This paper presents the results of a field study on five HVAC duct systems in France. The principal objectives of this work were a) to evaluate the extent of duct leakage in those systems; b) to compare design and actual fan flow rates; c) to evaluate the thermal energy losses in the air ducts. The ductwork airtightness, measured by fan pressurisation method, was found to be in general well above the leakiest EUROVENT 2/2 class (class A), yielding unacceptable uncontrolled airflows. Fan airflow rates were measured with the constant injection tracer gas method. In one case, the actual airflow rate was found to be only 30% of the design airflow rate. Temperature changes along the ducts were measured with autonomous data loggers installed in the ductwork, typically for a period of 3 days. Average delivery effectiveness were found to be typically on the order of 65% to 80%. This field study shows that significant energy losses can occur in HVAC ductwork systems and that some of the observed deficiencies can result in comfort issues.