Sludge, pump or motorised valve?

[Canitbefixed]

OK, has anyone had this problem?
I went on holiday for a week and turned my hot water system off.
I have a gas boiler with a tank, a controller, a motorised valve and a Grunfos pump.
When I came home the hot water would only heat up by bleeding the pump. Before this the motorised valve just kept on turning and would not park at the hot water position. Now, two days later, same problem.
I think it is the motorised valve switching, or sludge in the pump causing problems. Any ideas?
The boiler does fire for the few moments when the motorised switch is pointing at the hot water position on its travel round. Problem is, it just won't stop there.

Any advice greatly appreciated.

 

[Steve69the69Gas]

I'd start with the MV -prolly just the head needs replacing. But draining/flushing/inhibitor would be nice too.

 

[ChrisR]

If your MV "travels round" it sounds like an old blue Switchmaster - obsolete.??
Is it?
If not, and it's a 3 port, its resting state is HW.
But check you have water in the system. If you bleed a rad you should hear the header tank refilling.

 

[Canitbefixed]

Thanks folks, yes it is an old blue switchmaster.
Further this, with the valve at HW, I removed the pump centre disc to check it was running. The spindle was rotating, but relatively little water came out. I assume this again points to lack of water in the system.

 

[mickyg]

my guess:

Blocked cold feed.
Why?
You bled the pump, probably robbing water from another part of the system, a radiator more likely. Gradually over 2 days the air has built up again where the pump is.

 

[Canitbefixed]

Ahhh. Makes sense.
Please excuse my ignorance, but what is the sequence to bleed the air/refill the pump?
Once it ran (two days ago) I did run the central heating up to check all the rads got hot, they did.

 

[mickyg]

easy enough to check. start draining down the system and see if the header tank starts emptying. If it doesn't, blocked cold feed.

 

[Canitbefixed]

Ok folks, will check this out. Many thanks for your suggestions.

Well, cold feed wasnt blocked, BUT when I removed the cover to the MV it was pretty damp with condensation.

It looks like water has got into it and was shorting things out so not switching properly. I've isolated everything over the weekend while it dried out, and it now seems to be switching ok.

Perhap water seeped through the D shaft into the unit because the pump stuck after a period of inactivity.

Will watch and see.