Finding the proper size and type will depend on the flow rate—measured by a GPM (gallons per minute) number—that each fixture needs. Every application has a standard flow rate that must be added up in order to calculate the hot water demands for your entire home. For instance, if someone is using a sink with a 1.5 GPM at the same time another person is running a shower with a 2.0 GPM, the flow rate for the tankless unit would need to be at least 3.5 gallons per minute. You will have to add up the flow rate for all the applications in the house to get the minimum GPM figure for your tankless water heater.
In addition to flow rates, tankless hot water heaters are also measured by how much the water temperature needs to rise as it moves through the heating unit. You can determine the temperature rise for each application by subtracting the temperature of water coming in from the desired temperature going out. Once you add those together with the overall flow rates, you will know which tankless water heater can handle your overall hot water needs.
Before you buy an on demand hot water heater, it is best to talk to a professional installer. While the flow rates and temperature rise for most household appliances are fairly standard, these numbers can vary because of several factors that professionals are trained to calculate. Size is not the only factor to consider when shopping for a tankless water heater. Fuel type and efficiency should also be factored in to your purchase, which is another reason to talk to a licensed Sammamish installer.