Heating or Air Conditioning Duct Ice-up, Duct Flood Damage, Duct Water Leaks & Mold. WATER & ICE IN DUCT WORK - CONTENTS: Sources & causes of water or condensation inside or on the outside of air conditioning ducts. Frost or ice build-up in heating or air conditioning ducts is often mistaken for a roof leak. Central HVAC humidifiers may also leak into air handlers and ductwork.