Noisy Central Heating Pump

[madluka]

In our upstairs cupboard along with the immersion heater/tank is a grundfos UPS 15-50x18 pump which is making groaning/grinding/vibrating noises and is getting EXTREMELY hot.
It would appear to be a faulty pump, but what confuses me is how the pump seems to be active when the boiler downstairs is only on the pilot?
I had to set the boiler thermostat downstairs to OFF to shut the pump up last night. This morning I put the thermostat on 3 (the maximum is 5) and the pump seems to be quiet still.
The pump electrical supply has an isolator switch, which has always been in the ON position for years - is that how it should remain? I assume with the pump not on at all if the boiler was boiling then a disaster would happen?

 

[kevplumb]

the boiler might have a pump overrun fitted

if its suddenly gone quiet are you sure its working

 

[Paul Barker]

This is one of those ones we need to be there to appreciate properly.

One scenario in my head is you have gravity hot water and the isolation switch for the pump gets turned on in the winter when you need heating. i.e. very poor mnanual control.

Secondly a scenario could be that the f/e tank has run dry as the pump is high up in the system it could be dry. I've come across idiots who turned off the service valve to the f/e because of water coming out of the overflow. They have sold the house and a year down the line someone like you calls someone like us, we find that the oh stat on the boiler has come in to play, and have to trace it back. Or sometimes it's simply the ball valve stayes up where the float was once bobbed up to.

Could be anything really.