Boiler pressure rising indefinitely when heating is on

Hello,

Recently moved to a new rental place. The Vaillant EcoTec Pro 24 boiler was empty (I think for quite a while), refilled it via the loop to 1.5 bar & bled the radiators to remove any air.
One of the valves (the smaller handle) in the filling loops leaks very slightly but I don't think it's related.

The issue - when the heating is on the pressure goes from 1.5 bar to 3.9 bar (max on the scale) in about 15 minutes.
I didn't notice it the first time & found the boiler with an F.22 error after an hour. Restarting the boiler at the fuse cleared it up. The next day I emptied & refilled the system again - same thing, but turned off the heating when it hit 3.2 this time.

- Turning off the heating when it's above 3.5 bar it drops back to below 2 bar after an hour or so
- Radiators (3 total) do heat up like they should during the test.
- Both the water & radiator temp is set at 60C, hot water comfort mode is on.
- Hot water works ok when heating is off - pressure doesn't rise when e.g. showering and nothing else abnormal noticed.

Is there anything else I can do or check before going to the landlord about this?
And thank you, this forum is a lifesaver!

 

Your boilers expansion vessel needs re-charging/replaced, you will need to get your landlord to arrange this