Contractor focus
Building regulations and ductwork leakage testing
The Building Regulations require ductwork leakage testing, with implications for designers, installers, and maintenance contractors. Peter Rogers, Chairman of the B&ES Ductwork Group Technical Committee, explains
from ducted dis- tribution systems is an important consideration in the design and operation of ventilation and air conditioning systems. A duct- work system that has limited air leakage, within defined limits, will ensure that the design char- acteristics of the system can be maintained. It will also ensure that energy and operational costs are kept to a minimum.
Leakage testing
Building Regulation ADL2A for new buildings and regulation ADL2B for existing buildings state that leakage testing “should be carried out in accordance with the procedures set out in B&ES DW/144.” Recognised as the stan- dard against which the quality of ductwork manufacture and instal- lation should be measured, DW/144: Specification for Sheet Metal Ductwork stipulates that leakage testing of high-pressure ductwork is mandatory. For cost reasons, testing of ductwork designed to operate at low and medium pressures or velocities is required only when stipulated in individual job specifications. Air leakage from sheet metal ducts occurs at the seams and joints and is therefore proportion- al to the total surface area of the ductwork and related to the air pressure in the system. Although there is no precise formula for calculating the level of air loss, it is generally accepted that leakage will increase in proportion to pressure to the power of 0.65. As there is no direct relation- ship between the volume of air conveyed and the surface area of the ductwork system required to match the building configuration, it is difficult to express air leak- age as a percentage of total air volume. Similarly, the operating pressure will vary throughout the system and, as leakage is related to pressure, the calculations are
complex. It is generally accepted, however, that, in typical, good quality systems, the leakage under operating conditions will be in the region of six per cent for low pressure systems (class A), three per cent for medium pressure systems (Class B), two per cent for high pressure sys- tems (Class C) and just 0.5 per cent for the highest pressure sys- tems (Class D).
For Class C and D systems - with a maximum air velocity of 40m/s and static pressure limits of 2,000 Pa positive (compared with 500 Pa for Class A and 1,000 Pa for Class B) and 750 Pa nega- tive – permissible air leakage is only 0.001 and 0.003 litres per second per square metre of duct surface area. For Class B, the per- missible leakage rate is 0.009 litres per second per square metre and, for Class A, it is 0.027.
Considerations during design, manufacture and installation At the design stage, it is possible to forecast with reasonable accu- racy the total loss from a system by calculating the operating pres- sure in each section, the surface area of the ductwork in each cor- responding pressure section and the allowable loss at the operat- ing pressure for each section.
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intended and satisfy the specified pressure classification. There is more information about manu- facturing requirements, with detailed illustrations, in DW/144. At the installation stage, the extent of the ductwork to be test- ed and the method of selection (where not included in the design specification) should be deter- mined between the system design- er and the ductwork contractor. Testing must be satisfactorily com- pleted before insulation or enclo- sure of the ductwork and before any terminal units are fitted. The system designer may, for example, indicate that a particu- lar system is tested as follows: a) High pressure ducts – tested. b) Medium pressure ducts — 10 per cent of the ductwork should be selected at random and tested.
c) Low pressure – untested.
DW/144 specifies permissible leakage figures. It is important to note that system designers can achieve significant cost savings by matching operating pressures throughout the system to con- structional standards and appro- priate air leakage testing. This means the practice of specifying construction standards for whole duct systems based on fan dis- charge pressures may incur unnecessary costs.
In manufacturing, the integrity of ductwork depends on the suc- cessful application of the correct sealant, gaskets or tape and suit- ability for operating temperatures up to 70°C. Materials used should be suitable for the purpose
The testing environment Leakage testing should always be done under positive pressure even when the ductwork is to operate under negative pressure. This is because, if a duct under a negative pressure fails a leakage test, it is impossible to identify the leakage paths. It is also worth noting that there can be a risk of explosion if ductwork is blanked off prior to testing to prevent the spread of contamination if paint vapours have not fully dispersed from within an enclosed section of ductwork. B&ES members have cited examples of explo- sions where inspection lamps have shattered within the con- fines of a duct where cutting or drilling has caused sparks. To avoid the pitfalls of leakage testing and to find out permissible leakage rates and minimum acceptable accuracy of test rigs, DW/144 from B&ES Publications is an indispensable reference source with easy step by step pro- cedures and a sample test sheet.
HVR | July 2014 | 11
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