Question about Grundfos MQ3-45 1 HP Pressure Booster Pump

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My pump, used to pressurise the water in the house, keeps coming on every 2 mins approx even though there is no water being used in my system. I have checked for leaks and am suspicous that the pressure detection mechanism in the pump is faulty. How can i check this ?

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If you have valves at the pre tank you can run it till it shuts down then close the inlet valve and the outlet valve. Wait and see if it goes on again. If so you have a problem in the pressure tank or the switch mech. If not open the outlet valve and wait. If it starts you have a leak somewhere in the house. If not open the intake valve and see what happens. If it kicks on then you have a problem with you check od backflow valve. If this is a shallow well pump it could be in the supply line also. Good luck.

Posted on Feb 01, 2010

Testimonial: "Simple logical explanation and elimination instructions. Thank you"

  • Kieran Ruppert May 13, 2010

    It's hard to say what caused the problem. When the pump dosn't prime after 24 hours it shuts down till reset. when you pulled it off you shut down all power. This reset the pump so it will run now but the orignal problem may still be there.

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I had the same problem. By design this pump turns on when the pressure in the house drops below 29psi OR when there is flow through the pump. It doesn't turn off due to pressure, it turns off when all flow through the pump stops. So, the Grundfos tech rep said my flow sensor is bad and they would sell me a new one for $75. I elected to buy an new pump ($420). Recently, I tested the old pump and it is now working!

Posted on May 13, 2010

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