Pass a motion at your next strata committee meeting that you intend to pursue the appropriate by-law with the new owner if the current owner hasn’t agreed to the by-law. In that case, the new owners will be given the choice of agreeing to the by-law or having the hot water system removed at their expense.
Write to the current owner informing them that this decision has been made, and that it is in the minutes of the strata committee and reminding them that, by law, this information must be passed on to the new owner as it might affect their decision on whether or not to purchase the unit.
That way, neither the current owner or purchaser can say they weren’t informed.