In Sweden, according to the trade guidelines VVS AMA, on-site pressure tests need only cover 10% of the duct surface area round ducts, and 20% for rectangular ducts. Denmark has similar rules. In Finland, 20% of the duct surface area is tested in the case of Class C airtightness, and 10% in the case of Class D or better.
All parts of the duct system are equally important in terms of potential air leakage. In the main duct, near the fan, the operating pressure is higher than the rest of the duct system, so leakage points there have a higher leakage flow rate. On the other hand, field studies of leaky duct systems have shown that most of the total leakage flow occurs in branch ducts near the terminals. This is because branch ducts constitute a very large surface area compared to the main duct, and branch ducts tend to have more joints between components (e.g. tee junctions, VAV-boxes, flexible ducts, plenum boxes) which creates numerous opportunities for leakage [see reference 24 in Paper P187 below].For the above reason, all parts of a duct system should be subject to leakage tests. To save time, one can test select multiple test sections at random (each of approx 25 m² and never less than 10 m²), constituting a small fraction of the total duct surface area. Test sections can be isolated from the rest of the duct system by e.g. pumping up an air bladder (balloon) with a hand pump.
For more information you can check-out the following:
- Information Paper P187 “Duct System Air Leakage — How Scandinavia tackled the problem” by P.G.Schild & J.Railio, on the BUILDUP website
- Webevent presentation “Duct System Leakage Testing — Methods, requirements, handing over” by P.G.Schild, available on the website www.asiepi.eu