Abstract: A tankless water heater with an impeller flowmeter having multiple K factors significantly improving the accuracy of flowmeter readings, particularly at low water flow rates, such as under 1 gallon per minute. Rather than use a single K factor impeller flowmeter in a tankless water heater that is particularly inaccurate at low flow rates, this disclosure provides an impeller flowmeter with multiple K factors to obtain precise flow rate readings to precisely control heating of the water at low flow rates. The flowmeter has an onboard memory with multiple K factors stored for a controller to access and read. These multiple K factors are established for flowrates across the entire dynamic range of the flowmeter at the time it is manufactured. Just enough K factors are determined to provide good curve fitting. The flowmeter onboard memory is programmed with the multiple K factors.