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First Man on the Moon

djthxdjthx Member Posts: 52
Can't believe I'm going where no man has gone before, i.e., the first to post on this forum.  Anyhow, what is the most efficient way to produce DHW?  I thought that tankless was the way to go, until I installed one in my home.  Aside from having to have sufficient flow, you'll always get that "cold sandwich" effect in between showers.  Is there a better way?  Also, is there a way to recapture any of the heat that goes down the drain along with the hot water?


  • Mark EathertonMark Eatherton Member Posts: 5,843
    My personal fav's

    For DHW production with a modcon appliance is to use a revers indirect (Turbomax or Ergomax).

    For drain waste heat recovery,

    There are others out there, but I am personally familiar with this one. It can recover up to 50% of the heat available.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • EricAuneEricAune Member Posts: 432

    The sandwiching can be reduced.  I have found very positive results in eliminating the hot/cold sandwich by using a thermostatic re-circ.  This my not be possible for your house, if its not you could install a small electric water heater after the tankless.  I have installed small 6 gallon tank to act as a buffer/storage on the outlet for some customers to cut down on the sandwiching, its works really well.  (no need to wire the tank)  They are reletively inexpensive, easy to install and will likely last a long time.

    I have been using Rinnai's tankless units with very good reports.  The Navien has a model with a built-in buffer tank to curb this issue. 
    "If you don't like change, your going to like irrelevance even less"
  • djthxdjthx Member Posts: 52
    Indirect vs Direct DHW tank

    Is an indirect dhw tank, even one installed off of a modcon, more efficient than a direct, high efficient hw tank?  The fact that you have to heat one source to indirectly heat another one doesn't sound too efficient.  From what I understand, you can get a 95% efficient direct hw tank.  Wouldn't this be the way to go?  What are the advantages of going with an indirect tank?
  • djthxdjthx Member Posts: 52
    Electric Tank

    Any idea how much energy a 6 gallon electric tank consumes?  
  • Jean-David BeyerJean-David Beyer Member Posts: 2,656
    It seems to me, as a non-professsional, ...

    ... that to decide between direct hot water heater and indirect requires more information than is available to justify a general answer. I am not a heating professional, but I think about these issues a lot.

    I used to have an oil-fired hot water boiler whose aquastat kept the water between 130F and 140F. (In theory, this would promote condensing, and thus rust out the boiler, but it did not; I guess I was lucky.) Basically, the boiler was always on during the heating season. And I had an electric hot water heater. Now running an indirect hot water heater off that would not have made sense. It used over a gallon of oil a day to keep the thing hot, and I would not wish to do that during the summer.

    But now I have a cold-start boiler that holds about 3 quarts of water, and an indirect fired hot water heater. I notice that it fires for hot water only once or twice a day for 5 to 10 minutes depending on how I adjust things. In addition to the water in the boiler, the DWH holds 6 gallons in its water jacket (tank within a tank design), and maybe a gallon in the pipes between the two. So 8 gallons of water. Once the hot water heater's thermostat is satisfied, the circulator to it continues to run for 2 minutes so the left-over heat in the boiler can be transferred to the hot water heater. Furthermore, the boiler has combustion air from outside and exhausts to the outside. A small blower does this, so there is no air flow through the boiler when it is not firing. That seems quite efficient to me.

    If I had a gas fired water heater, the exhaust pipe would probably go out the cheminy

    and thus there would be a cooling draft through it when it was not heating, lowering overall efficiency even if it worked out to 95% when it is firing. I have not studied high efficiency gas water heaters, so maybe they have heat exchangers as efficient as my boiler, and control the draft through it to be zero when not heating. If so, my guess is that their efficiencies could be close.
  • DanHolohanDanHolohan Member, Moderator, Administrator Posts: 14,908
    Sears says

    2,000 Kilowattt Hours per Year for their Kenmore unit.
    Retired and loving it.
  • EricAuneEricAune Member Posts: 432
    edited March 2010
    electric tank

    I don't actually wire the tank, just pipe it in series. It will not totally eliminate the problem, but it does cut it down significantly.  As the cold water that passes through the tankless enters the tank the temperature is mixed.

    This does pose an issue when the house has sat idle for extended periods as the water in will cool.  you could wire it for that reason alone, most are only 120V.  I have not had any complaints, actually I first tried this as a remedy for a couple complaining about the sandwiching issue.  They are happy with the results, oh and by the way, their gas bill dropped significantly because they were using a (correctly sized) 180,000 Btu cast iron boiler and an indirect.  The new tankless was properly sized and doesn't require the behemoth boiler to fire for a small domestic load....all year.
    "If you don't like change, your going to like irrelevance even less"
  • djthxdjthx Member Posts: 52

    for the info.
  • TonySTonyS Member Posts: 849
    How much flow do you need?

    Navien makes a unit with a small buffer tank, about a half gallon that eliminates the cold sandwich. It is not a true tankless because you now have heat loss from the small tank. The efficiency is 98% and they can be tied together with as many as you need to fill your flow needs.
  • WaterHeaterGuyWaterHeaterGuy Member Posts: 80

    The Eternal unit has a similar design... 2.2 gallons of stored water eliminates the "sandwich".

    As for which system is more efficient (indirect w/ modcon boiler -vs- standard atmospheric) the answer is frustrating, but usage makes a huge difference in comparring the two.
  • Hey_ObieHey_Obie Member Posts: 66
    What is a sandwich

    I can't figure out what the sandwich problem is with an inderect hot water. To me, the hot water comes out of the tank mixed and goes on its way. But it sounds to me like you guys are saying, the water goes out hot with cold in between hot. Do I have it right?
  • JamesplumbJamesplumb Member Posts: 2

    The "sandwich" is from an "On-Demand" type tank-less water heater, not an indirect. An indirect can have temperature stacking but that different. The sandwich is from the tank not storing hot water so a flow sensor turns the gas valve on to the appropriate level to create 120 or so. If you turn off the water for about 30 seconds or so the unit thinks you are done with hot water, then if you turn hot water on again you get a section of cold water running through the line for a bit until the flow sensor realizes to turn everything on again. I call it a cold water burp but either way its a pain.
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