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DHW sizing question

brad. 55 gallons of storage? he gets to use all of it? really?

you know better than that, come on ;)

If his tank is 55 gallons, at 140 degrees, and we need 110 at the fixture, he's got 30 x 8.4 x 55 = 12.6kBTUs stored.

If incoming water is 50 degrees, and he's using 14 GPM of 110 at the fixture, that's 60 x 8.4 x 14 = about 7kBTUs/minute. He's depleted (under 110) in less than two minutes with no input using the full flow rate of the shower (not 4).

nitpicky, I know, but ultimately the only way to deal with this stuff is to get back to BTUs. It's all about BTUs in, BTUs out, and BTUs in storage.

Start talking GPM and you start muddying the waters, IMHO, because you're missing an important factor, temperature. for instance a 100 gallon turbomax full of 180 vs the same size regular indirect at 140 has something like 3 times as much usable heat stored...


  • Dan C._4
    Dan C._4 Member Posts: 70
    DHW sizing question

    I have a shower with 4 body sprays, one shower head, one handheld that can all run at the same time. The total GPM is 14. I have very limited space to work with and they will not give up any more space. I wanted go with Rinnai heaters but when I called them they said I would need four of them. So now I am looking at HTProducts Phoenix 199-55. It's 199000 BTU 55 gallons storage first hour rating of 295. How many minutes of hot water could they epect to have if I use this tank? I figured 21 minutes but I don't think I am doing it correctly. Thanks

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  • Brad White_185
    Brad White_185 Member Posts: 265
    If you consider that

    Your 55 gallons initial storage at temperature will be depleted in just under four minutes (55 gallons/14 GPM). While this is draining, you will be firing upon the first sign of tank drop below setpoint.

    The firing capacity, assuming about 80% efficient, can keep up with 4 GPM at an 80 degree rise.

    (Put another way, your first hour rating is 295. Subtract the 55 gallons starting hot storage and you have 240 gallons per hour generation capacity, again, 4 GPM which is a good cross-check.)

    Now, think of it this way: Your first minute of flow will probably trigger the burner so will make up essentially 4 GPM while you are drawing out 14. You have 3 minutes left in the tank at that point so after another minute you have subtracted 14 gallons but added four.

    You are down at a rate of minus 10 GPM... continue that curve and you will run out of water about 6 or 7 minutes from tank re-start (or have to just keep the pecker-only attachment on) until the tank re-charges.

    Once the tank is down, that is about what you are left with unless you soap up for about 14 minutes and start the process again. There is some allowance for the normal mix-down at the shower valve as Jack pointed out, which prolongs the time available.

    Remember, the key term is first HOUR. Your flow rate times minutes exceeds that as an hourly rate by a factor of over three.

    My guess is that they were never in the Navy or are compensating for years of ship-board coffee-can showers.

  • Jack_42
    Jack_42 Member Posts: 9
    You don't need 4 Rinnais

    You need three. At a 70F rise the R75LSi will make 4.3gpm. The R94LSi will make 4.8. At that rate if making 120f water you will mix down to the shower temp of 110-112. You will be right on the shower output. That will be max but I'd go with three. Certainly the 4 will give a bit of extra capacity for the rest of the house. Space, money, expectations... The problem with storage for a shower like this is that you maintain the water temp all day long and size for the single fixture. With the Rinnais you end up with the capacity when it required but the rest of the time, the system will modulate to the absolute demand.
  • Dan C._4
    Dan C._4 Member Posts: 70

    it's 80 degree temp rise and they need water hotter than 115

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  • Dan C._4
    Dan C._4 Member Posts: 70

    so it won't work. I called the HTP rep and he said it would. I didn't believe him which is why I asked the question. Thanks

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  • Jack
    Jack Member Posts: 1,046

    Then you will need four. The bigger issue for the customer is the monthly operating cost. Remember they cannot write off their monthly utility bill but they can finance the equipment cost in the mortgage. Assuming two people are using this fixture per day with all the holes running how long are they going to actually have that high flow tap open. With the on-demand they can run it all day long, so they gain the comfort of the big cost/flow fixture. When the system is off, and I don't care how much you like your shower, at some point you have to get out of it and go pay for it, the system will draw no energy. As well, it will modulate across its firing range throughout the day giving excellent operational efficiency. Remember, they picked the shower. Now they have to feed it and allow you to provide what is necessary to confidently do so.

