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PLANNING AHEAD FOR RENEWABLE ENERGY The proposed revisions to Part L of the Building Regulations are due to come into force on 1 January 2006. Although at the time of writing the final version of the regulations has not been published, the versions issued for consultation suggest that the aim of the new regulations will be to reduce the carbon emissions of new buildings by about 25 per cent compared with the current requirements. There will also be a commitment to continue to reduce this target by a further 20-30 per cent every five years.

It is likely that on-site renewable energy, which may be a requirement of future Part L revisions, will be necessary for compliance. This will affect buildings' architecture at a fundamental level because, coupled with reducing emissions criteria, there will be a greater emphasis on improvements to the existing building stock.

This means that when buildings designed to today's apparently onerous standards are refurbished in 10, 15 or 20 years' time, they will need to be upgraded to meet more stringent requirements. Even if compliance with today's regulations can be achieved without any on-site renewables, it may therefore be necessary to retrofit such technologies as part of a future refurbishment.

Building-integrated photovoltaics, despite their long payback periods, are a popular sustainable option because they make a very visible statement of a building's 'green' credentials. The technologies available can produce a peak electrical output of up to 140W/m 2, which translates into an average daily output of 280 Watt-hours. A potential pitfall of building-integrated photovoltaics is the number of points of responsibility they have. The panels can be manufactured, formed into glazing elements, installed and connected electrically by four different organisations.

Biomass fuels, such as dedicated energy crops, are becoming more widely available for use in buildings as a fuel for heating-system boilers. Although such fuels are theoretically carbon neutral, delivery and storage need to be considered. Security of supply can also be an issue, and some of the low-carbon benefits can be undone if the fuel has to be transported over a long distance.

Solar water heating employs solar collectors, generally roof-mounted, to capture energy from the sun and use it to heat domestic hot water. Systems are most efficient if they are sized to generate no more than 60 per cent of hot water demand and therefore supplement rather than replace a conventional hot water system. Payback periods are typically 30-50 years but domestic-sized systems are available for under £5,000.

Small-scale wind power is another very visible technology, currently in its infancy, but with huge potential to improve the carbon performance of the UK's building stock. This country has the best wind resource in Europe and most of this resource is yet to be exploited. Unlike some of the other renewable technologies, this can be considered as very much a 'bolt-on' element.

AIRTIGHTNESS AND ITS CONSEQUENCES Pressure testing of buildings is a relatively new phenomenon.

Ten years ago few people in the construction industry had encountered the large fan rigs which are used to carry out the tests. But as regulatory improvements in U-values began to produce diminishing returns, and the huge energy wastage caused by leaky buildings became more widely appreciated, the government acted quickly.

Under current Building Regulations, non-domestic buildings of less than 1,000m 2 and all dwellings are exempt from building air-leakage tests as long as they use approved airtightness details. But the new version of Part L is likely to make pressure testing mandatory for all non-domestic buildings and a proportion of all residential developments.

The reason is simple. A leaky building equals a high infiltration rate, resulting in higher heating loads in winter and higher cooling loads in summer. This leads to greater fossil fuel consumption and higher carbon emissions.

Occupant comfort in a leaky building can become hugely compromised by this.

The construction industry is only beginning to get to grips with the problem. Who is responsible for making the building pass the test? Is it a design and detailing issue, or a construction one? In truth it is both - and can therefore result in contractual disputes.

The test itself involves connecting a very large trailer-mounted fan to an existing opening in the building facade, via flexible ductwork and a wooden template. The fan blows air into the building and a pressure difference builds up between the inside and outside. This pressure difference, and the flow-rate of the air are measured and from this data - and information about the size of the building - the air tightness of the building can be calculated accurately.

As well as institutions such as the Building Research Establishment and the Building Services Research and Information Association, a number of firms have sprung up around the country to fulfil the need, not just to carry out the tests, but also to provide a design-review and a site-monitoring service.

Appointing one of these independent consultants seems to greatly increase the chances of passing the test first time and reduce the potential for arguments over responsibility later. But many clients are unwilling to pay for an 'extra' consultant to carry out work they see as lying within the existing design and construction team's remit. This leaves architects to decide whether to employ a specialist or tackle the hazardous area of airtightness detailing themselves.

