"Andrew" wrote in message
...
On 05/08/2019 09:05, Harry Bloomfield wrote:
Chris Green formulated the question :
Is there a size of pipe above which a syphon won't work because the
water empties out of the down pipe without 'sucking' the water above
down? It works well with small pipe because, I assume, surface
tension helps to prevent the water from 'dropping out'.

Correct!

Air has to be prevented from being sucked up the discharge pipe, by
having the discharge under water, or the syphon will be immediately lost
with a larger pipe.

My toilet cistern doesn't seem to have this problem

But doesn't have as large a pipe as you would use to drain that dam quickly.