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

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  • Why would you want to put a tankless WH in line with building hot water? Commented Apr 26, 2023 at 22:03
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    @JimStewart I'm curious too. They can be useful if it's a long hot line and you want quick hot water, or as a booster. Otherwise it's a lot to do for usually little benefit.
    – KMJ
    Commented Apr 26, 2023 at 22:39
  • Not an exact duplicate, but worth reading: diy.stackexchange.com/questions/176654/… Commented Apr 26, 2023 at 22:50

2 Answers 2

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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.

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    As an alternative, sink locations are great spots for a small point-of-use heater. They're pretty common in commercial construction. Typically they're a few gallons and run off a normal 120V 15A or 20A circuit. They are effectively using the water as a battery.
    – KMJ
    Commented Apr 27, 2023 at 16:05
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    @KMJ Oh yes, using water as a "hot, wet battery" would be the better answer actually. Commented Apr 27, 2023 at 21:20
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    The nearest big box to me stocks and sells tank heaters from 2.5gal to 20gal. They go on a 15A or 20A 120V circuit, super easy to run.
    – KMJ
    Commented Apr 27, 2023 at 21:27
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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.

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  • If you think 160 is normal for a hot water heater I don't want to use the water in your house! Even 140 can cause significant burns in a few seconds. I assume you're talking about both of those in combination with a tempering valve of some sort.
    – KMJ
    Commented Apr 26, 2023 at 20:36
  • @KMJ I agree 160 or even 140 is WAY TOO HOT! It may be OK to run the WH at 160, just so it has a mixing valve to reduce the temp to 120 before it goes to the plumbing. I run my WH at about 120-125 and have never had a problem with "nasty stuff". Commented Apr 26, 2023 at 20:42
  • Sorry, looks like the typical numbers are 140 (for hot enough to kill germs) and 120 (not too hot for safety). But yes, at those temperatures need mixing valve. And in my house I go to the cooler side. Commented Apr 26, 2023 at 20:56
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    Appreciate the edit, those are more sensible temperatures.
    – KMJ
    Commented Apr 26, 2023 at 22:38

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