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is not in use, these valves are open to the atmosphere, and radiators and pipes contain air. When a heating cycle begins, the boiler produces steam, which expands and displaces the air in the system. The air exits the system through the air-venting valves on the radiators and on the steam pipes themselves. The thermostatic valves close when they become hot; in the most common kind, the vapor pressure of a small amount of alcohol in the valve exerts the force to actuate the valve and prevent steam from leaving the radiator. When the valve

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