Many architects, already on tight fees, choose the latter course and end up learning about the consequences of poor airtightness the hard way.

But contractors are becoming increasingly aware of what is at stake and are frequently choosing to obtain specialist advice themselves.

Keith Horsley is an executive engineer with Hoare Lea Consulting Engineers AIR DISTRIBUTION - THE ARCHITECT'S GUIDE TO GRILLES AND DIFFUSERS An architect's ideal HVAC system would probably be completely invisible, writes Steve Price. Unfortunately for service engineers and architects, most systems require visible apparatus to enable conditioned air to be introduced into and circulated around a space. For the system to work properly, this has to be done in a controlled way, at the right velocity, in the right direction, and at the right place in the room. This is particularly true if the air is performing a heating and/or cooling function rather than just providing fresh air. Although the heating and cooling plant may be selected properly, poor air distribution can affect the system which controls the temperature and/or avoids draughts. This can lead to complaints from the occupants.

The most common location for diffusers is either in the ceiling or at high level in a wall. Diffusers in these locations rely on introducing a supply of air to the room at a controlled temperature in such a way that it mixes with air from the room to produce an even, comfortable temperature in the occupied zone. For a cooling application, the diffuser must be selected so as to ensure the air stream 'sticks' to the ceiling (the coanda effect), for long enough to entrain and mix with air from the room, rather than the cold supply air immediately 'dumping' straight down from the diffuser without mixing.

Supply diffuser selection is equally important in heating to avoid the warm supply air stratifying and remaining at a high level where it will not heat the occupied zone. It is necessary to introduce warm air at a high enough velocity to reach a wall or collide with the air stream from an adjacent diffuser, forcing the warm air down into the occupied space.

The selection of diffusers involves choosing the number and arrangement of the correct type and size for the volume of air that needs to be supplied to the space. It is important to have the correct 'throw', that is, the distance from the diffuser at which the velocity of the air has reduced to a standard value, and to ensure the selected diffuser does not have an excessive pressure drop or produce too much noise.

Since heating and cooling applications have conflicting air-distribution requirements, and most diffusers are used for both heating and cooling at different times, the process is inevitably a compromise.

The two most common types of diffuser for ceiling applications are the square, (also known as the louvre-face, or multi-cone diffuser), and the slot diffuser.

The louvre-face diffuser consists of angled slats which direct the air. They are suitable for rooms with moderate heating and cooling loads.

The slot diffuser is a linear element consisting of one or more slots. Each slot usually contains moveable blades which can direct the air and provide the ability to adjust the direction of discharge during commissioning to obtain the best performance. Slot diffusers can handle slightly higher loads than louvre-face models.

Swirl diffusers, available in circular or square versions, create a circular air pattern which has better mixing properties and therefore enables greater volumes of air to be introduced into a space without causing draughts. This is particularly useful where high occupancies or equipment use lead to higher than normal cooling loads. There are also adjustable versions of the swirl diffuser that use motors or expanding wax elements to adjust the direction of discharge and optimise heating and cooling performance.

Displacement diffusers work in a completely different way to the other types mentioned above. They aim to introduce air at a low level at a temperature just below that of the room and use natural buoyancy forces to allow the supply air to displace, rather than mix with, the air in the room as it is warmed by internal heat sources. Since there is no requirement to introduce the air in any particular direction or at a particular velocity, (other than low enough to avoid mixing), it is possible to conceal displacement diffusers behind a decorative front panel, provided that the panel has a sufficient free area to allow the air to flow through at a low enough velocity.

Another increasingly popular method of introducing air to a space is textile ductwork, sometimes known as fabric 'socks'. This consists of a fabric tube, either circular, semicircular or quarter-circular in cross section, suspended from a metal framework. The fabric is sufficiently permeable to allow air to flow through it, evenly along its length. The system can have cost benefits because it does the job of both ductwork and diffusers and, because it is a low velocity system, there are no problems with noise or draughts. Small plastic nozzles can be inset into the fabric.

These provide very good mixing and enable the system to handle very high air-change rates without creating noise or discomfort. This is not an invisible system, but with a range of colours available, it can brighten a room as well as ensuring it is well ventilated.

The importance of selecting the correct type of diffuser for the application cannot be overstated and, in all but the simplest applications, specialist advice should be sought at an early stage.

Steve Price is a director of Price Technical

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