Ensuring the MVHR system is located to allow access to change filters

• Flexible ductwork should be kept to a minimum and all ductwork in unheated spaces should be insulated to a standard similar to the building envelope

• Ducts and grilles should be sized to minimise system-generated noise

• Commissioning should be undertaken in accordance with the recommendations of the government’s Domestic Ventilation Compliance Guide by a competent person.

The report was also critical of user controls and user advice for MVHR units. It recommends:

• Easy to use, clear, intuitive controls for occupants to encourage the correct operation for different external weather conditions.

• Systems should indicate clearly whether the unit has failed, or is working in normal, boost or bypass mode; and also when maintenance is needed.

• User instructions issued with new MVHR units should give simple clear guidance on operation including during summer and winter, advice on issues such as opening windows, and unambiguous instructions for maintenance.

What is the industry doing?

“The report identifies a number of market development challenges; not least the need to ensure that systems are well designed and competently installed,” says Kelly Butler, marketing director at product manufacturer organisation British Electrotechnical and Allied Manufacturers Association.

BEAMA has launched an approved contractor scheme for MHVR systems. This is being promoted by mechanical ventilation manufacturers and is written by building services engineering training provider BPEC

The course, targeted at specifiers and installers, explains how to design, install and commission a domestic ventilation system. Having passed the course, an operative can sign up to an approved contractor scheme, under which he is responsible for completing a checklist to ensure an installation meets the performance laid out in the original design. Once the installation is signed off, the scheme’s SAP assessor will apply a reduced “in-use” factor to the SAP assessment for the installation, dropping the system’s specific fan power penalty from 40% to 30%, to help it meet compliance criteria.

As well as the approved contactor scheme, BEAMA is also promoting the adoption of a method by which a housebuilder gives a copy of the site-based checklist to the manufacturer on behalf of the house owner. This enables the manufacturer to begin a dialogue with the customer about replacing the filters and sending reminders about the unit’s service and maintenance requirements. The details of how the system would work, particularly the data protection aspects of the proposal, are still being thrashed out.