faulty water pump

[fred]

We store water in a large tank and access it via an electric pressure pump. THis type has an 8 litre water container mounted on it which the pump pressurises. Lately when a tap is turned on the pressure drops quickly. If the tap is turned off for a minute or two the pressure comes back but repeats the above. All this without the pump cutting in. Eventually the pump will cut in but this behaviour is maintained. If the pump cut in as soon as the pressure dropped I could understand it. What I cant understand is how it builds up pressure without the pump cutting in. Anyone any ideas.

[tabbypurr]

Presumably there's a pipe blockage somewhere.

NT

[Roger Mills]

Since water is pretty incompressible, I assume that there's a gas-filled accumulator in or next to the 8 litre container to provide some compressibility?

Sound to me as if there is a serious blockage between the container and the tap. With the tap closed, the pipe will be pressurised but the pressure will be lost as soon as you open the tap. When you close the tap, the accumulator will cause water to flow - slowly - into the pipe to re-pressurise it.

How many taps does this supply feed? If more than one, do they all suffer from the same problem? If there's only one tap - or only one with the problem - I would take it apart and look at its washer. Chances are that it's got detached and jammed in the works.

[harry]

ump. THis type has an 8 litre water container mounted on it which the pump pressurises.

s turned off for a minute or two the pressure comes back but repeats the ab ove. All this without the pump cutting in. Eventually the pump will cut in but this behaviour is maintained.

t. What I cant understand is how it builds up pressure without the pump cut ting in.

Yes. Or the accumulator has lost it's gas. It may have a diaphragm similar to pressure vessels in a central heating sy stem. Similar to this:-

[YorkshireDave]

replying to harry, YorkshireDave wrote: More than likely the pressure vessel diaphragm has deteriorated and split or it has simply lost its pressure - as tyres do because rubber is not leakproof. At one end of the vessel there is a point for testing and pumping up. Use a normal tyre pressure gauge and check it. I would estimate it should have around 2bar of pressure but the label on it should be accurate.

The point of the vessel is to give the initial push to the water and then the pump kicks in. The pump is started by a float activating a hall effect or reed switch. Occasionally these get some grit in and get caught so pull it apart and clean it out at the same time you check the pressure vessel.

If when you check the vessel you get water out of the air valve, then it's beggared and a new one (POTABLE TYPE) is needed. DO NOT put the cheaper version in, that's used on heating systems, as your water will be contaminated and it will last 5 minutes.

HTH

[harry]

The main purpose of the accumulator is to store a little water so that the pump is not constantly cycling on and off.

The water pressure switch also ahs a large hysteresis (often adjustable)to help with this too.

Cheap cold water accumulators are usually plastic lined steel to slow down corrosion. Expensive ones are stainless steel.