I was planning to install a mini tank water heater under the kitchen sink. It's just to be able to wash hands quickly without waiting for the hot water to come in from main heater. I don't have enough space to install one because I have a water filtering system with a tank which used a lot of space in the cabinet.

Then I was thinking to get one of those tankless water heaters because they take less space. I don't have a 220v and don't want to spend the money for an electrician to put one under the sink.

Are the 110v tankless water heaters any good for getting instant warm water for washing hands? I don't need hot water from it. Just a comfortable temperature for washing hands quickly. Also the faucet needs to continue working as before with getting hot water from the main heater.

My preference would be for a 55 degree rise ("very cold" say 40F up to 95F) and I'd really rather use the regular tap for this, not a side tap. I do not actually know the flow rate of my regular tap (is there a typical value?) but I'd prefer flow not be impeded and I can use it as normal.

If you can also recommend one with the setup I described, that would be great.

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    So what is your desired flow rate, in liters per minute or gallons per minute? Also what is your cold water temperature and what temperatures will you find acceptable? May 3 at 0:01
  • Would you want to feed this 120 V tankless with the cold water or the hot water supply? May 3 at 1:33
  • @Jim Stewart Feed it from the hot water supply so they're all one system. Now that I think more about it, I am wondering if the tankless heater would automatically shut itself off once the incoming water is hot, coming from the main heater. Maybe depending on the model. May 3 at 1:41
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    There'd be no point to feeding the tankless from the hot. The thing about tankless is that if it works while waiting for hot water, it works at all times and you don't need the other heater at all! "That was easy" As such, most "daisy chaining" tanked and tankless heaters are an exercise in stupidity. Speaking of that, "This is not a scientific experiment where precise information is needed." It is if you want it to work. You don't get a free pass on physics. And you have a physics problem you do not understand, and the only way to win is understand it. May 3 at 3:26
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    @Tony_Henrich I understand perfectly well what your IDEAL situation would be. I also know perfectly well that this is thermodynamically impossible. I am trying to educate you on the "why" of the matter, so you can decide for yourself on which direction you would like to compromise. a) side tap b) tanked heater c) LARGE 240v feed or d) abandon project. You have given no guidance on that, since you say every direction of compromise is unacceptable, and you have been curt and rude to everyone who suggests it. Shame on you. May 3 at 17:54

2 Answers 2


No, they're really not. True tankless on 1500W (max plug-in power) is actually a hard problem, because physics is against you. You cannot deliver enough BTUs to do the job.

That's why they're not much of a product.

The common ones are not tankless.

Most "under sink" heaters sold are not tankless at all, they have a little tank about the size of a 2-liter Coke bottle which they keep pre-heated. Those are small, which is a requirement you have, but run much too hot for your use. They are for tea. Or they have a 2.5 to 6 gallon tank for washing, but those are too large for your application. I have a 4-gallon at the lodge; it's over a foot in diameter and 18" tall.

Note that the compact "tea sized" tankless impede flow too much to be inline, and their minimum setting is far too hot. The larger 3-6 gallon tanked heaters have acceptable flow for main-tap use, but are quite larger than you have room for.

Can tankless work on existing wires?

You have the constriction "I don't have a 220v and don't want to spend the money for an electrician to put one under the sink". So what if we don't? What if we try to work with the wires in the walls? TLDR: impossible at main-tap flow rates, but you could use a "side tap" to limit the water flow to that which would be possible.

You can do the math. It takes 1 BTU of energy to heat 1 pound (pint) of water 1 degree F. So if you want to warm 1 pint from 50F to 90F, that is 40 BTU. Now think about your flow rate, how many gallons per minute do you want to flow? A gallon is 8 pints and 8.3 pounds. The faster the flow, the more BTUs you need.

1 watt for 1 hour gives 3.41 BTU.

1 watt for 1 second gives 0.00095 BTU.

Most of those heaters are 1500 watts. 1500 watts can give 1.4 BTUs per second, or 85 BTU per minute.

Now you see plain why this isn't going to work on main-tap flow. At 2 GPM flow (16 pints/min) you are dividing 85 BTU by 16, and getting about 5 degrees of heat rise.

However, as a side-tap it could be a player - at 0.25 GPM flow (2 pints/min) we are 85/2 = 42.5 degree rise. At 0.375 GPM (3 pints/min) we have 27.5 degree rise. Both are sad even for a side tap, but better than nothing.

Now if the heater has a dedicated circuit just for it, then it cannot be overstated how easy it is to convert a 120V circuit to a 240V circuit (at the same amperage!!!) 15A wire (#14) can give 3600W, 20A wire (#12) can give 4800W. Crunch the numbers based on your inlet/outlet temp and desired flow, but it seems like a respectable side-tap could happen - 3600W giving 48 degree rise at 0.5 GPM (still side-tap flow).

We're still quite some distance from it being viable on main tap.

What if power were no object?

If you want tankless to work on the main tap at normal flows, power cannot be an object. But you must balance the heat rise you are willing to accept with the energy required to get that rise.

