The ductwork airtightness test is made by attaching a measured  airflow to the system and measure the static pressure inside. The idea is that the flow going in is the same as the leakage when the pressure is stable. The leakage flow is then divided by the ductwork surface area so that a big system can be compared with a small system. The procedure is similar to building airtightness test but with less flow, other pressures, other size criteria and limits.

Ductwork air tightness can be tested for certification and purchasing purposes described in:

• EN 12237 for circular metallic ducts
• EN 1507 for rectangular metallic ducts
• EN 15727 for technical components
• EN 17192 for nonmetallic ducts

and for a fully installed system in:

• EN 12599 for handing over a ventilation system

All standards have the same mathematical equations to define the limits for the airtightness classes. The limit is defined as the leakage flow at a certain pressure. The leakage flow is also defined at 1m² surface area so that a big system can be compared with a small system.

It is easy to calculate the surface area for a system which is simplified to the length of the centerlines at each dimension multiplied with the circumference. The standards have also taken a normal system into consideration and limited the amount of joints. If a system has an exceptional  amount of joints compared to a normal system the system is out of scope and can not be defined in the standards. The definition of normal systems is different for example in metallic and non metallic standards.

For single components it is more difficult to define the surface area because some products are small and some large. Here the area is defined as a virtual area with 1 meter duct on the product. It becomes even more complicated when a product has more than 2 joints which are different in size or the product has no joints like an access door.

The actual test is made by attaching a measured  airflow to the system and measure the static pressure inside. The idea is that the flow going in is the same as the leakage when the pressure is stable.

For certification of the product/system  the supplier has to state a span of pressures that the system will work and test several pressures between the limits. In the ready built system the test is preferably done at the pressure that is going to be in the system when used. It can also be tested at a predefined pressure like in Sweden  it is always 400 Pa positive pressure then it is easy to compare and perform.