CHICAGO - The CASV is a demand controlled exhaust system used to vent boilers and water heaters. It maintains a precise draft by constantly adjusting the exhaust rate to meet current needs. According to the company, it can provide substantial savings over gravity systems and offer significantly better draft control. The chimney fan is installed at the termination point on the exterior of the building. The heating appliances will operate at a higher efficiency and standby losses can be kept at a minimum.