    I would suggest that you start looking for other locations for the system which will provide you with adequate space/performance. I know that is easy to throw out there to you but again, you have to be able to give them the system they need even if they "Won't give you any more room". They put you in the position of having to perform without adequate facitilites to do so. They will not remember that you didn't have the space when they run out of hw. It will not be their fault;)

    I was on a job recently where in a very large house they put the Rinnais up in the attic (there are attics and then there are attics and this one I could live in), built a small insulated enclosure around them, vented with short vents thru the roof, cut a register in the second floor ceiling to provide gravity heat to the enclosure and downfed the individual vent stacks. Pretty cool.

    Good luck on this. Let us know which way you go!
  • Dan C._4
    Dan C._4 Member Posts: 70

    maybe I could put in that same water heater and a storage tank somewhere else. Ijsut need to let them know how long they can run the hot water before running out. That is what i don't know how to figure out. My other option is putting a wall hung boiler in the basement and an 80 gallon horizontal indirect in a closet on the fifth floor but it would be a ton of work and it probably still wouldn't be enough. THere isn't many other options of they won't give me any more space. Sometimes people think I am a magician. Also whatever I come up with is extra cost because the contractor never figured for this. That is the other problem.

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  • Jack
    Jack Member Posts: 1,046
    Just don't

    let the homeowners and GC's problem become your problem, because that is the way these things tend to go. Regardless of whatever equipment gets used I would suggest that you layout the options, both from an equipment standpoint and a necessary location standpoint and insist on a meeting with the owners and GC to discuss the good, better and best options and come up with a firm agreement on what, where, when and who pays! Not having that meeting increases your exposure. Again, Good luck on this!

  • You're in trouble regardless of what you do....

    People who build these drive thru body/car wash showers obviously have NO consideration for the environment.

    I have an alledged earth hugger in the peoples republic of Boulder who has an all geo home. We put 2 tons of DHW heating in at his request. He is adamant about no letting us hook up to the gas, sitting right there. Part of the irony is that he makes his living managing the drilling of gas and oil wells...

    Anyway, we forwarned him about DHW shortages. His response was to put in more storage. We complied and told him that he just bought himself maybe 6 to 10 minutes of additional time in his drive thru shower. To compound things, this is the sole source for DHW in this building, and he has 3 units that he rents out to students.

    In any case, I told him his water consumption was unconscienable, and that I hoped he was caught and red flagged by the city inspectors. It is against the IECC code to flow at more than 2.5 GPM from any shower. I guess they could be interpreting the code to their liking, but the final say so is up to the inspector.

    This battle will never end. I just know it...

  • laurence salvatore_2
    laurence salvatore_2 Member Posts: 86
    copiouse hot water

    I think Noritz has a commercial tankless that will keep up with your shower. Also I think the large domestic ones from Noritz do 9 gallons a minute. Just a distant recollection from a job we spec'ed but did not get
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,545
    How about...

    ... putting in one or more GFX shower heat exchangers? That could reduce the need for such a large heat input without any nasty side effects.

    Yours, Larry
  • Brad White_185
    Brad White_185 Member Posts: 265
    Temperature is a factor Rob

    Temperature is mentioned and yes at a level you are right, it is all BTU's but I converted it to flow (at temperature) to better explain the depletion and dilution. Yes, I could have done a better job but the net result is the same, depleting 14 GPM at temperature while recovering four GPM by the burn, leaving a depletion rate of 10 GPM. Excuse my poor math :)
  • JohnNY
    JohnNY Member Posts: 2,917
    How about this craziness....?

    I got into trouble when a client put a giant "car wash" shower in right after I sold him a boiler and water heater that I said would take care of any demand his home was (previously) capable of.

    After much debate: No more floor space, not buying another boiler, larger indirect not the answer, etc...

    We decided to install a gas-fired tankless heater to pre-heat the water to 90° before it got to the indirect, where it was heated to 150°, effectively storing BTUs, then mixing it down to 125° for domestic use.

    I've attached a simplified drawing showing the basic setup, but there's actually a bit more to it to compensate for higher flow conditions, blah blah blah.

    The new shower uses 11gpm and the client is very happy with the results.

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