Leave enough room for it to be serviced in all directions. Put a 12"x12" louver or grate to allow combustion air 6" up from the floor and one 6" below the ceiling on the wall to the boiler room or make the door to the boiler room a louvered door. If you get a direct vent appliance, you wont have to worry about combustion air from the room. Leave room for servicing. Service people are not contortionists and it really pisses us off when people don't allow room for us to fit. If you can't allow 4' in front or sides, make a panel in the wall that comes off so we can replace the combustion chamber later.think about the service person or when the boiler breaks down and he can't get to it, it will cost you hundreds of dollars to repair the demolition job he'll do to your new wall or he may just walk away. Seems like your thinking about the air to the boiler but did you think about the 240 lbs man who has to get to it? Do not block off the combustion air openings with all your clutter as you pile crap on and around the boiler cause you now have a new place to place your junk.if you have your boiler serviced, please clear out all the crap you put within 8' of the boiler so they don't have to walk or climb over stuff.. Just being honest, whenever I show up to a job and the customer knew days or weeks in advance that I was comming and didn't bother to clean an area around the work areas and I have to move things in order to do the work, things will get broken.sorry for ranting but this question touched a nerve.service people want to help and for the most part love what they do but we will not be treated as common peasants and be expected to work in filth, clutter or made to work in unsafe places. We can be more efficient and do better work if we have room to work with good lighting. With all that said, use the above rules and you should have no issues.