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Thread: Load Calculations Tankless Electric Water Heaters

  1. #1
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    Load Calculations Tankless Electric Water Heaters

    Hi,

    We are working on a building remodel for barracks. Our requirements for hot water are to provide 1200 GPH, because it is assumed all residents will be showering at the same time. In reality, the residents are on rotating shifts and the most that would be showing at a time are 30%.

    The question is this:
    If we have tankless electric water heaters to provide the full demanded amount of hot water but there is never a scenario when they would all be running at the same time, can I apply a demand factor to the load for load calculations? If so, what numbers would I use?

    I am looking at approximately 650A Full Load Amp Draw at 208V, 3 Ph.

    Thanks!

  2. #2
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    Military installations require practice drills, training excercises, and other roll-call events that require everyone to be present and accounted for, at the same time.

    Not a good idea to force 2/3 of tenants to be present for roll-call inspections without a shower.
    Roger Ramjet NoFixNoPay

  3. #3
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    Our requirements for hot water are to provide 1200 GPH

    at what temp rise??

    Every millitary contract I've ever worked they want what they ask for, you get no favors for telling them different unless it will IMPROVE performance AND REDUCE cost at the same time.

  4. #4
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    I cannot think of a worse application of tankless water heaters than a ton of people that need to take hot showers at virtually the same time. 1200 gallons per hour is only 20 gallons a minute enough for maybe 30 showers with a 50-50 hot / cold mix. maybe that's all the barracks needs, I don't know it's size.

    no you cannot put a demand Factor on this, you're going to need to size the service for the full calculated load. pretty much all other options are better than a tankless set up here... Gas powered water heaters if available, running a commercial type water heater at a high temp and using mixing valves to bring the hot water back down to a hundred and twenty-five degrees, half a dozen 80 gallon residential type water heaters... I take it natural gas is not available otherwise the on demands would be gas-fired
    Electricians do it until it Hertz!

  5. #5
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    how many showers?
    you need to size demand at 100% based on water supply fixture units wsfu
    this must be done for all fixtures

    and make it a recirculating loop
    agreed pos or instantaneous are not good for this application
    you want industrial heaters with circ pump(s)
    The difference between genius and stupidity is that genius has its limits.

  6. #6
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    Ok, Thanks guys.

  7. #7
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    Quote Originally Posted by deb4523 View Post
    Hi,

    We are working on a building remodel for barracks. Our requirements for hot water are to provide 1200 GPH, because it is assumed all residents will be showering at the same time. In reality, the residents are on rotating shifts and the most that would be showing at a time are 30%.

    The question is this:
    If we have tankless electric water heaters to provide the full demanded amount of hot water but there is never a scenario when they would all be running at the same time, can I apply a demand factor to the load for load calculations? If so, what numbers would I use?

    I am looking at approximately 650A Full Load Amp Draw at 208V, 3 Ph.

    Thanks!
    650A at 208V is 230kw or just under 800,000 btu/hr. That will get 20 gpm 50 deg F water to about 130 Deg F. So you mix it 50/50 and it is about 40 gpm at 90 deg F. That might be enough for 8 guys to shower at the same time.
    Bob

  8. #8
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    a low flow shower head is 2 gpm
    so 40 gpm would supply 20 heads
    ???
    The difference between genius and stupidity is that genius has its limits.

  9. #9
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    Quote Originally Posted by Ingenieur View Post
    a low flow shower head is 2 gpm
    so 40 gpm would supply 20 heads
    ???
    maybe in your world but no one leaves those stupid washers in place. they take them out so you can have a real shower.
    Bob

  10. #10
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    Quote Originally Posted by petersonra View Post
    maybe in your world but no one leaves those stupid washers in place. they take them out so you can have a real shower.
    federal facility
    destruction of gov property
    felony
    The difference between genius and stupidity is that genius has its limits.

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