What is First Hour Rating (FHR)?

FHR is the volume a storage water heater can deliver in the first hour of operation starting with a full hot tank, combining usable tank volume and recovery.

How do inlet water temperatures affect sizing?

Colder inlet temperatures increase the temperature rise (ΔT) required and reduce capacity for both tanks and tankless. The calculator lets you set seasonal inlet temperatures.

Water Heater Size & Recovery Time Calculator

Water Heater Size & Recovery Time

Accurately size tank, tankless, or heat pump water heaters for your household’s peak hour. Build simultaneous draws, set temperatures, and run a minute‑by‑minute simulation. See timeline, outlet temperature and capacity charts, recovery time, FHR estimates, and actionable suggestions—plus CSV/PDF exports and a shareable URL for saving scenarios. Includes presets, units, and mixing‑valve modeling options.

Inputs

Quick Mode

Heater Type

Occupants

Peak Window (minutes)

Desired Shower Temp (°F)

Tank/Outlet Setpoint (°F)

Inlet/Cold Temp (°F)

Units

Tank Size (gal)

Usable Drawdown (0.65–0.80)

Input (Gas BTU/hr)

Input (Electric kW)

Efficiency / COP

Altitude Derate (%)

Simultaneous Draws

Add overlapping fixtures in the peak window. Defaults assume WaterSense flows; you can edit per row.

Fixture	Start (min)	Duration (min)	Flow	Temp (°F)

Recommendations

Awaiting simulation

Suggested Tank Size

—

Target FHR and margin vs your scenario.

Tankless Requirement

—

GPM @ ΔT and input BTU/hr or kW.

Recovery Time

—

Time to 90% and to full after your peak.

Timeline Visualizer

Tank State & Outlet Temperature

Scenario Details

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