Assessing the impact of pressure drop and leak distribution on ductwork airtightness measurements

Abstract

Ductwork airtightness test protocols implicitly assume that the pressure across the leaks is identical for all leaks. In fact, the pressure difference at the leaks decreases in absolute value as the distance from the moving air increases, due to friction and dynamic losses. This paper aims to quantify the impact of these pressure losses on airtightness test results, and thereby the measurement error generated by the constant pressure assumption. To this end, we developed an analytical model and performed measurements on an experimental set-up that reproduces residential ductwork at 1:1 scale with dynamic losses that can be modified using dampers. We compared the results obtained using the analytical model and the experimental set-up. The analytical model makes it possible to calculate the maximum “test length” for which the error due to pressure drop remains below 5%. According to this model, the impact of pressure losses on the measured flow rate is very small for airtight ductwork (Class C). In this case, the model shows that a length of 260 m between the measuring device and the far end of the ductwork can be tested at one time, even if the ductwork is quite narrow (average diameter of 200 mm) and the test pressure very high (1000 Pa). However, when testing very leaky ductwork (3*Class A) with an average diameter of 400 mm and a test pressure of 200 Pa, the distance between the measuring device and the far end of the ductwork must be no more than 67 m to limit the error to less than 5%. This model can be used to produce charts giving the maximum length to be tested as a function of the pressure drop and the expected ductwork airtightness class.