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On-demand with pre-heated hot water

Marc K.
Marc K. Member Posts: 15
I have a client living in a remote location, who wants to use a propane fired on demand water heater.  Issues: (1) Electricity provided by solar and/or gasoline generator, so low electrical use is necessary. (2) Water to on-demand unit will be pre-heated (either solar or wood stove or both), so a unit that can accomodate varying hot water temps. (including temps hot enough to go directly to fixtures) is required.  Any recommendations out there?



  • Jack
    Jack Member Posts: 1,047
    What you have to watch out for

    in these apps with tankless is: You will be sending some rocking' high temps to the unit from the solar tank and wood stove. So high that you can be driving the limits on the unit. I like to by-pass the tankless when the tank has sufficient temp to supply the demand. Then when the temp drops to 100 or so you bring the water to the tankless for peaking. I think Caleffi makes such a unit. Your other concern is to get a unit with the lowest flow requirement to initiate operation and the lowest btu input. I'm a fan of Rinnai, so bias noted, but you can get a unit (R75) that will fire down to .45 rpm and 10kbtu. Given the "warm" water temps you will be supplying to the tankless you need those low minimums. Power consumption on these is quite low...as long as you install it so the freeze protection circuit is not actuated. That means a side wall vent with several feet of pipe grading down to prevent cold air spillage into the cabinet. Properly done these are great systems.
  • icesailor
    icesailor Member Posts: 7,265
    On Demand, Off Grid:

    Maybe you should check out one of these. They are battery powered ignition. 2 D Cells.

    I have a portable one for in my barn for a horse that has sensitive teeth and won't drink cold water.

    They have a flow switch and will not run without flow.

    The company is in Puerto Rico and remember, if you have no power other than a 12 volt battery, a small 12 volt pump in a stream and a grill tank will give you a hot shower.

  • Gordan
    Gordan Member Posts: 891
    Water-cooled gas generator?

    If he's going to be running a gas generator, it seems wasteful to let go of all of the heat that thing will generate. If he used a water-cooled generator then he could use solar preheat with solar electric and generator waste heat when the generator is running.
  • kcopp
    kcopp Member Posts: 4,443
  • Marc K.
    Marc K. Member Posts: 15
    Bosch 520 unit

    I actually found that as I was looking through some Bosch literature yesterday.  I need to download the specs to see what kind of water pressure requirements are necessary for it to work correctly.  The well is using a 12V pump! If  anyone has experience with this unit (in remote, non-grid locations), I'd love to hear about it.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    I have used the BOSCH

    unit, in an off-grid application.  Seemed to work well, but is certainly not as refined as a Rinnai. Required little flow for it to fire.  No expierence with pre-heated water in it though.  The capacitor ignition system is on the cold side, and I don't know how it would fair with 140+ water temps fed through it.  Keep in mind a Rinnai at full output (full fan) is only around 100 watts.  The place I worked at had an inverter and battery bank setup charged with a remote yanmar diesel generator, as well as an on-site Honda. 

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,338
    Is their PV system...

    ... big enough to accommodate a heat pump water heater?  Run it in heat pump only mode.  Just a thought.

This discussion has been closed.