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There are other questions such as this that ask about a point of use heater to provide quicker hot water, but they don't seem to fit what I want.

Our kitchen sink takes a while to get hot, and I would like this to be more immediate. If I use a small tank water heater (2.5 gal) under the sink, and connect that to the hot water, it seems that I would get 2.5 gallons of hot water immediately, followed by 20 seconds of cool water (the time it normally takes to get hot water) that would then gradually warm back up, right?

What I would love to have is an inline tankless, plug in (120v) water heater that would immediately provide warm (ok if not super hot) water, and then automatically turn off once hot water from the tank arrives so that the water is not over heated.

It seems like such an obvious solution, but I haven't been able to find such a device. Is there a reason why this does not exist?

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    I thought any good quality electric tankless water heater would do what you want. I expect that they would have temperature feedback control circuitry to regulate the power input in response to flow and input water temperature to avoid output water over the set temperature. See gamut.com/p/… – Jim Stewart Oct 23 '17 at 20:08
  • @JimStewart, I guess that's part of my question. I haven't seen them explicitly state that they work safely with hot water input, but maybe they do just by the nature of their design. I just can't have even a second or two of scalding water if I'm washing hands or doing dishes. Oh, and that linked heater requires 30A. I'd like to have a 15A solution so I can use an existing outlet. – JPhi1618 Oct 23 '17 at 20:13
  • Water from our kitchen faucet takes a while to get hot and really it doesn't get hot enough to adequately disinfect dishes. We have a 115 kBTU 1st gen central nat gas tankless w heater. The hot line in un-insulated and goes under the slab! I have thought about doing what you are suggesting, but haven't done so. I heat a kettle of water on the stove and pour it over the dishes in a basin in the sink. I use a brush on a medium length handle to avoid putting my hands in the hot water. The most dangerous thing about this is carrying the kettle of scalding water from the stove to the sink. – Jim Stewart Oct 23 '17 at 20:21
  • If there were children or unruly dogs underfoot I would not do what I'm doing. If the handle of the kettle would release, there could be an awful injury. Some people catch the initial cold water from the hot tap (kitchen or shower) in a bucket and use it later in the garden. – Jim Stewart Oct 23 '17 at 20:26
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I have thought about what you are proposing, but at no expense and installation trouble I just use a kettle on the stove. I recognize the risk of messing with a kettle of scalding water once a day. Now that I think about it I will start holding the kettle over the counter when transferring it the 7' to the sink.

I think any good quality electric tankless water heater would do what you want. I expect that they would have temperature feedback control circuitry to regulate the power input in response to flow and input water temperature to avoid output water over the set temperature.

In my case the 120-V 20-A dishwasher circuit in the next cabinet could be used to power an under-sink tankless WH. The dishwasher was taken out in kitchen remodel 15 years ago and the fact is that 38 years ago we used it a few times but never after (noisy builder's model). I have a good dedicated circuit sitting idle, breaker turned off.

  • But the kettle takes time to heat, right? Good idea for very hot water, but I don't think it's going to be faster than just waiting for hot water from the tap. Of course I'm lazily trying to save 20 seconds, not a few minutes like some homes have to endure. – JPhi1618 Oct 23 '17 at 21:04
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The tankless water heaters I have installed have thermostats. When the water exiting the full sized or under counter model gets to the set temp it quits heating. I have installed electric point of use models in bathrooms and kitchens from memory the hottest setting was 130-140f as the water gets warmer from the primary water heater the point of use unit shuts down no over heating at all. The first time I installed this setup I was concerned as you are about scalding. I measured the temp set at 115f and turned the water on full hot at first the flow of cold water was more than the small unit could heat only about 110-112 within 3 minutes the water was running 120f. I checked another hot tap and the water was at 120. I turned up the small unit and the water never exceeded the set point. I have done this test with several brands and they all preformed close to the same. One test I did on a system that had a gas tankless and electric point of use was to run more hot than the gas tankless could keep up with and measure the point of use then turned of the extra tap so all the hot water went to the point of use electric, it did overshoot by 2 or 3 degrees but that was worst case. I don't know if the smaller gas tankless heaters would respond as quickly as the electric because I have not tested that but I recommend the smaller point of use heaters for bathrooms that are a long way from the waterheater I think this is more efficient than recirculation systems and the small electrics are reasonable to wire in not major power hogs like the whole house electric units.

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TLDR: they all do that, it's a safety feature to avoid scalding.

It takes a lot of power to make heat.

You won't get much heat out of a 120V on-demand heater unless you really, really, really restrict water flow, like, you could get a cup of tea out of it if you're patient. Don't even bother with on-demand unless you are willing to run 240V to the heater and at least 20A preferably 30.

An on-demand heater made to run alone on a 15A circuit, is 1440W. It will only embarrass itself and its entire class of heaters. You will be disappointed, swear them off for good, and that will be a shame because they are wonderful when you size them properly.

As others discuss, they have safety controls to prevent outputting scalding water. You should not set your on-demand heater to scalding to avoid legionella, legionella can't grow in an on-demand. You should set your tank heater to kill legionella.

  • So you wouldn't even bother with something like this: Chronomite ML-15? IT says the 15 unit will give a 28 degree rise on 1 GPM. At least I would have ~80-90 degree water immediately? I guess that's not very hot, and 1GPM is like a faucet only turned halfway on... – JPhi1618 Oct 24 '17 at 16:24
  • @JPhi1618 a great example of what you should do, because you are running a respectable amount of power to the heater. That one lets you use 12AWG (good for 16A continuous). Of course sourcing 277V is a little tricky in a home, you'd need to boost 240V with a 36V/20A transformer. – Harper Oct 24 '17 at 17:02
  • Good catch, I didn’t know that was 277v... I’m not even sure where in the world uses that. I’d be stuck using the 20A 120v model and that one is very anemic... not even a listed rise for 1.0GPM... – JPhi1618 Oct 24 '17 at 17:04
  • @JPhi1618 well, you need a dedicated circuit anyway even for a 20A 120V model, so put two pieces of black tape on the whites, land it on a 2-pole breaker instead of a 1-pole, and use the 240V model. Double the heat and it only costs you 2 pieces of tape. 277V is widely used in commercial, it's delivered as 480 3-phase wye, which can drive 480 delta loads and also 277V leg to neutral. Commonly used in lighting where you can put 40kw of lighting load on twelve #12 wires in a 3/4" conduit. Very efficient. – Harper Oct 24 '17 at 18:15

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