HVAC IN A CHANGING CLIMATE
In its 2003 Energy White Paper, the government set itself the target of reducing CO 2 emissions 60 per cent by 2050.
Buildings contribute more than 45 per cent of the UK's total CO 2 emissions, so tough energy-efficiency standards for new buildings were inevitable.
These arrived in Approved Documents L1A (for dwellings) and L2A (for buildings other than dwellings), which came into effect in April this year.
The country's building stock is renewed at a rate of around 1 per cent per year.
Only 4 per cent of commercial and industrial buildings in England and Wales are less than three years old, and more than 80 per cent were built before 1990. It is clear that to make inroads into reducing buildings' carbon emissions, something has to be done about improving the energy-efficiency of existing buildings as well as of new ones.
So now we have Approved Documents L1B and L2B.
Rather than imposing standards for all building owners, these assess and control the carbon emissions of existing buildings when they are refurbished or extended.
Designers working on today's of-fice refurbishments are likely to be dealing with 1960s, 1970s or early 1980s buildings. Many will be poorly insulated; almost all will contain energy-hungry HVAC systems that may have been badly maintained. None were designed for the amount of heat-producing IT equipment that is now common and a large number will have limited floor-to-ceiling height.
Satisfying a developer who wants buildings like this turned into office space of the quality (and rental level) of new buildings, for minimum capital outlay, is hard enough, but now designers also have to satisfy Approved Document L2B's CO 2 emissions targets and the little understood spectre of 'consequential improvements'.
CLIMATE CONTROL Many alternatives are available for conditioning an office building, but for refurbishments, the constraints of the existing building can drastically reduce the number of practical options.
One could start by questioning the need for mechanical ventilation and cooling, and try to engineer natural ventilation. This is a tough challenge in a new building, even if the client is prepared to accept a less rigid temperature specification than the 24¦C or even 22¦2¦C beloved of letting agents. But in an existing building, all sorts of factors can conspire against natural ventilation: the depth of the oorplate; a slab soffit that cannot be exposed easily or safely; a low floor-to-ceiling height; high solar gains; a shortage of windows; or even windows that don't open.
Many of these obstacles can be overcome. Ventilation shafts can be formed through the oor slab in deep-plan areas to provide a natural path for exhaust air up to a roof-level terminal (companies such as Monodraught and Passivent offer such systems); fixed windows can be replaced with opening lights; external solar shading can be added; entire facades can be replaced with a more highly engineered system to reduce solar gains.
Another alternative - and one that can achieve 24¦C on a warm summer's day - is a mixed-mode system. This allows natural ventilation to be used when it can maintain acceptable conditions, with the back-up of a mechanical system that comes into operation when necessary.
Even at the height of summer, natural ventilation systems can contribute by providing nighttime cooling to cut the loads on the mechanical cooling system during the day.
Such systems can offer big energy savings without making too much of a sacrifice on temperature control. Mitsubishi has joined forces with Passivent to market 'mixed-mode cooling' systems. The companies know that controls are key to such systems realising their potential energy savings; poor controls could increase energy use if the cooling comes on when the windows are open.
Unfortunately, it is often pretty much impossible to design a natural or mixedmode ventilation system for an existing building. When the brief or building constraints indicate the need for a mechanical, indoor system providing heating, cooling and ventilation, the options will include: a four- or two-pipe fan coil system; a displacement ventilation system, with or without chilled ceiling panels or chilled beams; a variable refrigerant volume system; or under oor air-conditioning.
Each system has its own requirements in terms of oor and ceiling void depth. If, in an existing building, these are -xed, it will not be possible to install certain systems.
A system based on displacement ventilation is widely considered the next best option after a natural ventilation system, because of its good ventilation effectiveness and energy ef-ciency. But such systems require a deeper oor void than is found in many older buildings. Increasing the void may not be an option and the alternative, oor-standing displacement diffusers, take up valuable space.
If a reasonable oor void can be achieved but there is not enough space for chilled beams or ceilings, then underoor airconditioning systems, available from companies such as Hiross and Denco, could be an option.
Floor-standing airconditioning units (CAMs), supplied with chilled and lowtemperature hot water, and with fresh air from a central air handling unit, are located throughout the office space.
The oor void is divided into supply and return-air plenums. The CAMs draw air from the return-air plenum and deliver conditioned air into the supply plenum. Fan terminals are set into the oor in the supply plenums and introduce air into the space above, regulated by a control system.
Fans supply air at temperatures as low as 14¦C, so have to be positioned away from desks to avoid causing cold draughts.
Despite the alternatives on offer, chilled-water fan-coil systems will probably be with us for some time. Such systems have a reputation for being energy-guzzling monstrosities that provide poor air quality.
