Blowing hot and cold


Whether you believe the two things are linked or not, the vagaries of the weather this year have ensured that global warming is at the forefront of everybody’s mind, and people are finally treating it as a serious threat. Reducing carbon dioxide emissions that are produced by the consumption of fossil fuels is the only way we can slow down the effects, and that means making ourselves more energy efficient.

One of the biggest culprits in the consumption of energy in the UK is the heating of our buildings, and this has for some time been the focus of attention in the drive to cut carbon dioxide emissions. What is less widely acknowledged is the slow but steady increase in demand for air conditioning to maintain the level of comfort in our buildings. Ironically, this is a demand that could escalate precisely because of climate change, and that could prove to be just as damaging as heating in terms of energy consumption.

In recognition of this issue, the latest changes to the Building Regulations call for limits to the effects of passive solar gain through design. However, this could be difficult to achieve in a building that incorporates a large area of glass, for example, so other steps to manage the situation become vital. Improving the energy efficiency of ductwork is one of those steps, and this will naturally be an effective move whether the ducting is for heating or for air cooling systems.

The new Approved Documents L2A and 2B look at three different aspects of ductwork performance. The first issue tackled is that of the energy and carbon dioxide emission savings to be made from a change in ductwork insulation standards. The new Approved Documents refer both directly and indirectly to the new “TIMSA Guide to Achieving Compliance with Part L of the Building Regulations for Domestic and Non-Domestic Heating, Cooling and Ventilation” (the TIMSA Guide) for the thickness of insulation required on ductwork carrying either chilled or warm air.

The guide in question, produced by the Thermal Insulation Manufacturers & Suppliers Association (TIMSA), is available to download at www.timsa.org.uk/TIMSAHVACGuidance.pdf, and provides a single definitive point of reference for all duct and pipework insulation thicknesses. Figure 1 shows the thicknesses of insulation that the TIMSA Guide and hence the new Approved Documents require for Ductwork carrying chilled air.

Having good levels of insulation is an obvious aspect of improving the energy performance of ductwork, but it is only one issue. Air-tightness also has a significant effect on the overall energy efficiency of and carbon dioxide emissions from a ductwork system and this is taken into account by the second and third appearances of ductwork in the requirements of the new Approved Documents.

The question of airtightness is the main unavoidable feature of the new Approved Document L2A and appears as an option in ADL2B. This forms an important part of the calculation of the overall carbon dioxide emissions rate of the building, which is one of the fundamental requirements of the recent changes. One of the factors taken into account by the calculation procedure is that of ductwork air-leakage. The point of this second issue is that lower air leakage rates will improve energy efficiency, lower the carbon dioxide emissions rate of the building in question and therefore assist in the building complying with the requirements of the Approved Documents.

The third appearance of ductwork in the Approved Documents is as follows. The design standards set out in ADL2A state that in order to limit air leakage, ventilation ductwork should be made and assembled in accordance with the specifications given in HVCA DW 144 and tested in accordance with HVCA DW 143 on systems served by fans with a design flow rate greater than 1m3/s. It also includes those sections of ductwork where the pressure class is such that DW 143 recommends testing and the carbon dioxide emissions rate of the building design assumes a leakage rate for a given section of ductwork that is lower than the standard defined in DW 144 for its particular pressure class.

Furthermore, Approved Document L2A states that if a ductwork system fails to meet the leakage standard, remedial work should be carried out as necessary to achieve satisfactory performance in re-tests and further ductwork sections should be tested as set out in DW 143.

Clearly it is vital to reduce the risk of ductwork failing to achieve the designed air-leakage rate as this could lead to project delays and penalties and it is equally important to know what air-leakage rate can be assumed for the purposes of calculating the carbon dioxide emissions rate of the building. Pre-insulated ductwork is generally more airtight than the galvanised steel alternative and therefore at less risk of failure. However, exactly how airtight it is and what benefits might accrue from that need to be quantified.

Kingspan Insulation commissioned an independent study by the Building Services Research and Information Association (BSRIA) to examine the different air leakage rates of traditional galvanised sheet steel ductwork and pre-insulated ductwork systems and ascertain how much difference air tightness can make.

The test examined 100 metre long runs of ductwork with cross sectional internal dimensions of 600 x 600 mm. The results for pre-insulated ductwork show a reduction in air leakage and consequently flow rate of 11.6%, saving 31% of the energy required to run a fan in an HVAC system with a typical design flow rate of 7.5 m/s and hence also giving a 31% saving in carbon dioxide emissions. At lower flow rates there are potentially even greater savings to be made.

Further work by Bucknall Austin concluded that for a 100 metre long run of ductwork, with cross sectional internal dimensions of 600 x 600 mm and an air velocity of 7.5 m/s, the pre-insulated ductwork could save 3,100 kW/hr per annum, which equates to a saving of 1.3 tonnes of carbon dioxide emissions per year when compared with galvanised sheet steel.

In conclusion, pre-insulated ductwork can lower the risk of HVAC ductwork failing to achieve compliance with the air-leakage testing requirements of the new ADL2A and ADL2B. Furthermore, it can save over 30% of the energy required to run a fan in a HVAC system and hence will reduce the carbon dioxide emissions of the system.

The KoolDuct System of pre-insulated ducting from Kingspan Insulation is lightweight, takes up less space and can be installed up to three times faster than sheet metal ducting – not including the further time taken to lag the latter. KoolDuct will soon be available in a 33 mm thickness to suit the demands of the TIMSA Guide. So there’s no need to blow hot and cold over the new regulations, pre-insulated ductwork can help to meet the demands and keep temperatures and energy consumption on an even keel.

Kingspan Insulation has produced an energy white paper titled “Overeating Resources?” which outlines the findings of both the BSRIA and the Bucknall Austin reports. Full copies of the reports are available upon request from the Kingspan Insulation Marketing Department on 0870 733 8333.

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