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There is quite long pipe going from the gas water heater to our kitchen sink. This means that there is a delay before hot water actually becomes hot, especially in winter months.

Now I saw that there are mini electric heaters that are quite affordable (in the 100-200USD range). Some come with small tank (120V) and some are tankless (240V). I am thinking of putting such electric heater below the kitchen sink in series with the gas heater.

  1. Would using such "heaters in series" setup be a good idea to get instantenously hot water at the kitchen sink?
  2. If yes, What would be pros and cons of going tankless?
  3. Would the electric heater be able to correctly adjust output water temperature (because the inlet temperature will increase once the cooled water from pipes is drained).
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I have also considered doing this, but have not so far (inertia). One way to do this is to add a separate single faucet for the electric water heater and T off the cold supply. That way when you use the hot you don't leave hot water abandoned in the piping. If you T off the hot supply, you will leave hot water in the lines when you are finished with the hot, but you will have a full flow of hotter water.

A small tank can allow you to have on tap at gallon or two of very hot water even using the 120 V circuit which is probably there to power a disposer. If like a lot of people you have removed the disposer, you could use the receptacle for this. If your disposer receptacle is on a dedicated 20-A circuit, this would work very well, but if not, you could use the disposer wall switch to turn off the heater when high power appliances are being used with other receptacles on this circuit.

A tankless heater would have to be at least 30 A at 240 V to give a decent flow of very hot water. You would have to have add the cost and trouble of adding this circuit to the project, but this would be ideal.

EDIT

One of these very small tanks with its own spigot (presumably rated for very hot water) might be the best choice. You don't want to run very hot water (200 F) through a standard faucet.

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    This answer hits the highlights. A 240V/30A tankless will give you unlimited hot water and you can tee it off the cold water so you don't waste energy abandonding hot water in a long pipe. A mini-tank will give you very hot water (like for tea) for a short time until depletion, by which time the big tank hot water arrives, but that'll be more cost wasteful. The tankless will actually save money. – Harper Nov 10 '18 at 22:06
  • Interesting point about adding dedicated hot water faucet. I have a kettle that has keep-hot function. If this dedicated faucet would be able to deliver boiling water, then I could probably throw out that kettle too. The delayed hot water mostly annoys me when I have to rinse a single plate. Trying to look from economics angle here ... I guess it depends from water, gas, and electricity prices; and also hot water usage pattern (whether one uses hot water for long duration a few times a day or more frequent short runs). Is there consensus in plumber circles which is better option? – Hans Solo Nov 10 '18 at 22:36
  • We keep a 2.5-quart kettle on the electric range for scalding water. We have a central gas fired tankless WH and I can't stand to draw small amounts of hot. A kettle with a keep hot function would be better because on the stove one does forget and let the water boil furiously. But my wife doesn't want to clutter our small kitchen counter with an electric kettle. Even if you have a thermostatted burner, there is some risk with swinging a heavy kettle over to the sink. – Jim Stewart Nov 10 '18 at 22:52
  • Edited my answer. – Jim Stewart Nov 12 '18 at 21:45
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The small inexpensive instant hot water heaters for under the sink are intended to be used with your COLD water supply, not the hot, because it is intended to provide you with the equivalent of an instant kettle, i.e. for tea or for cooking. You should never use your hot water supply for anything that will be ingested, the hot water heater tank and pipes leach dissolved metals and chemicals into the water. Not a problem for washing etc., it's just not a good idea for eating.

That said, there is no reason why it CAN'T be used to "boost" your hot water supply instantly, but you may be disappointed with the flow rate on the ones that go under the sink. They come out like a drinking fountain, again because the INTENT is for making a cup of tea or not waiting to boil water for noodles. If you DID do this for your hot water supply, it would still have a thermostatic control to limit the maximum temperature, so one the hot water gets to it, it just works less or turns the heater elements off.

The other way to deal with this issue is to have another pipe run back from your furthest faucet to where the water heater is, and you install a circulation pump that runs hot water through your pipes throughout the house. You put that pump on a timer so it starts circulating the water just before you wake up and stops once everyone leaves the house, then starts again when people get home. If you do this, take the time to insulate the pipes as well. In fact if you have not already insulated your pipes, I would start there, you may be pleased with the results of just that step alone.

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I have done both. I have added both tankless and tanked water heaters under sinks. The only negative with the tanked unit it takes up a massive amount of space so if space is an issue this will be a no go from the start. I have installed quite a few tanked units in the 3 to 7 gallon range based on the size of the existing plumbing and the length with the goal that once all the cold water enters the tank the water temp will not drop below 120 F. This will provide for a temp that varies if you run the hot more than ~ 50% of the tank volume. After that it starts warming back up the pro to this solution is there are 120v heaters so no new wiring may be a possibly.

With electric tankless the con is the power required, there are a few 120v units out there but I think these are a waste of time the flow rate is so small on a dedicated 20 amp circuit that it is better to run a 30 amp 240v circuit and use a larger tankless unit tha can supply the needed volume of water at the desired temp until the water from the main heater arrives. At this point the electric will start reducing the power to the tankless heater and there is very little variation in temp. The pro here is the tankless electrics are small compared to a tanked unit. Both units require re plumbing so the cost there is a wash, many times your service panel and water heater are close together so if it is a long run it will cost more for the power. What is the best option? It depends on how much space you have and your goal for the system. I have successfully installed a gas tankless main water heater but the owner wanted hotter water in the bath, we added a electric tankless that adds additional heat to the water just prior to the point of use and this solution was what several owners still have. I had 1 customer that had his entire service upgraded to get the a huge electric tankless it was not hot enough and the largest tankless I could fit in his bath still did not get the water hot enough he went back to a main tanked heater but kept the tankless in the bath. In short I think a electric tankless at the point of use for a sink in the bath or kitchen is great and will provide instant hot water enough for even teenagers. Not so much at the shower because of higher flow rates. So base your decision on the the amount of space and go for it.

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