Over-draft Control - Green Sustainable Venting Designs The MODS, Modulating Over-Draft Damper System™ is a demand controlled exhaust system that's designed for buildings that experience excessive chimney draft, which impacts the ability of the boilers and water heaters to operate efficiently. It maintains a precise draft by constantly adjusting the exhaust rate to meet current needs. The concept can provide substantial operating savings over gravity systems and offer significantly better draft control which reduces emissions. The MODS System can be combined with an MCAS, Modulating Combustion Air-supply System™ and can also work with a CASV/CASI, Chimney Automation System™