My mq 35 pump, has started to splutter and the pressure goes completely, occasionally coming back on its own and other times i have to reset the alarm... the pump is three years old and this has been happening for about three weeks

google Grundfos MQ3-45 check valve for more sites

copy and paste
https://rainwaterequipment.com/non-return-valve-for-grundfos-mq-3-45-or-mq-3-35-booster-pump/

https://www.pumpproducts.com/grundfos-96526663-non-return-valve-for-mq-booster-pumps.html



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- changes in hydraulic conditions (clogged pipes, clogged filter, ...)
- pressure switch failure
- flow sensor failure
(in normal condition the pump starts when the consumption is higher than 1.2 l/min or the pressure is lower than 2 bar)

https://www.manualslib.com/manual/1569763/Grundfos-Mq-3-35.html?page=1#manual

proper way is to install a water pressure regulator and due to potability reasons it needs to be brass ,bronze, stainless steel, or CPVC plastic , (no steel or cast metal) of course the less high tech way is to plumb a valve from pump outlet back to its inlet and adjust valve to max pressure setting you want at max demand , I don't recommend this method as it wont be able to give you a constant pressure regardless of demand as the first method will , Note: some water regulators are a "relieving" type and require special install methods and maybe three lines to it

Check the capacitor or the centrifugal switch.

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!)

Grundfos said the problem of on off every few seconds was air in the line. Then

later admitted it was the control unit. After replacing two units in 2 yrs I replaced

it with a standerd pump pressure switch.....Runs fine

MAKE SURE THE PUMP IS WIRED FOR 120/220. THE WIRE GAUGE
SHOULD BE #12 OR #10 IF 100 FEET.

There was possibly some damage done to the pump while it was operating dry and it in now not able to deliver the required flow of water. Pumps wear very quickly when allowed to run dry, even for a short time.

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.

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.