(a) DW/ Specification for sheet metal ductwork (low-medium- and high . shall be in accordance with the HVCA ductwork specification DW/ unless. The ASHRAE Fundamentals Handbook chapter on duct design and the SMACNA HVAC Duct Systems Design Manual contain additional information on . DW/ is recognised throughout the UK as the Standard Specification for ductwork manufacture and Smashing Cleaning Services LLC routinely work to ensure that our installations meet the standards in DW/ HVAC System Cleaning.
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The Building Regulations require ductwork leakage testing, with implications for designers, installers, and maintenance contractors. Air leakage from ducted distribution systems is an important consideration in the design and operation of ventilation and air conditioning systems.
Don’t Forget – TR/19 states that you will need a
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. 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 ew144 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.
dw1444 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 Athree per cent for medium pressure systems Class Btwo per cent for high pressure systems Class C and just 0.
For Class B, the permissible leakage rate is 0. 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. It is important hvacc 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.
Materials used should be suitable for the purpose intended and satisfy the specified pressure classification. 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.
DW standard – Pro-Duct Clean
Testing must be satisfactorily completed before insulation or enclosure of the ductwork and before any terminal units are fitted. 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.
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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.
The system designer may, for example, indicate that a particular system is tested as follows: The testing environment Leakage testing should always be done under positive pressure even when the ductwork is to operate under negative pressure.
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Duct work construction to DW/144
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Building regulations and ductwork leakage testing
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