Marey tankless water heaters come in many different shapes and sizes and there really is no "one size fits all" solution. Determining the proper tankless water heater to fit your application should be based on the number of fixtures that are to be used simultaneously, and their accompanying flow rates. Every home and every end user is different, so the sizing of a tankless water heater should be calculated to fit the needs of each home and user. While one home fitted with low flow fixtures and individuals who are very conscious about their water usage may only need one small tankless unit. Another home with old water hog fixtures and a large family wanting to use many fixtures simultaneously will probably need multiple large units to meet their needs. What it really comes down to is the end users preference when it comes to performance.
This is a really straight forward step, just go around your home and list the number of fixtures in your home. Separate them into groups of similar fixture types, such as shower heads, rest room faucets and kitchen/utility faucets. Don’t forget to list your machinery which use hot water as well, such as washing machines and dishwashers.
Now it’s time to figure out which of these fixtures/machinery will need hot water at the same time. Keep in mind the more simultaneous fixtures you need.
GPM or Gallons per minute is the standard flow rate measurement in the US and most of your faucets and showerheads will have their GPM flow rate stamped into the side. However, if the fixture does not have the flow rate listed on it or you cannot read it, you have two options; take an educated guess or manually calculate the flow rate.