POOL SUCTION PROBLEMS

[Malboro]

Hi please help.

My swimming pool (pump) had developed a unique problem. I have spoken to a number of pool shops but none can really identify what the cause might be.

My pool works perfectly under backwash/waste/rinse but as soon as I put it on filter there is insufficient suction to drive the pool cleaner and not much flow through the jets. initial thought was there is a blockage/problem in the filter, but the problem persists even if I bypass the filter.

The flow in the pump is 100% under backwash/waste/rinse but only about 50% under filter/bypass.
Thought that there might be a block in the return pipe, but surely the pump will still fill up?

Have done all the necessary checks in terms of leaks, seals etc. the multi-port valve is also new and replaced the seal.
Haven't yet dug up the underground pipe work though but that is my last resort unless someone has any suggestions.

[Denniswiseman]

Does it work OK when not vacuuming
Does the pump basket lose water when on filter, backwash or rinse