Is it newly installed or has it been working for a while till you noticed it?
SOURCE: Voltage at pump is 120 & air pressure in tank
it sounds like the pressure sensor or switch is bad.
SOURCE: 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.
SOURCE: My Grundfos MQ pump intermittantly continues to run
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.
SOURCE: I have a problem with the MQ-35 pump starting and
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
SOURCE: Grundfos pump leaking?
If it is starting and stopping, it most likely does have some type of leak. The check vavlve is normally installed on the suction side of the pump, and sometimes instead of a check vavlve or in addition to, there is a foot valve. A foot valve is just a check valve (usually with a screen) at the end of the line in the bottom of the cistern. If you have much suction lift, and the check valve leaks (or any leak in the suction piping) then the water will drop back into the cistern. This will also cause the discharge side of the pump system to drop in pressure, causing the pump to restart. You mentioned removeing the pressure tank, that tank keeps the pump from short cycling when you have low demand on the system. If the pump is higher than the cistern, and you don't see a check valve between cistern and pump then you most likely have a foot vavle in the cistern. Hopefully it's accessable by removing the pipe.
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Mine did that. Little known that the Grundfos MQ pump has a small pressure tank on the end. If the pressure is low and the air volume is therefore low, then any tiny loss of water results in pressure drop and pump cycles on.
To pump up the pressure bladder:
1. remove the rear cylindrical cover (swift bash with a fist downwards)
2. unscrew plastic cap on the end (about 2 inches diameter with tapered sides
3. attach pump to schrader valve and pump up (IIRC about 30psi)
Reassemble.
Suggest teflon tape to schrader valve stem to try to create a secondary airtight seal.
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