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Community and Q&A

Is there an instant hot water heater that I can add to a hot water line?

McHouse | Posted in Energy Efficiency and Durability on

I have the common problem of it taking several minutes for hot water to get to the kitchen sink from the water heater in the basement. Is there an instant hot water heater that can be installed on the hot water line at the kitchen sink that will heat up the incoming water until the incoming water is at the hot temperature required, at which point the instant heater would switch off and just pass through the already hot water?
Many thanks for any info

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  1. GBA Editor
    Martin Holladay | | #1

    The answer is yes.

    Google "instantaneous electric water heater" or "tankless electric water heater" to discover many brands.

  2. Expert Member
    Dana Dorsett | | #2

    The very high intermittent power draws of a tankless water heaters are abusive to the grid infrastructure, and are one of the least-green solutions to this type of problem. Some greener-better solutions are:

    1: Demand type recirculation loop, where a pump brings the water to the device much faster whenever water is being drawn, activated by either a flow detector or a manual button/switch, turning the pump off when the temperature of the water at the sink rises above some threshold. As a retrofit the cold water feed to the sink can usually serve as the return line.

    2: A small (1-2 gallon) under-sink electric tank, with the hot water from the remote water heater feeding the cold side of the local tank-let. With this solution short draws are always from the local tank, which re-heats the water whenever the tank temp drops, but longer draws see an initial dip in temperature with the dilution of the hot water in the tank by the tepid water in the plumbing, followed by the full temperature of the water coming from the other water heater. The local heater may come on for a short time during that dip, but most of the time it's just making up for the standby losses. The size of the tank needed depends on how much water volume is in the plumbing between the remote & local water heaters. A 100' of half-inch pipe is about a gallon, so with a 2 gallon tank kept at 140F ( with a thermostatic mixing valve reducing the temp to the sink to 110-115F), the gallon of room-temp water in 100' of pipe from the other tank isn't enough dilution to lower the temperature going to the sink, but with a 1-gallon water heater it might.

    Of the two the recirculation approach is usually the greenest. With either approach there will be some amount of heat abandoned in the plumbing between the two water heaters, but the small under-sink tank approach has a steady standby loss.

  3. user-2890856 | | #3

    There are retro fit hot water recirc systems available . They consist of a fitting that goes at a fixture location between the cold and hot supplies . When a circulator is powered the fitting , similar to a check valve , opens and pulls the wter from the cooled hot line back to the circ through the cold water line . This can be operated from a point of use switch or motion sensor so the circ only comes on for a short time . Much more efficient and energy saving than a point of use heater .

  4. McHouse | | #4

    Thank you! I feel like an idiot as I thought that hot water eventually coming into the instant heater would be a problem!

  5. Jon_R | | #5

    The small under-sink water heater will produce hot water much quicker and is more convenient than manually triggered demand re-circulation. Not having a mixing valve, I notice some drop.

    A demand re-circulation system (typically not achieving plug flow rates) heats up the water in the return pipe - causing some inefficiency/energy waste, especially with lots of brief flows.

    Would be nice to quantify standby costs (my guess is less than $2/month).

  6. David_Gregory_CZ3_CA | | #6

    friends - following up on this as the answer(s) seem too simple to me; perhaps my application is slightly different (bathroom sink, low flow, only several seconds of heating needed before water from HPWH arrives)

    Do _all_ MINI tankless instant water heaters (e.g. Stiebel Eltron 231045 Mini 2 Point of Use Tankless Electric Water Heater, 1.8 kW, 120V; or Instant Flow SR-20L/120 HTR-I from Chronomite labs) modulate the wattage as the incoming water warms, so as to not overheat? Or do some (which?) either just turn the heating element 'on' or 'off' when flow starts / stops (because they 'expect' cold water), or 'on' until some set input temperature is reached, in which case they would be either be overheating, once the hot water arrives, or uneven as the warm water begins to arrive, if plumbed to hot...? (I assume they have high temp controls to prevent scalding)

    Thanks for any input...I feel there's a place for this approach and it was dismissed too quickly...saving water (vs. running until warm), lower cost / complexity / noise vs. recirc; not putting (cooled-off) hot water into the cold water line (legionnaires? minerals/metals?)...

    1. PBP1 | | #7

      Eemax point of use include thermostatic models that switch the element on responsive to flow and off depending on the temperature of the incoming water (which is programmable). I believe there's a program setting for a max outlet temperature too. Thus, if you set the max outlet temperature, regardless of inlet temperature (as long as it is below the max outlet temp), the unit will "modulate". Look for "thermostatic" as many models do not have such controls.

      As a side note, the Eemax units have a relay that can be programmed to be always on such that you don't hear a snap (mechanical relay closing) once you turn a faucet on - that may expedite hot water by a fraction of a second. The downside, most all mechanical relays buzz, which is annoying. I set mine to operate by closing the relay responsive to sensing flow - so i hear a snap, followed by a release when the flow is either shut off or the inlet flow reaches the programmed temperature (my 2.4 kW heater is downstream a small electric tank heater (2.5 gal), which is downstream an on-demand gas heater).

      1. David_Gregory_CZ3_CA | | #8

        Thanks! Will look into those…

        My first experience with tankless was gas-fired in Poland and it was a miserable bath/shower between hot and cold…gas-fired in Chile a decade later not much better…it’s mostly about the spec I suppose…

        1. PBP1 | | #9

          You have quite the experience, I've been places with the shower head electric units with exposed wires ;-) Safety first - some places don't seem to pay attention to venting the small gas units adequately.

      2. mgensler | | #10

        I have an eemax thermostatic and would like to disable the relay from being always on. Could you share how to program that?

        1. PBP1 | | #11

          It took me a while to figure it out, I actually spoke with Eemax, they were helpful. They even sent out a new relay that I installed, while better, still had the buzz (seems inherent in design of most relays of that type regardless of mfg). I'm sensitive to noise and have a workdesk adjacent the backside of the kitchen sink.

          I was able to find my info from Eemax:

          START Excerpt

          Thanks for your time today, the SPEX2412T per the “non-silent mode” (or ECO or Eco mode) is actually now clicking on/off on-demand such that the contactor coil is not continuously energized/humming. The clicks are not too loud.

          The manual I have is EXO7200-121 Rev. A, p. 11 has the flow chart for settings.

          Thus, no more contactor humming, unless it occurs during the short period of time the unit is heating.

          END Excerpt

          Website with manual:

          At p. 11 of the manual - see the table and the flowchart - unfortunately, the manual does not explicitly relate "Non-Silent Mode" "NS" an "Ecomode" - but they're related. ECO is Electrical Cut Off.

          Table: ERROR CODES & UNITS
          From Advanced Menu, press and hold the “+” and “–“ buttons for 10 seconds.
          Non-Silent Mode NS Toggle non-silent operation

          Flowchart: Eco Mode (ECO) is the last option in the Error Codes & Units series.

          Hope that helps. I do like the click on/off as it gives me feedback as to when its heating or not heating.

          PS - if there's a power outage, I've had no need to reprogram, seems to hold the setting.

          1. mgensler | | #12

            That works! Thanks. Eemax also replaced my relay under warranty. They should have told me about this feature first. The new relay didn't make a difference. Cheers!

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