I am having a 60 gallon bathtub installed and have only a 30 gallon tank water heater. I am considering installing a 12 kw on-demand tankless water heater to serve just that bath. My husband, and our plumber (who seems to have no experience with these units) do not think it will do the job, but I have done the math and, though close, I think it will work.
We live in Florida and the tap water temp is around 70 degrees in the winter, 80 plus in the summer. The flow rate for the faucet is 2.5 GPM. When I apply the formula, (Kw x 6.83 divided GPM) it appears I would get a 32.8 degree rise in temp, which would be just barely enough in the winter and ample in the summer.
Does anyone have experience with these units for this sort of application? What happens if the flow rate exceeds the unit's ability to heat to the desired temperature? Does the water cool off, or does the flow decrease?
Any suggestions?