In message , Brian Reay
writes
I'm having a problem with our CH. Upstairs rads and hot water are fine,
downstairs rads are very slow to heat up (even with all other rads off). No
air in system, as far as I can tell and system was nicely balanced last
winter and worked fine- ie the problem probably isn't balancing.

Pump seems to be running- there is a slight vibration which you normally get
when it is BUT the only time I've seen this sort of thing before was when
the pump had failed (in our previous house).

So, I'm thinking the pump may be rotating but maybe the impellor is shot in
some way (broken shaft maybe). Is this a "normal" failure mode?

It can happen, yes

On a Grundfos pump, the shaft is ceramic, and can shear off


--
geoff