Tankless water heater in line with building hot water

Question

Will a tankless water heater at a bathroom sink on the hot water line shut off once the hot house hot water reaches it?

Answer 1

Yes, but it also won't do jack-all to heat water.

You need 40 amps of power per GPM of flow (9600W per GPM, or 80A per GPM if 120V). If you don't provide that, you will get tepid water and yet another tankless water heater project failure because of unrealistic design. It happens a lot.

People object and go "Oh, but this is such a small load! It's only an under-the-sink heater and the big heater will pick it up when hot water arrives". Nope, the cold water moving through it right now is unpersuaded by that emotional appeal. The only way to make it work electrically is for the tankless heater to have a stack of batteries inside it, so it can borrow from the batteries to produce 9600 x GPM watts in the moment and recharge later.

However, water itself makes a much better battery than batteries do, when heat is the ultimate objective*. And that's readily available in the marketplace, as a "tanked, not tankless, point-of-use heater" that runs on 1500 watts and keeps a gallon or three of hot water right under the sink. If the volume of the tank is 3-4 times the volume of the pipe from the water heater, it should mix well enough that you won't get a shot of cold water in the middle.

* We once crunched the numbers for a person who wanted to physically carry BTUs of heat from the house to a shed, and was thinking of using lead-acid batteries to run a resistive electric heater. The "BTUs per pound of weight" figure very much favored bringing a kettle of boiling water instead.

Answer 2

It depends on the temperature settings. For example, if the house hot water tank is set to 120 degrees and the tankless is set to 140 degrees, then the tankless will never stop running while you call for hot water. On the other hand, if the tankless is set to 120 degrees (plenty hot enough) and the tank is set to 140 degrees (to avoid growing nasty stuff) then the tankless should shut off as soon as the 140 degree water starts coming through.