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sizing formulas for DHW

the only formulas i've seen end by selecting the specific manufacturers' models without really explaining the science. i'd like to select my own heater and my own tank based on other factors. for example: if i go from a 100M btu boiler with 60 gallons of indirect storage, to a 140M btu with the same water demand (apartments) how much water do i need to store?  


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,841
    Loads, time factors and extrapolation...

    I believe most manufacturers use the Hunter Curve for calculating DHW loads, and it is all based on load potential. Take a 24 unit apartment complex for example. You know there are at minimum of two points of considerable DHW loads in each apartment (tub and shower and kitchen sink). If the kitchen sink /dishwasher draws 2 GPM, and the shower draws 2.5 GPM, of which 80% is hot, then its actual draw demand is 2 GPM. So, the draw factor is 4 GPM per apartment. Here's where it gets tricky. How many apartments will be running one or both fixtures and for how long?

    I can tell you what the time factor is for the shower (average shower time = 7.5 minutes), but kitchen sinks and dish washers are a big variable, as is "how many people per apartment?"

    You have to make a bunch of assumptions, and are biting off a large chunk of liability if your guess is under estimated. This is why I like deferring to manufacturers like RayPak's commercial sizing guide software. It takes altitude into consideration, and spits out a number, based on the Hunter Curve, as to what size of boiler and what size of storage tank. From there, you can work backwards, and vary either storage or fire power.

    For example, IF in the above apartment example, based on the RayPak program, IF it called out 334,000 btuH boiler input, at 82% efficiency, and a 100 gallon storage tank, assuming an 80% draw down without significant dilution, then you now have the base numbers to determine wether you have adequate capacity or not.

    82% 0f 334,000 btuH = 267,200

    80% of 100 gallons is 80 gallons. Assuming a holding temperature of 140 degrees F, and a 40 degree entering water temperature, then the storage tank represents 80*8.33*100 = 66,640 tbuH. This is first hour capacity, and will always be worst case scenario, other than continuous loads, where the storage tanks buffering ability is taken out of the picture. Add this number to the boilers output capacity, and you have the total btuH capacity necessary for the example.

    NOTE: This is all fictitious. If you have an apartment complex you want to size a system for, go to RayPak and download their free sizing software.

    If this is not commercial, but is residential, most water heater manufacturers have their own sizing charts. You can do the same thing with their output information.

    Hope this helps.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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