Water-heating planning tool
Planning comparison only, not product selection or installation design. Start with editable DOE worksheet planning averages in Consumer mode, or use Advanced for direct entry. The tool totals uses in the busiest single one-hour period. FHR is not nominal tank capacity.
Peak-hour planning
Planning comparison only — not product selection or installation design. The result is a minimum First Hour Rating (FHR) comparison target for a storage or heat-pump-with-tank heater. FHR is not tank capacity.
Plan responsibly
This project may be subject to local permit, zoning, site, or structural requirements.
FHR comparison only: The result is a First Hour Rating comparison target, not tank capacity, product selection, delivered performance, or installation design.
Scald hazard: Hot water can scald. Follow manufacturer instructions and qualified local requirements for temperature control and scald protection.
Water-heater installation: Model-code installation provisions do not convert peak-hour demand arithmetic into tank capacity, product selection, or an approved installation.
Source: ICC — 2018 IPC Chapter 5 Water Heaters · Reviewed 2026-08-24
Local requirements may differ. A model-code reference does not confirm the code edition, amendments, zoning rules, or permit requirements adopted where the project is located.
2018 IPC, Section Chapter 5: Water-heater installation. Source: ICC — 2018 IPC Chapter 5 Water Heaters. Reviewed 2026-08-24.
Local requirements may differ. Verify the adopted code, amendments, zoning rules, and permit requirements with the local authority.
Methodology
Consumer mode starts with DOE worksheet gallons-per-use values labeled as editable planning averages, not measurements or facts about the home. Replace them when you have a better measured assumption. Advanced keeps the same entries and offers direct event, additional-demand, and optional manufacturer-FHR fields. Only positive-count events are submitted. Each quantity is multiplied by its entered gallons per event; acknowledged additional peak-hour gallons are then added. Exact bounded scaled-integer arithmetic preserves up to six decimal places.
The DOE worksheet values are a starting handoff from builder-style planning to homeowner inputs. They remain editable; use a measured or product-specific gallons-per-use assumption when available.
Add only peak-hour gallons not already represented in event rows; otherwise demand will be counted twice.
Compare the result with manufacturer/EnergyGuide First Hour Rating. FHR is one-hour hot-water delivery beginning with a fully heated tank; it is not nominal tank capacity.
A qualified plumbing/heating installer must verify the appliance and installation with the manufacturer and authority having jurisdiction, including fuel, venting, combustion air, electrical, seismic restraint, relief, scald protection, drainage, structural support, local code, permits, and inspections. Hot water can scald; hand-test it and follow manufacturer and qualified/local requirements.
Worked examples
If two showers occur in the busiest hour and your measured planning assumption is 18.5 gallons each, enter 2 and edit the shower value to 18.5. The event contributes 37 gallons.
In Advanced, add a labeled event or acknowledged additional gallons for a project-specific use. Do not repeat gallons already represented by a Consumer row.
Common questions
No. It returns only a minimum manufacturer/EnergyGuide First Hour Rating comparison target, not tank gallons or a product recommendation.
No. DOE's method concerns the busiest single one-hour period, not total daily use.
No. The tool performs only an arithmetic comparison of the user-entered FHR and derived target; it does not verify or recommend a model.
No. Consumer mode labels them as editable DOE worksheet planning averages. They are starting assumptions, not measurements or facts about your home; replace them when you have better information.
Your Consumer entries stay in place. Advanced exposes direct event rows, optional additional peak-hour gallons with a no-double-count acknowledgment, and an optional manufacturer or EnergyGuide FHR comparison field.
Sources
Last reviewed: 2026-08-24
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