The combination brush is designed with two lengths of bristles. The shorter inner bristle is more rigid, helping to centralise the brush and also remove more stubborn deposits. The longer bristle is designed to reach into the corners of the ductwork where the shorter bristles don’t reach. The brush is fitted with a centraliser disc to improve air flow and improve the brushing effect. Brushes should be approximately 10% larger than the duct size so that they may rotate freely to remove dirt.