a pump , siphon drain valve and float controlled water inlet valve are located in a comparatively small water supply tank which is mounted below an opening formed in the floor pan of an evaporative cooler to locate the cooler &# 39 ; s water supply externally of its cabinet and are operative to recirculatingly supply water to the cooler and receive the returning water . the water , which increases in mineral concentration during such recirculation , is periodically drained from the tank by shutting off the pump so that the returning water will elevate the water level in the tank to prime the siphon drain valve . when the pump and water inlet supply are both shut off , the tank completely drains , and when the pump alone is shut off , tank draining , flushing and refilling is accomplished .