What is the thumb rule for water tank calculation?

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Aaron, this question relates to actual, real-life, sizing rather that energy modeling.

DHW sizing is both an Art and a Science. Figuring out the expected domestic water load "accurately" is obviously the biggest challenge. Load diversity is the wild assumption for new buildings.

(Side note: for existing buildings, metering is an excellent idea to get the actual load profile of the facility... My experience with that is that any other theoretical methods tend to grossly oversize the DHW systems for Multifamily buildings at least... Hunter curves/Fixture-unit methods are worse than the ASHRAE method but ASHRAE oversizes by 30+% already)

First, there are many options you can choose for your system setup (instantaneous, direct, indirect, etc.). Once you have settled on a system type, there are basically an infinity of combination of heater capacity (recovery rate) and storage that will satisfy the assumed load you previously calculated.

To quote the 2011 ASHRAE Handbook - HVAC Applications, Chapter 50 - Service Water Heating:

To serve a hot-water load adequately, the needs of both the peak energy withdrawal rate and total integrated energy delivery for end uses must be met. Meeting these needs can be done either by providing a heating rate large enough to meet the peak energy withdrawal rate of the system (and modulating that heating input for smaller loads), or by providing a lower heating rate combined with storage (from which the peak rates can be satisfied).

I suggest you take a close look at this chapter 50. Table 7 basically tells you that for an office building, the maximum hourly demand is 0.4 gal/person, the maximum daily is 2.0 gal/person, and the average daily is 1.0 gal/person. If it's an office building, you're lucky because it's way simpler than residential (and a much lesser usage)

Knowing whether you'll have a recirculation loop or not is important as well.

To quote an engineer who had a lot more wisdom than me: There is no right way to size a DHW system, only better or worse ways to approaching the task. Only the engineer with the greatest understanding of the system and who is making the least amount of assumptions will provide the best system. Experience is key.