Tankless Water heater Ground Water Temp

Discussion in 'Tankless Water Heater Forum' started by Alan Kish, Apr 4, 2021.

  1. Alan Kish

    Alan Kish New Member

    Apr 4, 2021
    I want to install a natural gas tankless water heater, converting from a standard tank type. My house is 2 story 3200 square feet with 2 occupants. My ground water temp is 42F. I live in a cold climate in North West Montana, I am anxious to get the right system, any recommendations or concerns? I was looking at Rinnai tankless systems.
  2. Bannerman

    Bannerman Well-Known Member

    Mar 19, 2014
    Ontario, Canada
    Consider a Navien NPE-240A2

    Gas input anywhere between 13,000 - 199,000 BTU/hr @ 96% efficiency.

    Stainless Steel primary and secondary heat exchangers.

    Built-in buffer tank and recirculation pump.

    120F - 43F = 78 degree temp rise. Rated at 5.2 GPM @ 75F rise and 4.9 GPM @ 80F rise

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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    First thing is to determine the maximum volume of hot water you will want at the same time, and then, at what outlet temperature. You probably don't want to be going to the unit or a remote and changing the settings frequently. Say washing dishes, uou want it fairly warm. You may have occasion to do a load of wash with hot, and unless your dishwasher has a heater in it (and if it does, when it needs to heat the water, that extends the cycle, maybe by a lot of time), your dishes won't come out clean unless it's near max (nominally 120-degrees in a home unless you can plumb directlyu the DW with a hotter branch).

    Water weighs about 8.4#/gallon. Take your GPM max you want to support, and the temperature difference between the incoming water and your desired set point. Multiply the gpm * 8.4 then * 60 minutes (heaters are rated at BTU/hour), then multiply that by the temperature rise you want, and (without taking into efficiency) that will give you the BTU requirements. My guess is that will be more than the largest one you can buy. The end result will be cooler water coming out, or it will throttle the volume, neither one is great.

    The ratings are done with the average incoming water temp of 50-degrees...your max temp at those same volumes will be less by how far below that your incoming water is.

    FWIW, when I thought about going tankless, I measured my incoming water after a cold spell, and it was barely above freezing...while still liquid, it read 32-degrees on my thermometer. So, consider that in your assessment. Southern NH isn't generally considered all that cold...Wyoming can be colder.
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