Boiler losing pressure

[simonlb]

We have an Alpha 240 Eco combi boiler which works fine for both heating and hot water. However, each time it has been used for heating it loses pressure and requires refilling from the filling loop. If the heating isn't used at all the pressure remains fine.
I've checked all the radiators and pipe connections at the bolier whilst the heating is running and can't see any leaks.
Can anyone shed any light on why this is happening?

 

[mlb3c]

check the prv pipe outside ex vessel might need a dose of looking at

 

[gas4you]

Do as Kev says, but what pressure are you re-pressurising it to each time?

Should be just over 1 bar cold.

 

[simonlb]

Just over 1 Bar.