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Water heater sizing
Storage tank size (gallons) for a commercial space given occupancy + use type. Rule-of-thumb hot-water demand based on ASHRAE Applications Handbook values. Sanity-check only — not a substitute for a real plumbing-engineer design.
Daily hot-water demand
400 gal/day
@ 2 gal per person/unit/meal/visit
Tank size to order
140 gal
Exact: 140 gal; rounded up to nearest 10 gal
Rule-of-thumb: tank carries 30-40% of daily demand; recovery rate covers the rest. Bump higher for usage with concentrated peaks (a school's lunch hour). For tank vs tankless, instant-recovery vs slow-recovery, mixing-valve setpoints, etc, defer to the plumbing engineer.
Hot-water demand assumptions (gal/person/day)
- Office: 2 gal/person/day
- Residential (multi-family): 40 gal/unit/day
- Restaurant: 2.4 gal/meal-served
- School (day): 1.8 gal/student
- Healthcare (outpatient): 5 gal/visit
Storage is sized for ~30-40% of daily demand to handle peak hour. Recovery rate (BTU/h × efficiency ÷ 8.3) does the rest. The calc returns a tank size that covers peak with reasonable recovery.
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Forward runs water-heater sizing, pipe-size-for-gpm, fixture-units-to-gpm, drain-pipe sizing for DFU, plus the broader NEC + IBC + IMC calcs and Procore + Autodesk + OneDrive lookups — over one phone number.