This thesis is based on studies undertaken during the period 1998-2003 at the HVAC laboratory of Helsinki University of Technology.The thesis focuses on dust accumulation in, and removal from, recently installed supply air ducts and on the bristle behaviour of rotating duct cleaning brushes. The results of dust accumulation, measured using three different methods, were compared and the amount of dust in newly installed air ducts was evaluated. The vacuum tests was found to be an efficient method of collecting dust samples on the duct surface. The vacuum test and the gravimetric tape method gave approximatively the same results when the measured dust accumulation on the duct surfaces was 0.1-1.0 g/m2. The gravimetric and optical tape methods can be used to evaluate cleanliness of new ducts when the amount of dust accumulation is 0.1-1.0 g/m2.The effect of protection measures on dust accumulation was studied. The efficiency of two dry air duct cleaning methods applied to new air ducts was compared.A mathematical model to simulate the behaviour of a single bristle of a rotating duct cleaning brush was developed.