A duct leakage tester is a diagnostic tool designed to measure the air tightness of forced air heating, ventilating and air-conditioning (HVAC) ductwork. A duct leakage tester consists of a calibrated fan for measuring an air flow rate and a pressure sensing device to measure the pressure created by the fan flow. The combinations of pressure and fan flow measurements are used to determine the ductwork air tightness. The air tightness of ductwork is useful knowledge when trying to increase energy conservation.
Uses of Duct Leakage Testing
Duct leakage testers are used in residential single family, residential multi-family and commercial buildings that have a forced air delivery systems for heating and cooling.
How Duct Leakage Testers Work
A basic duct leakage testing system includes three components – a calibrated fan, a register sealing system and a device to measure fan flow and building pressure. Supply registers or return air grills are sealed using adhesive tapes, cardboard, or non-adhesive reusable seals (Vent Cap Systems). One register or return is left unsealed. The calibrated fan is then connected to that unsealed register. Pressure is monitored in one of the branches of the ductwork while the calibrated fan delivers air into the system. As air is delivered into the ductwork, pressure builds and forces air out of all of the holes in the various ductwork connections or through the seams and joints of the furnace or air-conditioner. The tighter the ductwork system (e.g. fewer holes), the less air you need from the fan to create a change in the ductwork pressure.
A duct leakage test can be performed by either pressurizing or depressurizing the ductwork. Ductwork that is outside the building envelope, meaning in an unconditioned attic or crawlspace, should be pressurized so as to not bring in unwanted contaminates.
Duct tester air tightness measurements are presented in a number of different formats including but not limited to:
Air Flow (CFM)
CFM25 is defined as the air flow (in cubic feet per minute) needed to create a 25 Pascal pressure change in the ductwork. CFM25 is one of the most basic measurements of ductwork air tightness. A pressure of 25 Pa is equal to 0.1 inches (0.25 cm) of water column.
Leakage area estimates are a useful way to visualize the cumulative size of all leaks or holes in the ductwork. There is a variety of standard calculation methods used to calculate leakage areas.
Commercial and industrial ductwork is often tested to standards developed by the Sheet Metal Air Conditioning Contractors’ National Association, SMACNA. Ductwork is subjected to higher pressures and then given a rating or classification rather than a leakage estimate.