Building Tight, Ventilating Right - 40
Ventilation, in all its forms, is about a lot more than fresh air. As homes become ever more airtight there is the irony that increasing thought has to be given to how they are ventilated, since a constant supply of fresh air is vital for the health of both the occupants and the building’s fabric.
Without ventilation, there will be a build up of condensation, pollutants and odours and the safe and sustained performance of some combustion appliances cannot be guaranteed.
Importantly, be the homes new build or retrofit, the ventilation has to be controlled: as the adage goes it is about building tight, ventilating right. Effective ventilation must be achieved by design rather than accident and the latest revisions of Approved Documents Part F (Means of Ventilation) and Part L (Conservation of Fuel and Power) of the Building Regulations, which came into force on 1 October, underscore this.
Part F and Part L are intrinsically linked explains Lee Nurse, chairman of TEHVA’s (The Electric Heating & Ventilation Association) ventilation committee and marketing director at Vent-Axia. “Both documents include a number of major revisions that include minimum energy efficiency levels for all ventilation systems. The launch of Part L’s new Domestic Building Services Compliance Guide highlights ventilation performance levels. Here for the first time a specific fan power requirement of less than 0.5 watt/sec is included for intermittent fans used in new build developments.”
To further lower dwelling emission levels, homes need to be increasingly airtight but not at the cost of good air quality. Changes to Part F include guidelines for airtight properties with infiltration rates tighter than 5m3hour/m2 at 50pascals. Where intermittent or passive stack ventilation systems are employed in airtight dwellings the guidance increases background ventilation rates by 50 per cent.
Nurse believes this looks set to cause some developers to re-evaluate their designs and move any new planning applications away from intermittent fans since the previous provisions in Approved Document F 2006 have already been difficult to achieve when using trickle ventilators in windows. “Our belief is that new regulations will clarify any grey areas to ensure that, as buildings become more airtight, ventilation levels are maintained,” says Nurse.
William Wright, energy and sustainability consultant at Inbuilt, is more circumspect. “The new Approved Document Part F gives many welcome revisions and guidance in specifying and commissioning. However, research on the old Part F has shown that ventilation issues were often ignored or poorly implemented in practice, causing indoor air quality problems. Good ventilation is as much about the implementation as the theory.”
Clearly housebuilders must now consider how the revisions will affect ventilation strategies and the impact on air quality and occupant comfort. As intermittent fans fall out of favour, changes to Part F and Part L look set to increase the uptake of continuous ventilation since it performs better in SAP (Standard Assessment Procedure), is easier to specify and easier to standardise, as trickle vents are not required.
In the short-term it seems likely that there will be increased adoption of whole house Mechanical Extract Ventilation (MEV) systems and decentralised Mechanical Extract Ventilation (dMEV) systems where individual fans in different rooms operate continuously. In the longer term, as airtightness requirements and Code for Sustainable Homes levels rise towards 2016, Mechanical Ventilation with Heat Recovery (MVHR) will become increasingly prevalent.
“MVHR or continuous extract can be advantageous in achieving current building regulations but MVHR will become practically indispensable in economically achieving the carbon savings for Code Level four,” says Wright. “As a broader understanding of issues around MVHR is gained by UK industry, it will more easily be designed into our buildings from the outset.”
Typically whole house, multi-room ducted MVHR systems combine supply and extract ventilation in one unit and use a heat exchanger to extract heat that would otherwise be exhausted to the outside.
Wright highlights noise, positioning of the unit and maintenance as key considerations. “If the fans are noisy, occupants may be inclined to try to turn the ventilation rate down or even turn the unit off.
The new domestic ventilation compliance guide gives particular attention to the issue in designing the ducts to be quiet. Ultimately, best practice may require baffling on the ducts as is sometimes used in Passivhaus and this can be quite bulky.
“With MVHR finding its way into apartments and smaller houses, due attention must be paid to where the unit is placed, with room for ductwork and consideration of noise issues. MVHR might need a dedicated cupboard in the dwelling if a particularly efficient, therefore large, unit is needed for Code compliance, although less efficient units can mount to the ceiling. The ducts to the outside air should be kept shorter for greater efficiency, which may mean mounting the unit in the roof void, assuming there is one,” explains Wright.
When it comes to maintenance there is one particularly worrying issue to consider. The NHBC Foundation review, ‘Indoor air quality in highly energy efficient homes’, states: “Recent BRE discussions with UK manufacturers of MVHR systems suggest that there is no market for replacement filters with several reporting no filter sales at all. This suggests that maintenance is not being undertaken – even at the most basic level.” Wright says air filters must be replaced regularly to maintain efficiency and prevent build up of pollutants. “The occupants need to know how to do this or it must be scheduled by the maintainers. Housing associations will need to think about the accessibility of MVHR units for maintenance. An operation and maintenance manual will be distributed to private occupiers, but how many will take on the obligation to maintain the system over the years?”
John Kelly, marketing manager at Airflow Developments, says MVHR units are becoming increasingly more efficient, recovering over 90 per cent of heat generated within the building that would otherwise be wasted. “This is usually the damp extract air from the wet rooms – kitchens and bathrooms – of a dwelling. What would normally be lost is, in fact, a valuable resource to warm the fresh, filtered incoming air from outside and distribute it to the living areas of a dwelling.”
MVHR reduces excessive moisture in the air so it combats condensation and subsequent mould growth, saving money on long and short-term maintenance and decoration. The resulting better indoor air quality also has the dual health benefits of reducing microscopic fungal growth and eliminating the conditions in which house dust mites thrive, both of which are linked to allergic reactions and asthma.
Kelly warns that care needs to be taken when selecting a heat recovery system. “It is important to ensure that this is combined with an air distribution system that enables it to operate with optimum efficiency. Ducting, pipework and fixings must be of high quality and installed correctly.
“Traditional methods like flexible ducting are easily torn, high on system resistance and are often squeezed around bends and between joists, further reducing air flow. Likewise, plastic flat ducting is often ill-fitting, with sharp bends causing dust traps, time consuming to install and wasteful of materials.”
Another key feature to be aware of is ‘summer bypass’ and, at Inbuilt, Wright warns that not all units provide this. “Summer bypass means that heat is not being brought back into the building when not needed. However, we have come across several MVHR units coming to the market that omit summer bypass as a cost saving exercise.
Summer bypass is not explicitly required by building regulations, and omitting it could cause significant overheating problems in summer leading to complaints from purchasers.”
Another point to consider is that, for the first time, Part F requires post-completion testing of ventilation equipment to ensure it not only delivers the required airflow, but does it efficiently and quietly. “Post-installation performance policing is critical to ensure air quality in increasingly airtight homes. This is especially important with the increased adoption of highly efficient ventilation systems, like MVHR, which require trained competent installers,” believes Vent-Axia’s Lee Nurse.
Traditionally it has been the electrical contractor who has installed ventilation equipment. One interesting consequence of the new Regulations is likely to be a change in the contractor base because, with the pipework required for ducting and the knowledge and calculations needed to comply with the Regulations, the skill set is much more akin to plumbing.
Bovis Homes is one housebuilder which often employs MVHR. Michael Black, group development director, says that the company has sought to ensure that it has the most efficient systems installed whilst, at the same time, ensuring their operation and maintenance requirements are not difficult for its customers.
“In fact, we’ve just completed drafting an installation guide with a major MVHR manufacturer to assist our site teams and subcontractors in ensuring that the systems are properly installed and commissioned.”
Roger Hunt.