To lose pressure either a leak, or the expansion tank has failed and when it warms up it opens the relief valve.
Either way it clearly needs fixing, however I found my Worcester Bosch worked with no pressure, always seemed to need topping up, likely one of the lock shield valves leaking, but clearly if too much things around the leak can rot, so even if it does not harm boiler still needs finding.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!