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Commercial Hot Water Demand Calculator

Peak hot water demand and the storage and heat input it implies for hotel rooms, apartments and restaurant seats, with recovery credited against the peak.

Inputs

Occupancy

Count rooms, not beds.

A design value from the demand table you are using. The common figure for hotels and motels is around 4 gal/hr per room, but confirm it: this number drives the whole calculation.

Count units. Use 0 if the project has no residential units.

Again a design value from your demand table. Apartments are commonly carried near 3 gal/hr per unit, but that varies with the fixture package.

Peak seating, not daily covers.

A design value from your demand table. Full-service dining runs higher than counter service, and a dishwashing kitchen usually dominates the load.

Laundry, commercial kitchen beyond the seat count, process use, janitorial or anything else you are carrying. Leave 0 if nothing else applies.

Design basis

Delivery temperature minus incoming water temperature. Commercial estimates are often carried at a 100 F rise; confirm the convention used by your method, since it scales the heat input directly.

How long the peak demand lasts before falling back to normal use. The classic demand-table method implies about one hour.

Credit the storage tank only with what the heater cannot cover. Leave 0 to size storage for the entire peak, which is the conservative answer.

Result

Peak hot water demand
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Storage required after recovery credit
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Heat input to meet the peak flow
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Same input for gas equipment
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Apartment contribution
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Hotel or motel contribution
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Gallons drawn during the peak
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Gallons the heater recovers during the peak
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Heat input used by that recovery
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Restaurant contribution
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—
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The input that decides this
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Where this stops applying
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Calculation details
Design temperature rise
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Storage share of the peak draw
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What to check

  • Per-unit gallons per hour from the demand table your design follows
  • Peak duration and the recovery the selected heater provides
  • Local plumbing code requirements for storage and Legionella control

Standards referenced

Screening estimate only. The result panel lists the limits that apply to this calculation. Verify nameplate data, rates, groundwater temperatures and installer quotes before deciding, and have gas, venting and electrical installations checked against the local code by a licensed contractor.

Saved calculations

Keep the assumptions with the result.

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