Mold can grow on the accumulated dust and foreign material that accumulates in supply and return duct work. If humidity level increase, mold growth within 24 hours inside air ducts. HVAC coils can also malfunction and push excessive moisture into the duct work, which will also cause mold growth. The final cause of mold growth in duct work are whole house humidifiers installed in HVAC systems; often these can malfunction and push too much moisture into a system.