The invention relates to a volatile-liquid-filled, encapsulated diaphragm valve for the thermal control of condensate water drains. The capsule consists of two unhardened turnings which are welded together, the lower capsule part having a threaded stem to be screwed into the bore of the condensate drain. In the outlet passage in the threaded stem of the lower capsule part there is press-fitted a hardened valve seat body having valve seats at both ends, the seat cone on the outlet side, which cooperates with a check valve body, being prolonged in the outlet passage all the way to the end of the threaded stem, terminating in a narrow edge. The entire conical flare forms the chamber for the movement of the check valve body, which is limited by a sieve plate. Above the threaded stem the capsule bottom part's diameter reaches all the way to the diaphragm clamping zone, while the lateral inlet ports in the bottom part consist of bores having an inside diameter corresponding to the wall thickness.