Dedicated Tankless for Commercial D/W

Discussion in 'Tankless Water Heater Forum' started by P.Dieter, Apr 4, 2014.

  1. P.Dieter

    P.Dieter Remodel Contactor

    Messages:
    16
    Location:
    Seattle
    I just installed a dishwasher in our catering facility and pushed our existing elect. 40 gal hot water heater over the top (large 3 compartment sink and HE laundry also). We figured this would be the case.

    I was thinking I was just going to get a larger HW heater but then thought maybe just pull off a line to an on demand tankless just for the D/W.

    ½" pipe only available, 110 or 220vt (prefer 110)
    D/W uses .93 gal per 90 sec cycle (complete)
    would like to run >120 degree water

    What say ye olde plumbing gurus of The Love?

    thx
  2. Justin B. Williams

    Justin B. Williams New Member

    Messages:
    6
    Location:
    Pennsylvania
    possibly a rihanni RU98i
    or
    Rheem RTGH-95 abou the equiv
    or
    Navien NPE-210A
    all with a recrc line installed to assure almost instant hot water at your .93 GPC rating should be able to get >120°
  3. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    Napkin math:

    0.93 gallons x 8.34 lbs/gallon = 7.76lbs.

    Assuming a wintertime temperure rise of 100F (from 40F up to 140F, since you want to be >120F), that's 7.76lb x 100F= 775 BTU. If delivered evenly over 90 seconds (which it isn't) that's 8.6 BTU/second.

    So, 8.6 BTU/sec. for 3600 seconds in an hour is a heat rate of 30,960 BTU/hr.

    At 3.412 BTU/watt-hour, that's 9,074 watts.

    If your water flow rate is fairly continuous (probably not), you'd be fine with a 10KW tankless, but at the real non-continuous flow rates you'll need at least 20KW worth of tankless, which would require a dedicated 100 amp 220V breaker (and dedicated fat wiring sufficient for handling that 100 amps.)

    At typical 40 gallon 2-element tank type HW heater is good for 9,000 watts, and would be able to keep up with the dishwasher all day & night (assuming it takes at least 5-15 seconds average between loads.)

    Smaller 5-15 gallon point of use tanks are typically single-element, good for 1500-2000 watts. If the average duty cycle of the dishwasher is less than 50%, with a 15 gallon unit you could go about an hour, serving 15-20 loads in that hour, and you'd be just fine running it off a dedicated 20A/120V breaker. If it needs to do more than 20 loads in an hour you'd have to up the tank size, or find one with bigger (preferably multiple) heating elements. Don't know what your peak wash load rate is, and how many hours of dish washing you need to do continuously, but this is probably a solution (and if it works, it's a cheaper/better solution than a monster-output electric tankless.)

    It's probably worth timing how long it actually takes to run 5 loads, and estimating your worst case back-to-back number of loads before committing. You might also time how long the fill-bursts are for the water, to calculate the real heat rate requirement for a tankless solution.
    Last edited: May 30, 2014
  4. vomuqu

    vomuqu New Member

    Messages:
    3
    ECO 27 and Rinnai RU98i are all good choice to offer you instant hot water.
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