shedsrus – good pseudonym, and the answer to general question about the workings of Lot/Common Property is “in mysterious ways”.
The answer to your specific question however is a little simpler, in that if there is no Special By-Law stating differently on the subject, then if the hotwater heater screen is entirely freestanding and on your Property then it’s your responsibility, and if it’s on or attached to Common Property (such as to a perimeter wall of your Lot) then it’s the Owners Corporation’s (O/C) responsibility.
But, here comes one of those mysterious bits….. even if the screen is the O/C’s responsibility and you want to modify it and the O/C agrees, then you may go ahead and do that, but still at your expense.
So depending upon how often you anticipate replacing your hotwater service, it may be prudent to just leave the screen as it is.