But there are ways to mitigate the poor energy performance to assist in complying with Part L.
Variable-volume chilled water and low-temperature hot-water systems, using inverter drives in conjunction with two-port control valves, are becoming more common and are much more efficient than constantvolume systems. Several manufacturers, such as Grundfos, now produce pumps with integral inverter drives and controls to match the speed of the pump to the heating or cooling demand.
Much energy consumed by fan-coil systems goes to producing chilled water for cooling. This can be reduced by adding a free cooling facility to the chiller (particularly where there is a high wintertime cooling load, such as in deep-plan buildings) or through using a ground- or watersource heat pump in place of an air-cooled chiller.
Fan-coil systems are big energy users because each unit contains a fan that runs continuously when the system is on. Each fan only consumes a few dozen watts, but over the whole building this adds up.
Manufacturers such as Ability have developed electronically commutated fan-coil motors that run on direct current and are more efficient than their conventional counterparts; the motors consume less energy and contribute less heat gain to the airstream. The fans can also be set to switch off when the unit is in its dead-band between heating and cooling.
IN SEARCH OF HEIGHT One of the eternal conflicts on refurbishments, particularly in 1960s, 1970s and 1980s buildings, is between raising the ceiling to give a sense of space and lowering it to house building services in a costeffective, easy-to-maintain way.
To address this problem, most manufacturers of fan-coil units produce a special 'slimline' unit. Some, such as those from Benson Environmental, are now so slim (around 175mm from top to bottom), that it is not the fan-coil unit that causes problems with the necessary ceiling void depth, but the ductwork and pipework - particularly condensate drainage pipework, since this has to be laid to falls.
There is an important relationship between the number of vertical services risers provided and the depth of ceiling void required. Generally, the further from a riser a duct or pipe has to run, the larger it will be; the further, in the case of condensate, it will have to fall; and the more likely it is that it will cross over or under another duct or pipe, hence requiring a deeper ceiling void.
Often, when floor-toceiling height is particularly tight, there is a desire to dispense with the ceiling void altogether. The obvious answer is to use floor-standing perimeter fan-coil units, but this reduces the floor area available for letting and can lead to poor air distribution. A compromise in relatively shallow spaces is to place fan-coil units in a ceiling bulkhead around the core area and step up to a higher ceiling towards the perimeter.
Another way to deal with this problem is to use freely suspended multi-service beams, available from companies such as Trox. Although the height to the underside of these units may be relatively low, the fact that the ceiling is visible above and in between them can create an illusion of greater height.
CONSEQUENCES OF L2B Most refurbishment projects will fall into the categories covered by Approved Document L2B, which are: an extension, material alteration or change of use; the provision or extension of a controlled service or fitting; or the replacement or renovation of a thermal element.
There may be a requirement to carry out 'consequential improvements' to the existing building in cases where work involves: an extension to a building greater than 1,000mless than or equal to; the initial provision of a fixed building service; or the increase in capacity of an existing fixed building service.
To avoid increasing the capacity of existing cooling systems, it is worth examining the building's cooling loads in detail. Buildings designed 20 years ago are unlikely to have allowed equipment heat-gain values sufficient for today's offices. However, their lighting heat-gain allowances are likely to have been considerably higher than is necessary with today's lighting. This, coupled with measures to reduce solar gain, could allow consequential improvements to be avoided.
Works that qualify as consequential improvements include: replacing existing heating, cooling or air-handling systems or controls that are more than 15 years old; upgrading lighting systems to improve their energy efficiency; installing additional energy metering from companies such as ABB; upgrading windows or thermal elements with U-values worse than threshold values; and increasing on-site, low and zero-carbon energy-generation systems.
Spending on consequential improvements should be 'not less than 10 per cent of the cost of the principal works', but nothing need be done that is not 'technically, functionally and economically feasible'.
Note that the 10 per cent cost cap does not apply when installing a fixed building service for the first time.
This part of the Approved Document contains much that is confusing and open to interpretation. For example, how do you establish U-values in an existing building where no as-built information exists, to ascertain whether they meet the threshold values?
The requirement to spend 'not less than 10 per cent of the cost of the principal works' has also sparked debate. Some interpret this to mean that if doing all the things on the listof potential consequential improvements applicable to the building costs less than 10 per cent of the overall cost, you need spend nothing at all.
Others advocate examining the content of the principal works to see if anything you were already planning to do could contribute to the 10 per cent of consequential improvements.
The best advice is to get to know the Approved Document and consult your Building Control body at an early stage to avoid being hit with unexpected costs later.
Keith Horsley is an associate at Hoare Lea Consulting Engineers