Pump problems

[Mike Jackson]

Called to look at a problem on a pumped system today. House is well off the beaten track with a very poor mains pressure and flow. only the kitchen sink is on the mains. The rest are fed from a storage tank in the roof space of a single storey extension via a Grundfos MQ3-35 pump. This feeds an unvented cylinder and all other cold outlets. When an outlet is opened the flow is initially great but as the pressure drops and the pump kicks in the flow is greatly reduced and the pump continues to run for quite a long time after the outlet is shut.

This indicated to me that the pump is not working at it's full capacity and struggling to keep up/repressurise the pipework. I'm thinking either blocked inlet or faulty impeller as if the problem was with the outlet side the pump would keep cutting in and out. There is no obvious blockage but I haven't stripped the pump yet as i want to get a service kit in case I damage any seals.

Anyone got any other thoughts on a cause?

 

[Ch4 plumbing]

When the pump starts it doesn't open the PRV/TPRV does it? Just a thought.

 

[Chalked]

The MQ pumps shut off slowly.

 

[Mike Jackson]

It's not shutting off slowly so much as taking one hell of a long time to build the pressure back up.

 

[rpm]

Not familiar with the pump Mike, if me I would start with the inlet side of things and if possible measure pressure & flow rate is up to spec to see if the pump is being staved of supply.

 

[rpm]

Any signs of cavitation, and can you reduce the outlet flow on this pump to balance out the inlet & outlet?