Vokera Compact 24 - Explosive ignition 1st time.
 

I have just had a Vokera Compact 24 installed. I then couldn't get
hold of the installer and so I had to get it finished from somebody
else. The problem now is that no-one will sign off the boiler, as they
won't sign off someone else's work.

The boiler works fine but when it fires up for the first time it makes
a small bang. If I then turn the tap off and back on again it is fine
until i leave it for a couple of hours or so and I'm back to my small
bang again.

Any ideas as to what is causing the ignition problems the first time
around.

Secondly is the boiler still covered by warranty as the 2nd installer
said it wouldn't be until the boiler was signed off.

Thanks in advance for any comments.