The coin vent features special hygroscopic cellulose discs in the valve body. When dry the discs allow air to be released from the system. Once all the air is purged the discs would then come in contact with system water, where upon they would swell to create a positive shut-off. This continuous cycle prevents the radiators from becoming air bound and speeds the process of filling the system. The vent can also be manually shut-off by means of an internal ball check. The 417 coin vent is suitable for use in water and low pressure steam applications.