I own a rental duplex and have run into a water problem.?

I own a rental duplex and have run into a water problem. The house's laundry is hooked up to the #2 unit. #2 unit is complaining that the #1 unit is doing laundry and using the #2 water heater to heat #1's laundry water. She feels she should not have to pay the amount needed to heat that water for their laundry. She wants the laundry connected to #1 unit and insists she will just wash her clothes in cold water. Any advice on how to handle this??? Nothing was stated in the lease that the laundry was connected to unit #2 water, as we have not had this issue before...