blimey - I was getting a bit discombobulated with all the wherein's therein's and thereof there mate. I think all you can do mate is refer to the guidance that you know to be solid and reliable - that being what's in say BS5440:2 and the Gospel according to CORGI/Gas Safe.
I haven't got any of the gas safe books other than the USP booklet (been made redundant, so no money to buy any either), but in the 5th edition of the corgi manual (and I think it was in the previous versions) it does give some guidance on the matter in terms of what might be seen a interpretation or guidance on the provisions set down in bs5440 - nothing concrete, just a description of things to consider.
IMHO it's too easy to get bogged down thinking of trying to define what is/what is not a compartment. At the end of the day, I think if you're assessing an internal space and you could reasonably argue that this space is not subject to the same number of air changes per hour, or the same adventitious ventilation as you would expect in a normal room (i know this doesn't factor in RS installs), and it fits the definition described in the standards you need to be thinking "do I feel happier treating this as a compartment or not" or to put it another way "do I feel happy NOT treating it as a compartment?".
Imagine a 3 bed 1970s semi with a flat roofed enclosed pvc porch on the front or back. Now imagine there's a boiler fitted in there with a vertical flue - let's say it's room sealed fanned draught to avoid complications with minimum flue heights and adventitious ventilation. Granted, it's probably going to look terrible - boiler in a porch?! but it makes a point.
you could turn around and say, well, it aint a compartment, it's a porch.
but from a point of view of the installation, the environment in which the appliance is installed is vastly different to an installation in a typical room. We have a situation whereby we have nothing like the normal ventilation set up, the space is very much smaller and so we could not reasonably expect to have similar adventitious ventilation, and certainly if it turns out that this entrance to the house is not used on a regular basis, we could expect this porch to be closed up for extended periods - resulting in greatly reduced air changes. Couple this with heating from solar gains and you're going to be thinking, yep, it's likely to be getting a bit stuffy in there, and now someone's gone and stuck a boiler in there (which I think they say could emit as much as 750W of heat energy (probably cast iron heavyweight boilers) - not far off a 1 bar electric fire). it's not really going to help much eh? It's likely that the ambient temperature in there is going to be higher than in the normal rooms of the house on a regular basis. The appliance is intended to be installed for operation at the sort of ambient temperatures found in homes. So the question you then ask yourself is, "do I still feel happy treating it as a porch or should I be thinking this is more of a compartment installation?".
I know it's a bit wordy, and I can't sit here and claim I know all there is to know or anything (i'm sure there are blokes out there who've forgotten more than i know), but I've got little else to do but sit here and type and I hope it explains a bit of how I think about compartments, and what guidance I base that thought process upon.
Also be mindful of ensuring that there is provision to ensure stored articles are kept separate from the appliance (I think 5440 or it could be 6798 specifies a barrier posssibly mesh upto 13mm, affording a 75mm separation i think) unless the manufacturer says you don't need it.
