Bosch combi loosing pressure when heating is off

Hi there
Got a new boiler installed last Christmas. Initially we had a leak because the old expansion vessel was left and dicomissioned later as I was told boiler has an internal one.

All fine for a few months but now the boiler is loosing pressure when the heating is off. It looses overnight too, when we didn't even use hot water.

Can I still have a leak on the heating system or is this boiler related? I assumed the water doesn't run through the heating if it's off? I will call the company who installed it, but I want to have the info as I am pretty sure they will blame it on the house.

I have lined up a thermal image surveyor in case it's not the boiler, but something here does not add up to me

Thanks

 

That is the last thing to check. I will report tomorrow. Thanks

 

So I tried the heating. The copper pipe at the back of the house (I guess this is the prv) is dry but the condensing pipe is wet, I guess that is it's purpose.

Pressure went from 1.4 bar to 1.7 bar according to the boiler display, but from 1.4 to 2 according to a gauge I have on the filling loop. This was in the space of 45 min-1 hour. When turned off, it cooled to 1.6. The bag on the prv is dry.

I inspected all the rads and the one in the kitchen has a bit that is badly rusted. Could it be than even when the heating is off, there is a pore there where air comes in? I cant see water falling underneath but it's definitely going somewhere...

I will ring my plumber to come inspect that radiator and most likely change it...