Originally Posted By: Victor
Rainman is right on the money. When I have some water flowing through but not operating at siphon, I have to remove the vent cap and close a valve at the bottom. The water then fills the pipe and I put the vent cap back on. Then I open the valve and let the water flow. For some reason if I keep the vent cap in place while trying to allow the pipe to fill it never gets all of the air out of the system and the siphon doesn't start consistently.

Last thing - be very very careful around an open vent cap or the intake side of a siphon. It may have tremendously powerful suction associated with it.


If you cap the vent tube before letting the outlet pipe fill after being plugged, the trapped air has to be compressed. There is still air in the system unless the pond level is higher than the highest point in your crossover pipe, but it is a small enough volume that the weight of the water pulls it out when the high velocity flow of the siphon starts....the "inconsistancy" comes from the water flattening out and a large volume of trapped air causing a vacuum break.