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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1 Answer

Grundfos pump


Louise, since there is several different types and models of Grundfos pumps it would be hard to answer your question. To possible point you in the right direction below are a few links to where you can find some PDF documentation about a popular Grundfos pump the MQ series and direct link to the manufactures "Service Support".

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2 Answers

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Pressure is bleeding back through the pump or system to the tank or well or where ever the water is coming from. Should be some sort of one way valve to keep water from going back to the source, mounted somewhere so that once pressure is pumped up, it will hold that pressure in the part of the system that has the pressure switch that controls the pump.

That or you have a leak where you can't see it. (Actually you do, it is just internal and not leaking outside the pipes!)

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My Grundfos MQ pump won't start after it has been idle for 4 to 5 weeks. It is installed at my weekend place in the country. This has happened several times. Pump was replaced once but problem...


There should be a small cap on the motor end of the pump, take that off and you will see the stainless steel shaft with a **** in it for a flat bladed screw driver. You should not need to turn the water off. A few drops might come out, but it's safe to do it that way. Turn the shaft with the screw driver and that should loosen up the pump.

Hope this helps.

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1 Answer

I have a problem with the MQ-35 pump starting and running for about 10 minutes when there is constant demand (such as running a sprinkler) and it then alarms and shuts off for 30 minutes before it will run...


This sounds like a issue with air still being in the system and not being removed from the system to make things work properly. I did try to findthere number and was unable to locate it. The only other option for you is to return the product and find one with better support

May 04, 2010 | Grundfos MQ3-35 3/4 HP Pressure Booster...

2 Answers

With the water running the MQ pump runs intermittently


This is a problem with your bladder in the pressure tank or the pressure switch. Ether one will produce this cycling on and off. Good luck.

Mar 09, 2010 | Grundfos MQ3-35 3/4 HP Pressure Booster...

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