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
Air leakage from ducted distribution systems is an important consideration in the design and operation of ventilation and air conditioning systems. A ductwork system that has limited air leakage, within defined limits, will ensure that the design characteristics of the system can be maintained. It will also ensure that energy and operational costs are kept to a minimum.
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 standard against which the quality of ductwork manufacture and installation 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 proportional 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 relationship 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 leakage 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 systems (Class C) and just 0.5 per cent for the highest pressure systems (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 negative – permissible air leakage is only 0.001 and 0.003 litres per second per square metre of duct surface area. For Class B, the permissible 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 accuracy the total loss from a system by calculating the operating pressure in each section, the surface area of the ductwork in each corresponding pressure section and the allowable loss at the operating pressure for each section. 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 constructional standards and appropriate air leakage testing. This means the practice of specifying construction standards for whole duct systems based on fan discharge pressures may incur unnecessary costs.
In manufacturing, the integrity of ductwork depends on the successful application of the correct sealant, gaskets or tape and suitability for operating temperatures up to 70°C. Materials used should be suitable for the purpose intended and satisfy the specified pressure classification. There is more information about manufacturing requirements, with detailed illustrations, in DW/144.
At the installation stage, the extent of the ductwork to be tested and the method of selection (where not included in the design specification) should be determined between the system designer and the ductwork contractor. Testing must be satisfactorily completed before insulation or enclosure of the ductwork and before any terminal units are fitted.
The system designer may, for example, indicate that a particular 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.
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 explosions where inspection lamps have shattered within the confines 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 procedures and a sample test sheet.