On Monday, September 3, 2012 12:12:26 PM UTC+1, GB wrote:
On 03/09/2012 11:27, Onetap wrote:

On Monday, September 3, 2012 10:57:57 AM UTC+1, GB wrote:

The internal coil has probably failed; the crud is from the heating system.







I thought of that, but surely the OP would be forever topping up his



central heating system, and surely he'd have mentioned that? Assuming



it's an unvented system, ofc.



The pressure in the CH & the unvented cylinder would equalise, so there may not be a loss of pressure in normal use.

He did say; "This always happens if I depressurise and repressurise the tank."

The CH would leak into the tank when he had drained it, so this scenario sounds plausible.





Ah, gotcha!



The OP can test this hypothesis by depressurising the central heating

circuit. If it fills itself up (from the HW tank), then you are right.

The CH would lose pressure after the tank was drained, but it could be a slow leak.
It should be covered by the maker's guarantee, if the OP has the receipts, so it would be a cheap fix, compared to most faults.