Good question. Most municipalities require separate meters for different rental units under its housing code so as to prevent the very situation that you are dealing with.If your landlord failed to disclose that your rental would include the entirety of the unit's hot water bill, not only for you but for others, this fact which is a material consideration should have been disclosed to you before your signed the lease by the landlord. I would speak with the landlord in person about the situation to resolve it where your hot water bill would be substantially reduced for example by 2/3rds or that you will consider the lease subject to a rescission by you due to the landlord's failure to disclose the hot water charge orally or in writing. I would send a letter memorializing the conversation, keeping a copy for future need.