Grundfos pump oscillating

[finchy01]

Morning chaps,

I've been sent to look at a job. Sealed system with a big twin head self modulating grundfos pump. The pump is constantly modulating up and down, up and down and the pressure is up and down in time with the modulating. I e not worked on this type of pump before (I've been living under a rock for the last couple of years!) And just wondered if you can give me some tips on what to check?

Many thanks as always.

 

[oz-plumber]

Is there a by-pass on the system?

 

[finchy01]

Hi oz. Thanks for replying. It does have a bypass. A manual one which was shut when I arrived on site but opening that does nothing. It's still pulsing. I've check the pump valve too and they are fully open -1/4 turn obviously. It's a grundfos alpha to not a twin head as I was told!

 

[oz-plumber]

How close to the pump is the by-pass?

The pump could be over compensating itself if the by-pass valve is too close to the pump.
It could also be the same situation with the by-pass closed and the pump pushing water through the TRV's.
The pump may be oversized for the job and is trying to constantly rectify itself to suit demands.

 

[finchy01]

Thanks again oz. The bypass is very close to the pump so I'm liking your theory. It's a 3 floor office bock so I doubt the pump is oversized however I didn't get a chance to have a proper look around as I was being hounded to move onto the next job and arrange a time to go back when we have more time. I will double check the distances. I'm guessing the dial on the front of the pump is similar to the old 3 speeds on 15/50. Surely switching this down would compensate for the pump trying to work top hard?