MVHR for architects: Including it early in the design
As new build dwellings become increasingly air tight to reduce energy usage Mechanical Ventilation with Heat Recovery is growing in popularity. Richard Porteous, of Built Environment Technology Ltd, discusses how important it is to consider MVHR when designing the structure of a building to ensure the best outcomes.
Since the introduction of Part L of the Building Regulations (Conservation of Fuel and Power), Mechanical Ventilation with Heat Recovery (MVHR) has been growing in popularity. In the last few years as it has become increasingly tricky for housebuilders to reach Dwelling Emission Rates this growth has gained momentum with BEAMA reporting 24,000 MVHR fans were installed in the UK in 2012. In fact, the Zero Carbon Hub’s VIAQ Task Group states that MVHR use will continue to grow becoming the dominant form of ventilation in most new homes post-2016.
Even though the last amendments to Part L of the Building Regulations gained a mixed reaction with only a 6 per cent uplift in efficiency standards for new homes compared to 2010 standards MVHR still gained a significant boost with the introduction of the FEES (Fabric Energy Efficiency Standards). FEES focuses on the efficiency of the fabric of the building. Fabric first is the first stage in delivering an efficient home. Once a building is airtight, the focus then needs to change to ensure that the ventilation delivers the required indoor air quality to maintain people’s health, bringing MVHR to the fore.
With this fabric first agenda comes a move from intermittent to continuous ventilation since within airtight thermally efficient homes it becomes increasingly important to ventilate effectively and efficiently to ensure good indoor air quality. This is set to further drive the adoption of continuous Mechanical Ventilation with Heat Recovery (MVHR).
With this swing towards centralised MVHR systems an increasing number of architects and specifiers will be considering this as a ventilation strategy. However, it is important to gain expert advice from a ventilation specialist when specifying MVHR to ensure the best outcomes. Unlike intermittent unitary fans, whole house ventilation systems cannot be specified towards the end of a project because their ductwork needs to be considered in the structural design of a property.
Unfortunately some of the latest CPDs for architects and designers advise designing around the services. For electrical services the space can usually be accommodated by architects designing around them. However, for whole house mechanical ventilation systems this is a problem, since it is not possible to design around MVHR systems without detailed knowledge of duct sizes or ventilation design drawings. Moreover, the duct size is determined by the airflow calculations for Building Regulations Part F, which must be in accordance with the Designed Air Permeability of the envelope, so the message really is to think about MVHR as early as possible.
Similarly some new build projects forge ahead specifying MVHR as the ‘right’ choice for a thermally efficient property without thorough consideration. However, although it is quite easy to select MVHR as an option, it is more difficult to specify it correctly. Once MVHR has been selected, thus ticking the ventilation box, it seems in some projects it is quite forgotten until installation. In the meantime timber or eco joists are installed in a home without considering how the ductwork interacts with them. It is often not until it comes to installing the MVHR that a newly appointed ventilation specialist warns the developer that the joists do not allow enough space or angle for the MVHR ductwork.
As MVHR gains in popularity Posi and Eco joists are increasingly specified alongside since they offer easy access for the installation and maintenance of services. These types of “open-web” joists combine timber with aluminium struts in the middle allowing space for the ductwork to go straight through. However, these joists are not the silver bullet for MVHR installations which allow designers to ignore the detail in their ventilation strategy. Although these joists are a simple idea which can be effective, designers need to ensure the joists line up and are properly installed otherwise the ductwork will not fit through. Also designers sometimes specify the joists too small and so it is necessary to compromise by opting for100mm ductwork; unfortunately this then can then require an additional MVHR unit, with commensurate additional costs.
As well as considering joists it is essential to select the correct type of ducting for an MVHR system. To ensure the correct ventilation rates are achieved it is important to use rigid ducting for the majority of an MVHR installation and to minimise the use of bends. When installing ducting it is important routes take the path of least resistance. Plus it is important to size ducting correctly to suit the system you are using.
Rigid PVC ducting is probably the best value and easiest to install (though an anathema to Green specifiers). In contrast rigid galvanised metal ducting can be more costly, it is unwieldy and needs cutting with an angle grinder. In line with Building Regulations the use of flexible ducting must be kept to a minimum and any lengths must be short and taut. Flexible ducting may at first seem an easy installation option especially when space is tight but it is not recommended since it can be easily crushed and is highly resistive – these can be fatal issues.
Again advice on ductwork specification should be left to a ventilation expert. Often a specialist ventilation contractor is not specified by the designer on the drawings since the expectation is that the main contractor will deal with the subcontracting of the mechanical services. Unfortunately leaving the services to this late stage can cause problems such as noise issues and, at worst, a system that cannot be Commissioned for Building Regulations. Meanwhile, without detailed drawings for the MVHR systems at the design stages often developers can find SAP calculations can fail to add up at the end of the project, causing costly alterations to be made. To ensure the best outcome it is important for architects and designers to seek expert ventilation advice at the design stage.
About Richard Porteous
Richard Porteous is Senior Projects Manager at Built Environment Technology Ltd.