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Mod-Con's and High DWH demand with Low space Demand

If you really want to "soak up BTUs" from the boiler into the indirect, I'd suggest looking closely at Viessmann indirects. They're certainly expensive, but you get <I>very</I> generously sized, very low head heat exchangers. Their technical information regarding indirects is exceptional.

Find out if they <I>really</I> need "unlimited" hot water at 7-8 gpm--and more important if they're willing to pay for the required equipment... To a degree you can compensate for lower BTU input by increasing storage capacity (and also increasing storage temperature provided you use a mixer to reduce the temp).

My big shower can flow about 12 gpm with everything running. Currently it uses a stand-alone 90-gallon (80 mbh) water heater. While I rarely use it at anywhere near this flow, I have occasionally had a rather long, fairly hot "car wash" and have never run out of hot water. I'm not one for extremely hot showers (it uses thermostatic valves I usually set it at 100F or lower) but some who like extremely hot showers have used it and I'm almost certain they'll turn the temp down significantly when turning on lots of sprays.


  • JimL
    JimL Member Posts: 21
    Mod-cons, High DHW, Low space requirements

    I have a 2000 SF suite that's 100 percent radiant heated with climate panels. Space requirements are 55 KBTU but some fancy Kohler products in 2 super showers drive the DHW demand to about 7-8 GPM if all heads are pumping. I want to go Mod-Con but am not sure how to guarunteee unlimited DHW water supply. Potential DHW requirements are 210-240 KBTU figuring a 60 deg Delta T. My thoughts were to stage a Prestige Solo 100 and 175, and use a 60 or 80 Gal indirect for a buffer. The 110 would be more than adequate for space and the 175 would be the afterburner when needed. I want to be sure that the indirect can tranfer the full 250K BTU to DHW if needed. Given that most ratings are relative to first hour and recovery, how can I calculate the true efficiency of the indirect relative to heat exchange. If the 60 can x-fer the full 250K, is there any point in increasing the buffer size? There is also about 600 SF of snow-melt planned for the future.
  • hot_rod
    hot_rod Member Posts: 22,107
    Depend on loads

    and how they time out. Sometimes a big dump load with large storage will be fine. If the loads are more constant you may need the horsepower to drive it continously.

    Knowing, or guessing the customers use is the trick. Generally most residential loads are early morning and evening. Leaving plenty of time to recover a large storage.

    Or you could encourage them to reduce their DHW consumption and we all win :)

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ALH_4
    ALH_4 Member Posts: 1,790

    Depending on the job location, maybe solar domestic hot water preheat (120gal) would be a nice addition, particularly because they will use enough fuel and water for three average showers each time they shower. A simple temperature display in the bathroom showing the temp of the preheat tank and some experience with the system would let them know whether it was a day they should turn on all the jets or not. That doesn't seem too inconvenient.

    Just an idea.
  • Uni R_2
    Uni R_2 Member Posts: 589
    BTU recovery

    Are you considering the recovery of many of those BTUs with something along the lines of a GFX?
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    a 2ND unit ??

    Providing you have the gas available, Noritz makes a wall hung heater that you could use - N-132M. Spec attached. If you figure a 60 rise, start with 50F end with 110F water, you'll get 10.4 gpm.

    The nice thing about their units is, in theory, you can't over draw them. They demostrated this at a counter day here. If you try to draw 12 gpm, it won't give it to you. It maintains the outlet temperature (you program it) and it cuts down on the outlet pressure to maintain the target temperature. Facinating to watch it work.

    Burn only the fuel you need when making hot water.
  • jp_12
    jp_12 Member Posts: 1
    I agree more and more

    with systems that require high DHW needs, an on demand makes more and more sense.

    why store 80 gallons of 140F water for 23 hours just to fill the need for maybe an hours worth of showers a day?

    Time to rethink the need for these huge beasts.
  • Guy Woollard
    Guy Woollard Member Posts: 82

    If you were to use a single Solo 110 with a Phase III Smart or TR 120 gallon tank, you could get a first hour rating of 255 gallons. That would require firing the boiler at 194 degrees for the domestic side, allowing for a 90 degree rise in DHW.

    That would cover a full half hour of 8 GPM use.

    If you went to a single Solo 175 in the same arena, you would gain an additional 100 gallons, or 12 minutes.

    If your showers aren't going to last a half hour steadily, I would probably grab the lower modulating 110 for the heating efficiency.



    Guy Woollard
    N.E. Regional Sales Mgr
    Triangle Tube Corp.
  • Mark Hunt
    Mark Hunt Member Posts: 4,909
    The GB

    can be configured to allow the heating side to be limited to a lower max modulation while keeping the DHW demand at full output.

    I have a home that required a GB142-60 for DHW yet only required a Gb142-30 for heat. We lowered the max modulation for heating to 40% yet the boiler ramps up to full output for DHW.


    Mark H

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This discussion has been closed.