Authors and scientists Paul Francisco and Larry Palmiter proposed what they call "a new method for estimating duct leakage in residential homes" known as the nulling test. It is said to have several advantages over other common test methods.
According to the paper, "The Nulling Test: A New Measurement Technique for Estimating Duct Leakage in Residential Homes," the nulling test consists of two parts: measurement of unbalanced duct leakage and separation into supply and return components. As with the house pressure test, pressures across the building envelope are used. However, no assumptions are made as to the distribution of leakage within the envelope, and no equations are necessary.
"The test is predicated on the assumption that any change in the pressures across the building envelope after turning on the air handler is due to unbalanced duct leakage," according to the paper. "Unbalanced duct leakage can be viewed as analogous to either a supply fan (return-dominated leakage) or an exhaust fan (supply-dominated leakage.) As with ventilation fans, if another fan of the same flow but opposite direction is installed, there will be no net change in the pressures across the building envelope. A calibrated fan, referred to as a nulling fan, is then used to zero out the pressure change due to duct leakage."
The nulling test, according to the authors, makes no distinction between air that enters the building through registers or via leakage to the inside. The results are at operating conditions, "not as some test pressure that then needs to be scaled to an estimate of operating pressure."
The authors conclude: "The field test results are promising. There is no apparent bias relative to the best estimate, and the scatter is comparable to the duct pressurization test. All other methods tested have more scatter than either of these two methods. However the sample size of field tests is small, and further work needs to be done to determine the accuracy of the test over a wide range of homes."