Quiet with CH, Noisy with HW

[boilerbeasty]

My combi boiler runs quietly with CH, but when running the hot water I get some bubbling noises from the pump. Similar sound to when its pumping a bit of air after drain down/refill - but I don't see where air would come from. The water does get hot and is an OK flow rate.
The pump (Grundfos 15/50 with an automatic air vent) gets fairly hot to the touch - is this normal ? Its quiet enough and definately pumping when on CH mode.

I'm worried about rubbish in the pump and the CH circuit of the secondary heat exchanger. As well as the noise I think I can hear 'bits' in the pump.
Would using Fernox SC Restorer (the one in the mastic gun, I don't have a tank to put it in) do the trick in disolving any small bits of limescale which might be around ?

 

[oilman]

The noise with HW is air, and it's probably because there is no bleed point at the top of the coil in the cylinder. If there is, bleed it; if there isn't, it's worth fitting one.

 

[boilerbeasty]

There's no cylinder - the hot water comes direct from the boiler.

 

[oilman]

oo..er..I should have read your post more carefully.

Have you tried bleeding the pump?

 

[boilerbeasty]

Its got an automatic air vent on it - is this doing the same job ? I've tried taking out the plastic plug and seeing whether pressure is built up if I cover it with my finger for a couple of seconds, but I can't detect it.

Not sure how to see if the pump is OK ? I think its possible to try spinning it manually to see if it freely rotates but I'm not sure how to (draining the boiler and removing the power are probably involved I expect).

 

[oilman]

There's a big screw on the back of the pump. Turn the CH and HW off, and remove the screw. Water should drip from the hole. If the CH works, the pump will be running, so you dont't need to do anything else.

 

[boilerbeasty]

Is that the large metal one right in the center, with a 1cm+ slot in it ? The back of the pump is a bit inaccessible and has the automatic air vent on it.

 

[ChrisR]

Yes it's the "end" of the pump where the pipes aren't.

What boiler is it, how old, new problem or old?
Might even be the wrong pump, most have 15/60's - has it been changed?