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Steve Fontas
Steve Fontas Member Posts: 26
in Domestic Hot Water
Requirements are to make DHW for 6 adults 3 children, bathtubs, laundry, etc. Here's the kick. <strong>no space heat is required.</strong> Solar is already in the equation (120 gal storage) so what is your back-up? My gut tells me tankless is going to put quite a drain on the pocketbook. What do you folks think?
-S.

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  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    What fuel source...

    What are the associated fuel costs, and what kind of dump loads are you trying to handle?



    Then there's the pocket book issue. You can pay for efficiency now and not pay for inefficiency in fuel costs over time, or you can get by inexpensively, and pay continuously for your up front decision.



    You can't have high efficiency and low upfront budget costs. The two do not coincide with each other.



    Personally, if natural gas were available, I'd use THE highest efficiency product available that would meet the needs.



    And don't forget to take into consideration the 1 man 5 woman Olympic swimming pool sized jacuzzi that they intend to fill 2 times per year... (Got storage?)







    ME
    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.
  • Steve Fontas
    Steve Fontas Member Posts: 26
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    Gas is the fuel...

    dump load?
    -S.
  • jp_2
    jp_2 Member Posts: 1,935
    edited April 2011
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    dump load

    How many gallons a minute will be your worst case that has to be met?  Oh, and total gallons needed in any worst case time period.



    I would guess if you need no space heating you have a large available amount of solar ? 
  • Steve Fontas
    Steve Fontas Member Posts: 26
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    No space heat needed due to the design of this house

    Worst case for DHW maybe around 18 gpm That's three old school showers going at once... Could be a little higher if the decide to do laundry at the same time.  
    -S.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    Wow....

    18 guppies per minute is a LOT of fish...



    18 * 8.33 * 100 * 60 = 900,000 btuH OUT put from the water heating device. That pretty much eliminates any small residential electric heaters.



    Let's keep crunching numbers. If their showers will be equal to the national average, then 10 minute duration means 18* 8.33*100*10 = 150,000 tbuH output.



    Divide that number by the combustion efficiency, altitudinal derate if you are above 2000' and throw in a 30 gallon reverse indirect (Turbomax) and you're cooking with gas.



    Dump load actually pertains to a fixed quantity of water, like a 120 gallon soaking tub that needs to filled in short order, as opposed to taking an hour to fill.



    ME
    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.
  • Steve Fontas
    Steve Fontas Member Posts: 26
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    Thanks for the input Mark

    Electric was never an option. We are in NYC. Electric heating here is a dirty word...
    -S.
  • jp_2
    jp_2 Member Posts: 1,935
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    whats your plan?

    how much "back-up" do you think you need?   WE are from all parts of the country so we can't 'see' what your need is??



    what percentage do you think the solar can handle?



    if its major,  gas/oil  makes sense, if its minor, electric makes sense.  certainly solar could handle the whole thing, depending on available sun .  all depends on size.
  • Steve Fontas
    Steve Fontas Member Posts: 26
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    Not quite sure

    What to make of your post jp. What I am asking is simple. What would be the most economical back-up for a solar thermal system making DHW only. Gas is the only fuel option. The system has a 53% solar fraction and 120 gal of storage. It is located in NYC. Would you use tankless? A boiler? Solar cannot handle 100% nor should it or I would have a major problem on my hands in June. Now I wish the solar fraction was better, but I am just trying to help a customer that may be getting shafted by someone else..
    -S.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    Define economical...

    First cost economics or long term operating cost economics. THe two, as I said before, don't usually ride on the same bus together...



    The least expensive "item" that would have the ability to keep up with the Joneses showering/bathing habits would be a big ol' tankless, like a Takagi Mobius or equal. Lots of btu's in a relatively small box.



    The least expensive "Item to operate would be a good condensing water heater, of which there are numerous on the market, but they will not be cheap.



    ME
    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.
  • Steve Fontas
    Steve Fontas Member Posts: 26
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    Thanks for all the input

    Mark, do you have a preferred condensing water heater?
    -S.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    Knight Lochinvar...

    coupled with a reverse indirect. Treat the DHW heater as a space heating function, and it gives your control logic a lot more flexibility. Use a 10K sensor at the tank instead of a bang bang aquastat so the intelligence of the Lochinvar can "see" what's really going on in real time.



    For reverse indirect, I'd use a Thermomax at www.thermo2000.com



    As I posted in another thread, if you can do some real time monitoring, you can fine tune the need for BTU's, and possibly down size the physical plant.



    go here http://www.heatinghelp.com/forum-thread/136081/Fitness-club-dhw-retrofit-Pro-advice-needed



    ME
    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.
  • Steve Fontas
    Steve Fontas Member Posts: 26
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    Thanks Mark

    You're the man...
    -S.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    De nada...

    Pay it forward :-)

    ME
    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.
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