Tankless water heaters only heat water on demand, eliminating the standby losses associated with traditional tank-style water heaters. A Tankless water heater can provide hot water indefinitely – as long as demand does not exceed the heater’s flow capacity. There is a limit to the amount of hot water every tankless unit can produce in one minute. This gallons per minute (GPM) rating will be small for units designed to supply only a sink or two, but much larger for whole-house heaters. A heater’s GPM capability fluctuates based on the “temperature rise,” or the number of degrees by which the heater must increase the temperature of incoming water. 55°F ground water and a 105°F setpoint on a heater, for example, would require a 50°F temperature rise.
Tankless water heaters provide more hot water in warm climates and during warmer times of the year. See below for a chart from A.O. Smith that includes standard ground water temperatures across the United States. Use this information along with anticipated output temperature settings to calculate your temperature rise. Always size based on the coldest incoming water temperatures if the heater will be used year-round. Make sure that the heater you choose can provide sufficient flow to meet peak demand at the required temperature rise. Typical showers use between 2 and 2.5 GPM, so a properly-sized heater would need to be able to provide 4 to 5 GPM at the necessary temperature rise in order to handle two such simultaneous showers. Specification sheets for tankless heaters display charts that indicate the maximum flow rates across the range of possible temperature rises.