Energy Efficiency Experts

The dominant duck leakage is test is used to determine whether or not the leaks in your ducting system are from your supply (out to rooms) or return (into system) portion of your duct system to the outside..

A blower door test is done under normal conditions.

Then the air handler to the HVAC system is turned on.

If the number on the manometer goes positive or above the current reading the there is a greater return leakage. The supply is putting more air into the space than it is taking out.

If the number goes negative or below the current reading the greater the supply leakage. The return is taking more air out than the supply can push out.

The dominant duct leakage test does not tell you how much the system is leaking. It simply identifies which element is stronger, supply or return. 

If no number difference is recorded the leakage between supply and return is equal.