Before all boiler installations speak to your installer about what they are exactly going to do and what rooms in your house they will need access to. Find out if you need to make any preparations before your new boiler installation; covering up carpets, putting away delicate items and moving any furniture to provide access.
We've said it before but it's worth repeating: all installers carrying out boiler installations must be Gas Safe registered. If you're able to it is worth staying close by particularly towards the end of the central heating boiler installation, as you will need to make sure your installer does the following:
If you follow this, you should have a problem free gas boiler installation.

