Abstract: A venting and aerating arrangement for closed flow systems for water of fluctuating temperature is disclosed. A vent casing, defining an air collecting chamber, extends upwardly from a horizontal conduit of the system. A float controlled vent valve is arranged in the upper portion of the casing. A grid of spaced rods is arranged in the junction between casing and conduit and extends into the flow passage of the conduit to break up bubbles of air and other gases with the liberated gas ascending into the air collecting chamber. An improved valve construction is also disclosed. The valve has an exteriorly threaded valve body with an axially extending flow passage. A first axial extension on the valve body has a smaller diameter than the valve body proper. The valve body has a plane end surface forming a valve seat surrounding the flow passage. An exteriorly threaded cylindrical closure body of the same diameter as the valve body and having a second axial extension of smaller diameter is included in the valve.