The advantage of tiling first are that you don't have to cut as many tiles - just those at the room corners and around pipes. Certain areas will be easier to tile too.
The disadvantage is that you need more tiles. You will also need to make sure that all pipes are in the correct location but not connected. This will mean either having the water turned off or fitting isolation valves (of itself not a bad idea).
May I ask you to temper or clarify one adjective? Is this water literally BOILING hot (hotter than the tanks) or only SCALDING hot (as hot as the tanks, about 120*F)? If the tanks are also used for heating, then perhaps 180*F?
If the tanks are used both for heating and domestic hot water, then you have several large tempering valves, correct? These tempering valves mix hot and cold water together to achieve the industry-standard maximum 120*F for domestic hot water.
You're on city water, surely? High pressure, enough to push water to the top of the building, with pressure regulators on each floor?
This is the sort of thing that might happen if a tempering valve fails or is misadjusted, AND one or more of the tanks has developed a large air bubble. It's more likely to happen during periods when the municipal water supply pressure falls (because other nearby buildings are also using a lot of water). The hot-water tanks have been pressurized by high municipal pressure, including the one with the bubble, which makes it into a large surge tank. When municipal pressure falls, pressurized hot water backflows through the failed or misadjusted tempering valve into the cold-water system, where it's delivered to faucets, toilets, etc.
A really good plumber should be able to track down the source, but it'll take some time. They'd have to carefully measure the temperature of the pipes in the pipe chases, following the hottest pipes towards its hottest end until they come to the hottest spot in the cold-water system - that'd be the tempering valve in question. It'd be BEST if they could work while the occupants of the affected units were not in the building, so the water to their units would remain pretty static... making measurement easier.
The only other cause I can think of would be backflow (intentional or accidental) through a bathtub or laundry outlet, where hot and cold could be mixed without actually dispensing water - in the case of a bathtub (which has much larger water connections than a faucet or toilet), it'd require that the spout be blocked while both hot and cold were turned on. This would allow hot to backflow into the cold-water system.
I'm leaning very hard towards the tempering-valve problem, though, because otherwise we're talking about collaboration between numerous occupants in different units or a really odd set of coincidences.
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Cant you find a 22mm-15mm adapter? I live on the other side of the pond and to change sizes there are adapters both male and female to make the change.
I just googled a “22mm to 15mm pipe adapter” and a brass piece for $6.69 came up on the first page. adapter
We use national pipe thread so I don’t know if that would work but try it. It is a male thread so it should screw in and then female so your new piece would screw into that if the thread pitch is correct.