Duct leakage testing
Duct leakage is caused by poor workmanship in fabricating ducting and subsequent rough handling during transportation and fitting. When a HVAC system is designed, the engineer calculates the amount of air needed in each conditioned space. These figures are then added together to work out the total volume of air to be distributed by an air-handling system. Theoretically, the system can be adjusted to this setting so that it distributes this precise air volume. In practice, however, the system must be over-sized to allow for duct leakage. This is no longer an acceptable practice, because additional air volume requires an increased energy consumption – and increased bills for business.
Affect on Business
Proper installation and sealing of duct systems can cut leakage rates from 15 or 20% down to almost nothing, resulting in substantial operating cost savings. For medium or high pressure design (500 Pa and up) SMACNA (Sheet Metal and Air Conditioning Contractors National Association) specifies “Total allowable leakage should not exceed 1% of the total system design air flow rate”. However, with the computer controlled duct manufacturing machinery today, a quality fabricated duct system, properly installed and sealed should achieve leakage rates as low as 0.5%.
Airlab recommends sealing all sections of a duct system, regardless of pressure classification. Airlab has special equipment that pressurises duct systems (generally done in sections) and then measures the leakage from the ducting under test. If the leakage falls outside the specified allowance then the contractor seals the leaks from the ducts and the leakage re-measured. This process continues until the leakage falls below the engineer’s specified allowable leakage.
The same equipment can be used to test builders-work shafts and plenums that are being used to transport airflow as part of the air conditioning and ventilation system. These often have significant leakage due to numerous penetrations required for piping and electrical reticulation.