anyone got any ideas on why my rainwater tank pump wont supply water about 16 meters down a four mm line, yet it is powerful enough to shoot further than that out of a hose?
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Welcome to the Workshop community @MrRich. We trust that you'll get loads of helpful information, advice and inspiration for all your projects around the house and gardens from our wonderful members.
Let me tag the ever-helpful @TheSaltyreefer for you, although I suspect he will need a lot more information about your irrigation system setup in order to assist. Perhaps you could even share a couple of photos of your setup so we can see what you are working with?
Welcome to the workshop community @MrRich. My apologies for the slow response.
The issue could be the pump having a high flow but low pressure. To push water down 16 meters of 4mm pipe is a long distance and should generally be avoided if possible. For the main transfer line, I would recommend at least Holman 13mm x 25m Black Poly Irrigation Tube for moving water to locations. I would then branch off with 4mm line to where you need it with Poly Irrigation Barbed 4mm Joiners, keeping the lengths of 4mm as small as possible.
If you could add more information about what you are irrigating, then I would be more than happy to tailor a solution closer to your needs.