Further, the amount of power will be a huge fraction of total electrical load in the house (really). As such, it will require a standard NEC Article 220/CEC section 8 Load Calculation to determine whether the existing service can handle this new load. If not, then either a service upgrade will be required, or new "demand-side management" tech that has not quite arrived.

The designer carelessly undersizing a tankless system is the #1 way in which tankless heater installations fail. Invariably, the half-wit DIY-designing homeowner blames tankless tech, when the real problem is in the bathroom mirror. I hope I inspire an appropriate disdain for this careless character.

So let's crunch the numbers. My own sink flows 1.25 GPM, however it's a shadow of its former self. I would aim for 2 GPM out of an abundance of caution. Let's figure the wattage per degree rise on 2 GPM. Well, 2 GPM is 16.6 pounds per minute, so we need 16.6 BTUs (absolute) per degree rise per minute. That is 16.6*60 = 996 BTUs/hour per degree rise. Divide by 3.41 BTU/watt-hour and we get 292 watts per degree of rise at presumed worst-case 2 GPM for a kitchen sink (not a whole house).

So at 2 GPM if we want 10 degrees rise, 2920 watts. 50 degree rise, 14604 watts, and at OP's guess of 55 degree rise, 16000 watts given assumed 2 GPM flow since OP won't say.

(and those >6kW tankless start to get large, so back to the space problem).

Yeah, that's a lot. This is the tyranny of tankless water heating. And like I say, this is where homeowners cut corners and put in too small a system.

So how in blazes do the British do their "electric showers" on a measly 40 amps (9.5 KW)? By sharply limiting flow to 1 GPM (141 watts per BTU) or less. Low-flow even by California standards. But that's not going to work on a general tap unless constrict the tap flow all the time.

Limited choices...

Unfortunately in comments you have precluded every one of them, so I don't know what to tell you lol.

  • Use a tanked heater inline. However, the small/compact heaters (2 quart) are too low flow, leaving the 3-6 gallon tanks. Those take a lot of space, and as you say "I don't have enough space to install one because I have a water filtering system with a tank which used a lot of space in the cabinet." So cross that out.
  • Use a side-tap. This would allow you to limit flow to a rate low enough (0.25 GPM) for a small heater like an EEmax Spex1812T to work. However you have said "The water is going into the same main faucet where it gets mixed with cold water to produce warm water." so cross that off.
  • Bring big power. However the amount of power is considerable, and this requires a Load Calc plus service upgrade or demand-side management. Also, you say " I don't have a 220v and don't want to spend the money for an electrician to put one under the sink." so cross that off too.


What does that leave? Water circulation systems. That requires either a return pipe to be plumbed, which involves tearing up the house. Or a hinky, dangerous hack of bootlegging the cold water line to shove the cool/tepid water back to the heater. If anythign goes wrong with that, both lines will be hot, and a thermostatic mixing valve cannot protect you from scalding.

Other than that, I don't know what to tell you. If I were you, which I'm not, I would find the space for a 4 gallon tanked. I have one at the lodge and has sufficient flow and capacity to be a "reliever" until the main hot water arrives.

  • There are both tank and tankless water heaters. Amazon has a bunch of tankless heaters listed. I don't want hot water. I want warm water. My post is all about tankless heaters for producing warm water for washing hands. It's a specific need. I have not seen 1L tank heaters. The smallest were like 2 gallons which is still too big for me. May 3 at 0:39
  • The under the sink tank and tankless mini heaters are mainly used for getting instant hot/warm water instead of cold water. That's the common usage description on Amazon. You mentioned water being too hot for my use. The water is going into the same main faucet where it gets mixed with cold water to produce warm water. It seems you're confused with the water heaters which are used for making tea/coffee where they have their own small hot water only faucet. May 3 at 1:50
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    @Tony I explained the physics for you, so that you could crunch the numbers for yourself. I was expecting you would have done so, and thus realized the complete futility of doing this on the main hot water line at normal kitchen-faucet multi-GPM flows. The only way this happens is with a side tap. I knew this going in, and was sure you'd come to the same conclusion when you crunched the numbers. May 3 at 2:44
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    @Tony_Henrich -- have you looked at the tepid-water heaters used to feed eyewash stations? May 3 at 3:29
  • @ThreePhaseEel interesting, the EEmax LavAdvantage line could possibly work as a side-tap with enough flow restriction... the SPEX1812T has 35 degree F rise at 0.35 GPM. May 3 at 4:37

If you can turn down the temperature (most of these are set far too hot, just below boiling, for hand-washing), and are satisfied with perhaps a liter of hot water, it should work. It might take twenty minutes to warm up the next batch, after using up that stored.

Note that these units always uses electricity, as they cycle on -and-off to keep the water warm, but that shouldn't be excessive, at comfortable water temperatures.

Caveat: Be very careful when washing, as I've had a thermostat weld itself shut, creating boiling water!

  • Are you talking about tankless or tank heaters? I don't think a 110v tankless heater is able to heat running water going through it to boiling or even very hot. May 3 at 0:31

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