Up to 25% to 30% of heating and cooling can be lost through improperly sealed and leaky ducting.  High-efficiency furnaces and air conditioning can offer only mediocre efficiency performance if the ducts that they are attached to are delivering the conditioned air to where it isn't supposed to go.

Duct testing is accomplished through using a duct blaster fan and a blower door.  The duct blaster is attached to the ducting or plenum of the furnace and similar to a blower door test uses either pressurization or depressurization to determine and locate the amount of leakage that is within the duct system.

With this diagnostic tool, we can determine not only the total amount of leakage in the duct system but also whether the leak is outside of the thermal envelope (paying to heat/cool the outdoors).

A high-performance HVAC system should have very few duct leaks.  This test can determine whether the HVAC contractor completed their job properly to ensure that the system performs correctly or if they just sold a furnace and air conditioning system.

Efficiency certification programs and 2009/2012/2015/2018/2021 IECC code compliance require duct testing and stated levels of permitted duct leakage.

Has this HVAC system been performance-tested?  That is another fair question to ask a builder or homeowner prior to making a purchase decision.  Utility bills are an ongoing and long term proposition which factors into the overall "cost" of a building.