To install a tankless heater for water, there are few things you have to follow. Firstly, you have to check the temperature rise of the incoming water. It should be between from 41 degrees to 105.5. Kindly check if you can heat at least 5.1 gallons of water with that temperature. So, you'll require a tankless water heater which can generate at least a 61-degree temperature rise at 5.3 gallons of water every minute.