When the thermostat is set for heating, the chilled water pump is off. The pressure from the hot water pump moves the check ball to seal off the water in the chiller tank. See Fig. 17-5. Now the water in the chiller system is dormant. The hot water pump circulates the water from the hot water generator through the air handler and back to the generator through the water reservoir. During the heating cycle the reservoir also serves as a place to relieve air from the system. The tube from the top of the reservoir passes through the chiller tank and runs up to the distribution pan, which is open to atmospheric pressure.