Question about Grundfos MQ3-35 3/4 HP Pressure Booster Pump

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My Grundfos MQ pump intermittantly continues to run when there is no demand, especially after drawing off a small quantity of water. It then overheats. The plumber has replaced the pressure tank but the problem persists and after a bench test is unable to find the cause.

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  • Anonymous Apr 18, 2010

    Thanks for the two offers of advice. I got a good deal to replace the pump with a new one. The pump had only 6.5 years use for a single person household, well below expected life of 8 years of normal use.

  • B Louis Oct 02, 2017

    Full service manual describes troubleshooting for frequent start/stop:
    a) Leakage in suction pipe or air in the water. Check the water supply/suction pipe/O- rings on the inlet part.
    b) Too low or too high pressure in pressure tank pos. 42. Check pressure in pressure tank. The pressure must be 1.5 to 1.7 bar.
    c) Seized-up or missing non-return valve. Clean the valve or fit a new non-return valve.

    Exclude a). b) is most likely, as the pressure tank will slowly deflate like a car tire.

    MQ pressure tank (part 98893507) has a schrader valve for pressurizing. This is not illustrated or described clearly in any service manual. (Full text "Service Instructions" MQ 3-35, MQ 3-45 is not on Grundfos website.) There is just an obscure mention in the owner's manual fault finding "check pressure in pressure tank" and the table value of initial pressure when pump is off and line is depressurized of 1.5-1.7bar (25psi). (Working pressure should be 2.0-3.5 bar, since pump turns on at less than 2.0bar (30psi) and maximum pump pressure is 3.5bar (50psi).)

    Turn off pump. Depressurize the outlet plumbing. Knock off the pressure tank cover with a fist (see Grundfos service manual for photos). Black plastic cover on the end of the metal pressure tank inside is just a big valve cap. Unscrew. Pump up the valve with a bicycle pump to 25psi. Suggest using high quality sealing metal valve cap instead of original. Test. Check pressure periodically. Once a year maybe enough.

    My pressure tank had virtually zero pressure in it after >10 years. No wonder the motor kept cycling. (Needs a new motor capacitor too now. Standard 12.5µF start/run capacitor 35mm diameter with 4 spade tabs.)

    The pressure switch is highly reliable and best not to fiddle with. It's just a spring loaded plunger with a standard microswitch. I tested the microswitch and it seems fine.



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I have the same problem and have found that the inlet check valve has a broken shaft in the center of device. It is a multi-piece plastic unit and it looks as if it is not quite seating and therefore the internal pressure leaks slowly back into my low pressure suction line. The pump will start and the pressure will build then shut off for about 30 minutes then cycle again. I am going to replace the check valve.

Posted on Apr 18, 2010

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Check the pressure switch/flow switch.

Posted on Mar 06, 2010


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