In our building we have a large water tank located on the ground floor and a pump (this one: https://www.pedrollo.com/public/allegati/DG%20PED_EN_50Hz.pdf ) that feeds water to the smaller tanks (one for each apartment) that are located on the roof.

This pump is consistently activated every 5 seconds when it is set to its 3 bar setting. It does that 24/7. When I increase the setting it is activated more frequently and when I decrease the setting less frequently. After it is activated it runs for about 5 seconds and stops. It doesn't appear to overheat because when it stops the LED panel lights up as expected, showing the pressure that was reached.

This pump also feeds the water that goes directly to the apartments (the water that doesn't go to the roof tanks). This direct water correctly responds to the pump's setting which means that the pump correctly regulates the pressure.

Right after the output valve of the pump the water meters for all apartments are installed. When the pump is working I can see that none of the water meters are moving, which means no water is actually moving when the pump is working. (Of course, there are cases where some water meters are moving when that apartment's tank is being filled).

When I close the output valve of the pump it stops being activated.

The possibility of leaks is excluded because if the pump was filling leaks we would see some water meter moving.

The possibility of a failed input valve to the pump is excluded because if this was the case the pump would still activate when its output valve was closed.

The possibility of the air bladder being bad is excluded because the water pressure that the pump regulates appears correct.

Now, all the above conclusions are my non-plumber opinions. My question is: What could be the reason that the pump is activated so frequently?

2 Answers 2


"The possibility of the air bladder being bad is excluded because the water pressure that the pump regulates appears correct."

That is a false conclusion.

The symptoms are pretty much exactly consistent with a failed air bladder. The water pressure is controlled by a pressure switch. If the pressure switch is activating after 5 seconds of operation, it's because there is not sufficient pressure compliance (the function of the compressible air in an air bladder tank.) Operating in this way will lead to a short life for your pump. The bladder tank does not control the pressure. The switch which is activating every 5 seconds does.

With no functional air bladder, the amount of water needing to move to drop the pressure is very small - so small you might well not be able to detect it on the meters. The function of the air bladder is to make it possible for more water to move without the pressure immediately crashing, as it does with no functional air bladder. A properly set & functioning air bladder with a properly set pressure switch should support 60 seconds or more of water use before the pump switches back on.

  • I mostly agree, the OP needs to get a pressure gauge...maybe one of those you can attach to a hose bib someplace, to determine what's really happening. A failed bladder in a pressure tank is not uncommon and will def lead to short cycling when water is being used. Another major possibility is a failed check valve (foot valve) which allows water to flow back into the well. We need to know more about the setup, type of well, type of pump, plumbing schematics, etc. to provide a better answer. + Jul 8, 2021 at 13:48
  • Failed foot valve does not match the symptoms. Activation stops when output valve closed; also, with a failed foot valve, the pump would run for the time needed to fill the bladder tank (which should be 60+ seconds of operation) and then the water would flow back to the intake - not switch on/off every 5 seconds. There is no well, as stated in the question: intake water is in a tank at ground level.
    – Ecnerwal
    Jul 8, 2021 at 13:54
  • Ahh, I missed the part about the intake water is from a tank at ground level. (wonder where it gets it's water from?, BTW). Also, it is possible to air up a tank with a failed bladder by opening a faucet someplace and connecting a compressor to displace some of the water and add an air cushion. I did that to diagnose a failed bladder on my water system a few years ago. In the old days pressure tanks didn't even have bladders. one of my jobs as a kid was to add air to our house's pressure tank when the pump started short cycling....ahhh the good old days. Jul 8, 2021 at 15:33
  • "(wonder where it gets it's water from?, BTW)" Usually a truck, in my traveling experience. Sometimes an intermittent pipe system, but usually a truck. In some areas, it's stored rainwater, but those cisterns are more usually underground, and meters are less common.
    – Ecnerwal
    Jul 8, 2021 at 15:41
  • The pressure switch is not activating after 5 seconds of water flow, it's activating after 5 seconds of non-water-flow, it even does this at 3 AM, and it's so consistent, every X seconds, always.
    – george
    Jul 9, 2021 at 7:10

There has to be water flowing somewhere to cause the pressure switch to be activated. You stated that the pump stops cycling when the outlet valve on the pump is shut off. If that is the case, that indicates that there is water flowing back through the pump or that there is a water line that you have missed in your exploration. Can you install a pressure gauge on the pumps discharge line in the vicinity of the pressure switch to show what is happening to the pressure. The only other possibility that I can think of is that there is a malfunction in the pumps control system. It would be helpful if you could post a picture or 2 of the system so we could see what you are seeing.

  • I can't install a pressure gauge... I don't have the expertise and I don't want to possibly mess it up and cut water to the building. I don't think there is water flowing back from the pump because that water would still flow back when the outlet valve is closed. I will try to get some pictures but I am afraid they won't elucidate the problem any more.
    – george
    Jul 8, 2021 at 13:31

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