another boiler problem clue

[visionset]

Last autumn I put in a Vaillant combi, over the winter it needed weekly topping up and I have not found any leaks. Still none the wiser and now the rads have shut off, another clue presents itself. Now the boiler doesn't need topping up as often in fact it's been months since I did it last. I'd have thought there would be no change. I mean the whole system is still under the same pressure isn't it? Does it point to knackered expansion vessel? And no, the PRvalve is not weeping one bit.

[EricP]

Put the system on for a while to heat right up and then see if any pressure loss when cool.

Then empty the smallest rad, (bathroom?), and then repressurise and heat up again. When it cools see if any pressure has been lost as usual.

The empty rad will act as an expansion pressure vessel and no pressure loss will point to a defective one in the boiler.

[Ed Sirett]

The chances are that there is a tiny leak on a joint or valve which is only present when the water is hot. Since the topping up was only weekly it's quite possible that the size of the leak could have gone unnoticed, especially when the pipes were hot causing and leakage to evaporate.