Noisy CH pump

[wurmstrangler]

My Central Heating pump is very noisy BUT ONLY when heating the hot water cylinder.
When just heating the radiators it runs silently (as it should).
If heating both the hot water and the radiators, it is noisy.
Any ideas?

 

[dilalio]

Y plan or S plan?

 

[dilalio]

Air;
sludge;
Worn pump;
Faulty HW MCV.

 

[wurmstrangler]

Thanks dilalio. It's Y plan.
The pump can't be the problem because it runs perfectly without noise if it's just heating the rads.
I can't see how the MCV could affect it. I'm guessing air or sludge in the HW loop of the pipework, but given that it's a vented system, I can't see where air could collect.

 

[dilalio]

Well, if it's Y plan, then a three/mid port valve could be at fault if not fully opening to HW only or HW+CH; causing a flow restriction; it could be combined with sludge to exacerbate the symptoms you are experiencing!

 

[wurmstrangler]

Problem solved. Massive amounts of air in the system because the header tank in the loft was empty. The ballcock inlet valve was jammed shut with limescale.
Thanks for your help.

 

[dilalio]