Problems with pump in cistern

Hello dear forum community,

I will try to describe my problem:

In the garden, I have a cistern with a capacity of about 2,000 liters that collects rainwater (about 2.50m deep, the "emergency overflow" is positioned so that the water level can only rise to about 50 cm below the top edge, i.e. to about 2.0m height). Now I have bought a standard garden pump to be able to pump out the water there from time to time. For this purpose, a hose about 5-6m long is connected to the pump, which reaches into the cistern. At the open end of the hose, there is a type of non-return valve with a mesh screen, which on the one hand prevents the standing water in the hose from flowing back into the cistern when the pump is switched off and on the other hand is supposed to prevent dirt from entering the pump. When the pump sucks, the non-return valve is "sucked open" and water can be pumped from the cistern.

This has worked very well several times, but now it no longer works. Apparently, the valve no longer opens correctly. However, the valve should not actually be broken, because a test with a rain barrel (same height as the pump) ran for minutes without problems. Without the valve, however, the water runs out of the hose so quickly that a test is not possible. And I somehow cannot imagine that the pump (bought at the beginning of the year) suddenly cannot reach the height anymore. The pump pressure during the rain barrel test was very good.

Does anyone have any idea what I can test or do? Or do you need more detailed / better information?

Thanks for suggestions Martin

Was the cistern full when it worked and now is it empty? Besides the pump's delivery pressure, there is a maximum suction height. Submersible pumps are more suitable for that. But if I think about it... with such a small height difference, it shouldn't be a problem. Possibly the hose needs to be vented, i.e., filled with water, before the pump works again, because it can pump water up but does not work with air in the system. Also something that submersible pumps do not experience.

The cistern was full when we installed the pump and for some time we were able to water perfectly with it. Then, due to the dryness, the cistern slowly became empty and the pump was "idle". When the cistern was full again, we turned on the pump in good faith and then nothing happened...

In the meantime, I have already tried various things, but nothing has worked permanently:

- I have filled the hose and the pump completely with water several times (and this amount of water was also pumped by the pump without complaint, but nothing else)

- Since we live on a hill and the cistern (the upper edge) is located on the upper part of the slope, I placed the pump (as far as the hose allowed) a little downhill, i.e. the idea was that the pump would be at the water level of the cistern (or below). That also worked very well for a short time but ended quite quickly again... although I cannot exactly estimate whether the water level of the cistern (which visibly had dropped in the short time) then fell below the standing height of the pump.

The technical data of the pump are 750 watts power, 7.5m suction height, 55 l/min flow rate, 4.7 bar delivery pressure)

ps: I had also considered a submersible pump; there was one recently at Lidl, but it said "Max. particle size of 8mm" and our water is quite turbid, so I did not dare to try it

8mm particle size is very large. Normally, you put a leaf filter in the gutter; I've also seen a tea strainer, which only lets through about 1/2mm. And then you can run it through a second filter, and if the cistern is sealed, the water inside is of drinking water quality, biologically purified by a light sediment layer.

Is the hose end even submerged in the water? The pump seems to be working... Is a stone stuck in front of the hose end?

The end of the hose is at least one meter underwater, the only thing that might be is that the end of the hose is not vertical in the water, if that can make any difference. I might have to try tying something extra to weigh down the hose or filter...