Apparatus for removing residual water from a water system including a water supply pipe and a shut-off valve. The apparatus comprises a valve detachably mounted in the water supply pipe and including an air valve element controlling flow of supplied compressed air to the water supply pipe to blow out residual water from the pipe after the shut-off valve has been closed. The valve comprises a hollow body having an axis of symmetry with threads on the body for threadably engaging the pipe at a position therealong such that the body extends radially of the pipe and communication is established between the interior of the body and the interior of the pipe. A tool-engaging portion is formed on the body to allow the body to be threadably engaged and disengaged with the pipe by a tool. The air valve element is coaxially secured in the hollow body to control flow of compressed air to the pipe. The air valve element extends axially from the body and includes an actuating element to permit introduction of compressed air to the pipe via the hollow body.