If any ducts are outside the thermal envelope, a duct blaster test must be conducted to determine the Total Leakage
of the system.
A duct blaster (or duct blower) is portable fan that is installed
on a duct system. The registers and grilles are sealed and the fan
is used to create positive pressure in the
ducts, forcing air out of any leaks.
To prepare for the test:
The fan is installed on the air handler compartment or
a central return grille (see photo at top right)
All of the other registers and grilles are covered with tape
or plastic "duct mask"
A pressure probe is installed in one of the supply ducts
A high-accuracy gauge is used that is capable of measuring both the amount of duct pressure (in units of Pascals
- Pa) and the amount of air flowing through the fan (in cubic feet per minute - CFM).
The fan speed is increased to attain a pressure of 25 Pa in
the ducts. Leaky ducts will require a higher flow rate to
achieve this pressure than tight ducts.
The result of a Total Duct Leakage test is a measurement of CFM25: the amount of air flow, in cubic feet per minute, required to
pressurize the ducts to 25 Pa of pressure.
Here is a video showing the steps of a Total Duct Leakage test:
The 2018 IRC/IECC specifies different leakage limits depending on the
test conditions. In all cases the limits are based on the
conditioned floor area (CFA) of the house:
