New boiler - pressure dropping

[Last Plumber]

Vaillant Boilers, siphon the trap out in one go so I would expect the trap to fill and then empty. Even if it was water from a burst heat exchanger !

I would expect you to find more than a few teaspoonfulls to be honest but I cannot say for certain.

My advice is to get an Engineer to check it for you.

You shouldn't work on a gas appliance unless you're Gas Safe Registered.

 

[zzzjim]

.......
Sometimes, changing from open vented to sealed will show up leaks that went unnoticed before the change

Concrete floors , corrosion & expansion issues are the enemies !

(When Hot any slow leaks elsewhere may evaporate .. installer may have some tricks up his sleeve for this )