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Need Help figuring out gallons needed for water heater

Is there a simple method out there to use to gage the size of a domestic hot water storage tank without doing calcs?

Example: 13,000 s.f. one family dwelling – yes I know huge.

10.5 bath rooms

Approx 4 washing machines

2 kitchens

Approx 6 people living there.

There is (3) 120 gallon domestic hot water storage tanks

Total of 360 gallons

Piped all together.

Fuel is Gas.

Manufacture: Viessmen

Model: Vitocell-v 300

After looking this bad boy up it just looks top of the line.

Anyone think this is over kill?

I’m figuring a 50 gallon domestic water heater Is good enough for a 2500 s.f. house

With 4 people, kitchen, one washing machine.

Times 50 x 5 = 250 gallons tops


  • Larry Weingarten
    Larry Weingarten Member Posts: 3,331
    the simple rule of thumb...

    ... is 20 gallons of hot water per person per day.  This house and system prolly makes that usage before the people get up!

    You need to see how the house is being used and get a good idea of what the highest realistic hourly use is.  Some folks figure the highest use in 15 minutes is a better predictor and with a high recovery unit, that thinking is a good fit, meaning you will not need so much storage.

    Yours,  Larry
  • Jack
    Jack Member Posts: 1,047
    There are bathrooms...

    And then there are BATHROOMS! Do you have any high flow or body sprays in the place?
  • meplumber
    meplumber Member Posts: 678
    Jack is dead on.

    You cannot use the normal numbers for these kind of houses.

    I have a similar house that has almost 600 gallons of hot water indirects banked for different bathrooms.  Yes it is overkill, but the reality of all 10 baths being used at the same time does exist.  The folks that build these types of homes demand instant gratification.  Meaning, they will not stand for a cold shower. 

    Also, I have never done a house of this nature that didn't have showers with multiple body sprays, multiple dishwashers, large jacuzzi tubs, etc....

    We normally differ to a Professional Engineer on houses this large.  These structures require you to do the math.  And the math is far beyond my ability.

    Good Luck.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited March 2011
    Yes It's OverKill

    I just quoted a job. 36 Apartments. Existing 2 165,000 80 gallon Gas Water Heaters. Solution. Vitodens 200 WB2B105 with ONE Vitocell B-300 Dual Coil Tank.

    Did a hotel that the called for 1.5mil boilers and 4 120 Gal indirects. Was able to use 2 Vitocell B-300 Dual Coil tanks instead of 4 indirects.

    Somone never did there research and just put in what an engineer called for. What do you have for boilers?

    There was an error rendering this rich post.

  • MoneGarand
    MoneGarand Member Posts: 6
    13,000 s.f. house in New York

    This house has (2) hot water boilers fueled by gas.

    The manufacture of the boiler is Viessman.

    The model type is: Vitgas100

    Just beautiful to look at.

    the size of these boilers are bigger than ive ever seen for a house but maybe for this size house i dont think i would question the size. Although without figuring out the BTU's for this house it would be tough to tell if its the correct size for a 13,000 s.f. one family dwelling of a family of say 6.

    there are radiators through-out the house.
  • Lance
    Lance Member Posts: 271
    How much is enough?

    When it comes to sizing residential water heaters, I always compare my notes to the old rule that to supply an unlimited amount of hot water at 3 GPM, I need at least 150 MBH to supply it. As this energy input is not always available, storage is required. My best advice is to check with the manufactures like A O Smith or Bradford White. And in large places with long runs on pipe, use a recirculating pipe system and make it low velocity, well insulated pipe and properly controlled or you may end up with even lower storage reserves.
  • Jack
    Jack Member Posts: 1,047
    Another option

    A few years ago I visited a 10,000 sq ft'er that had 3 Rinnai water heaters. Had they plumbed it traditionally there would have been a million miles of copper throughout the building and recirc would have been necessary. They built a small insulated "dog house" in the attic at each vent stack and put one Rinnai in each spot. A short vent thru the roof and they downfed that stack. Minimal amount of labor and piping and excellent response at the tap. They punched a small register up thru the second floor ceiling for some heating and it worked great. It was an excellent solution.

    For this size house breaking the hot water system up into areas is best for tankless. Traditional piping and recirc can consume more energy in heat loss off the piping than the actual hot water demand costs. I am stunned at how few people insulate hot water lines.

    For the big showers storing hundreds of gallons makes no sense either...imho, but I am biased;). If I can put in 5 tankless water heaters delivering 25-30gpm, or whatever the required number is, you will have unlimited hot water for the 40 min shower ( as long as you have gas, elec, water & septic capacity, and I have a story about that too). Then, when you shut the shower off, to get out of it to go pay for it, you consume zero energy. Due to the ability to control all of the Rinnai's together you have modulation from 9500 btu -1,000,000btu. We can manifold up to 25 units. How big is the shower flow rate?

    De-centralize the big systems. Yes, you have to run significant gas lines, but the systems performance is better.
  • meplumber
    meplumber Member Posts: 678
    Love the Rinnai's Jack.


    I use them a lot.  However, the only negative, is that in these houses, my customers don't like to wait.  And we have fought the delay times. 

    As you know, the Architects never put the mech rooms in a convenient location. 

    You are right.  When the runs can be kept short, tankless is the way to go.  The problem that we ran into in the home that I was speaking of, was distance.  Due to code restrictions on building height, the house was sprawling.  So, we went with storage, insulated piping, and recirc.  Not the most efficient way, but it keeps Mr. Big Money happy.
  • Jack
    Jack Member Posts: 1,047
    I put that out there

    to prompt the thought that getting out of the mechanical room is not a bad thing. The water heater has tradtitionally been installed in the mechanical room because that is where the chimney was. Then it was because that was where the boiler was. I could care less about a chimney and the tankless doesn't require the boiler. Given that, why is the water heater in the mechanical room when it may not lend itself to the best comfort of those living there or the efficiency. I bring this up to all arches and GC's I talk to. I also tell them that they are wasting money on masonry chimneys. Why use 11th century technology to vent 21st century appliances. Save the money on the mason and pay the plumber more;)

    the best way to recirc any hot water system from an energy, water waste and time issue is to use the Metlund Design system with the motion sensors. Check it out at www.gothotwater.com I don't sell them. Practical, sensible recirc design.
  • meplumber
    meplumber Member Posts: 678
    Thanks for that Jack.

    I hadn't heard of the Metlund.

    Why does it seem that the larger the house, the smaller the relative design of the mechanical spaces?  Regardless of how often you communicate space needs to the architect and gc, I always seem to lose space just before the start date.  Usually to something essential like a home gym or a massage room.  Can't live without those.  Right?
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,331
    Jack is right...

    With such a spread out house, there is the possibility of having a few areas served by separate hot water systems.  Central core plumbing puts a heater close to a grouping of fixtures.  It might be here that tankless heaters work for big tub filling while tank type heaters work for very high or low flow uses.  As long as you're at it, Google "Gary Klein structured plumbing".   He's written some informative articles on how to design plumbing to get hot water to the tap fast and waste far less water/energy doing it.  Contact him.  He'd enjoy the challenge of taming such a beast.

    Yours,  Larry
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