Losing pressure constantly

Hi pressure loss can be caused because of 4 reasons expansion vessel, pressure release valve, leak or air in the system

I’m assuming this is a Combi boiler and all radiators have been bled correctly, paying particular attention to towel radiators and double radiators. Has the expansion vessel pressure been checked to ensure it is 1bar? This should have been done as part of commissioning. To do this you will need to drain the boiler first, then use an appropriate pump for the expansion vessel to test pressure is 1bar. If pressure is low increase until you have 1bar. If the expansion vessel is ok. It sound like a leak of some sort, this is usually obvious when it happens, ie damp patches on walls or ceilings. double check all areas. That then leaves a potential PRV fault. If the boiler is holding pressure this may not be likely. However, if it is recent install and the boiler has been registered with Vaillant, they should come out to inspect under the guarantee.