Taco 418 Hy-Vent 1/2" Female x 3/4" Male Float Style Automatic Air Vent, 150 psi Installed on water or stream radiators the Taco automatic coin vent is the ideal air purging device 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