Don’t be beaten by technology

The UK has no shortage of technology to build low carbon homes, but do we have the skills to make the most of it? Richard Partington isn’t quite convinced

MVHR airflows: but with a ventilation unit in the loft space – convenient for the builder not the householder.
MVHR airflows: but with a ventilation unit in the loft space – convenient for the builder not the householder.

The house building industry has undergone great change, but are we prepared for 2020 when the industry must deliver zero carbon homes – or, by European definitions, ‘nearly zero energy homes’? These will be very different to those being built now. They will probably rely on mechanical ventilation and heat reclamation, and will have low carbon technology generating energy, space heating and hot water.
Improvements in airtightness and insulation have led to the widespread use of mechanical ventilation to provide background air that would previously have been supplied by ‘leaky’ fabric and trickle ventilators in windows. The technology is relatively new to the UK but concerns already exist that homes with mechanical ventilation are complex to maintain and operate; and that the systems may not deliver their full potential. There are also fears of inadvertently inviting problems like overheating and poor indoor air quality. 

Keep it logical

In theory, mechanical ventilation with heat recovery (MVHR) can deliver plentiful fresh air, with no draughts and barely perceptible background noise. However, the way designers incorporate this technology and the physical space given to it is critical for its correct operation. Ductwork must run in logical positions and co-ordinate with structural and building fabric elements. Coupled with space provision, an effective design will anticipate maintenance requirements, access and the position and ‘useability’ of controls.

MVHR is generously rewarded in compliance tool SAP, based on design assumptions regarding the efficiency of the fan pushing air around the home, the airtightness of the house, and the efficiency of the heat exchanger. These assumptions can all be compromised by poor design, installation or understanding of the system in use. To deliver the benefits, the design and installation of the unit and its ductwork must follow best practice, supported by rigorous commissioning. Failure to get all the components right can have more profound consequences than unrealised energy savings. If the system is costly to run or intrusively noisy the user may turn it off – inviting condensation, health and air quality problems.

The NHBC Foundation has supported several studies into the efficacy of mechanical ventilation and other potential causes of underperformance of heating and services in new homes (see NHBC NF 41). A guide will be launched this month looking at the implications for small builders if future homes are to incorporate these complex services.

The guide suggests that instead of fitting the services and plumbing around the shell of the structure, the house should be designed around the services and decided in tandem with structure and layout. Otherwise component parts get adapted on site, reducing performance.

... and rigorous

The whole question of design and efficiency of services is not restricted to ventilation. The Energy Saving Trust’s field trials into heat pumps (see links below) revealed high variability in technology performance, caused by factors including poor design, inadequate installation, and a general misunderstanding of how the technology interacts with ‘real-life’ demands of an occupied home. For a heat pump to work efficiently the designer must understand the external conditions and heat capacity of the ground (for GSHP); the efficiency of the exchanger and pumps collecting and distributing heat; the characteristics of the emitters (radiators and underfloor coils); likely demand profiles for heat and water; and the interaction with other potentially competing technologies such as solar thermal collectors. That’s quite a complex system and the homes that do perform well have invariably been delivered by a competent team of designers and energy consultants retained through the build process and supported by an interested builder and dedicated client applying rigorous quality procedures throughout – the sort of rigor promoted by PassivHaus advocates.

Unfortunately, much innovation has been trialled in the affordable housing sector where conditions for grant funding dictate higher levels of performance under the Code for Sustainable Homes. Feedback from the Carbon Control and Comfort group of academics, affordable housing managers and users suggests this sector may be less capable of engaging with the operation and controls that new technology demands. And the delivery mechanisms used for affordable housing may not supply sufficiently robust solutions for the home occupier. There is a general consensus in the UK that our low and zero carbon industries struggle to acquire the wide spread of practical skills necessary to deliver performance in the field. No doubt Ecobuild will showcase a bewildering array of new technology this year, but do we have the designers, installers and housing managers to make best use of it?

Designing Homes for the 21st Century is written by Richard Partington Architects for the NHBC Foundation

More information
Energy Saving Trust (2010). Getting warmer: a field trial of heat pumps.

DECC (2012). Detailed analysis from the first phase of the Energy Saving Trust heat pump field trial.

Zero Carbon Hub (Aug 2010). Carbon Compliance for Tomorrow’s New Homes. Topic 3: Future Climate Change.

Good Homes Alliance (2011). Ventilation and good indoor air quality in low energy homes: finding proven good practice.

NHBC Foundation, Low and zero carbon homes understanding the performance challenge NF 41: