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Grunfoss MQ pump no steady pressure

scott w.
scott w. Member Posts: 137
Have a three story home that is at the top of a hill with a public water supply. Public water supply pressure is to low at entry in basemen something like 20 pounds of pressure . Water company claims OK pressure at the street meter down hill. Did a renovation several years ago and replaced the old pressure pump with a Grunfoss MQ. Grunfoss told me at the time with this pump the MQ, no pressure tank was needed in the house. Pump worked fine for several years until I left last the winter for six months. Before leaving had the water shut off, drained all of the pipes, (of course i shut the electric off to the pump before draining), heat (hot water) was left on in the house at 68 degrees all winter.

Pump used to run any time the water was turned on and kept a constant 65 pounds pressure. Now after my return the pump does not keep a steady pressure. While water is running pump runs and then shuts off. Water pressure fluctuates from 70 down to 35 pounds of pressure. I have had the plumbing firm who installed the system spend a day on this problem. Took the pump apart to see if any debris was caught in it, they called Grunfoss tech help line after that didn't work. Grunfoss advised do the same again take apart look for debris or broken part. That didn't work. Plumber said he didn't know what to do after that. I still have the the problem. What to try next????? I have a back flow preventer at the entrance of the water supply in basement. Could a back flow preventer be a problem? Plumber said no. The very first back flow preventer installed years ago was replaced after about a year. I don't remember having a pump problem back then.

Pluming company didn't charge as they couldn't, didn't fix the problem.

Any help would be appreciated to this frustrating problem.




  • The MQ is an on/off pressure booster pump, using a pressure switch to turn on/off.

    I would check two things - there is a very small expansion tank on the end of the pump (you'll have to remove the 4" diameter cap off the end of the pump). Charge the tank with air and see what happens.

    Another thing I might check is look for any restrictions on the inlet piping. Choking this off will effect the pump as well.
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