ferolli combi boiler help

[wsrxh20]

hi all, and thanks in advance for any help, always had to top up every month or so, but last week dropped every day, got 3star gas cover, they came out , check the boiler, said all ok , must be a leak.
i have lifted the floorboards and can not see any water, in any of the rooms.
the boilers is at least 14years old, i know its probably needing replaced , but where is the water i'am putting in going, it;s not going out the wall, or under the floor.
i only have 4 radiators and 1 towel heater.
any idea's?

 

[Terry7556]

theres a copper pipe going out the wall, may end in a hook shape.is it dry?or has it been dripping
does your pressure gauge rise when heatings on

 

[wsrxh20]

the outside pipe is dry, no drips , not sure if it rises when on , will check when i get home from work today, it's an optima if that helps any, the eng checked it over and told me will need to check under the fllors, but i could see they were also all dry

 

[wsrxh20]

i had a look, no the pressure not up when on, any idea where the water is going ?.

 

[wsrxh20]

seemed to stop dropping for a few days but started soon ,after, still cant find the leak