Why do you think the water heater is in someone else’s property, and is not on common property? I’m doubtful that is the case… It may be that an owner has previously taken over the area illegally.
The starting point is the strata plan, which you should obtain, which show the lot property and common property. I’ll bet that shows the room with the heaters as common property. It is hard to sell common property, and it is unlikely this would have been done for a room that is has shared equipment belonging to all owners. And certainly, if that happened, then there would be written evidence, and a by-law.
Get advice from your strata manager, or else consult a strata lawyer. If someone enclosed common property without authority, then now you are aware of that, then they will need to restore it to its original condition.