Header tank filling with boiling hot water and pressure dropping daily

Hi,

We have an issue whereby the expansion vessel on top of the tank is filling up with very hot water (to the top) and the pressure unit on top is running to empty on a daily basis, needing topping up. The water in the expansion vessel is very clean water but does have a distinctly rubber / 'fish' like smell. The expansion vessel is starting to bow a little bit with the volume of hot water, and the airing cupboard is getting rather hot...

It has happened before in the past and the boiler engineer fixed it, but cannot remember what was fixed, and is now thinking a replacement tank is in order!

It is about 5 years old.

Any ideas?

Many thanks,
Nick

Could just be a faulty ball valve.
The water is hot.
Mike

Thanks Mike, apparently I do not have one of those as it is a pressurised system?

Sounds as if the expansion tank has lost its pressure.
It may be possible to recharge it (pump it up)
Or replace it.
Diaphragm may have failed
Mike