Why must the refill tube always be kept above the water level? Last Update: Sep 03, 2002

A slight pressure differential exists between the top (at atmosheric pressure) and the bottom of the tank water. With the refill tube discharging into the overflow tube below the water level, the higher pressure at the base of the tank refilling tube (“hush tube”) allows an internal siphon action to exist for water to continuosly flow (in a reverse direction), from the base of the hush tube through the refill tube. When this happens, the fill valve will continue to run to relplenish the water draining to the overflow tube. Clipping the refill tube at the top of the overflow tube avoids the internal siphoning problem.