Can't remember the gory details (I'm sure it's on th'interwub somewhere) but generally one assesses possible locations for the boiler, excluding ones like (IIRC) the living room, and scores each candidate on a scale of points for factors such as length of flue required, necessity for a condensate pump, the new location being in a different place from where the boiler currently is and so on, and take the most favourable score. If even that is higher than a certain number you can make an exemption and fit a non-condensing boiler.