Firstly, it is important to check the boiler. You should find out how old it is, when it was last serviced and if it was serviced by a qualified technician. Boilers should be serviced at least once a year as this will help to identify any problems before they become more serious.
Ask to see evidence such as servicing reports – these should be left by the technician who completed the work. If there are any issues and you suspect the boiler may need replaced before you can move in, you should negotiate a price through your solicitor.