Start with the busiest hot-water period
List the fixtures and routines
Include showers, tub fillers, lavatories, kitchen use, dishwasher, washer, and any recirculation. Note which uses realistically overlap rather than adding every fixture in the building.
Identify the busiest period
A morning with two showers and an appliance may control sizing even if the home uses little hot water for the rest of the day.
Measure or verify flow
Use fixture ratings or controlled measurements. For tankless sizing, add the hot-water flow expected at the same time.
Define the temperature task
Choose a safe delivered temperature and determine the design incoming-water temperature. Their difference is the temperature rise a tankless unit must provide. [2]
Check the utilities and installation
Confirm gas input, electrical capacity, venting, airflow, condensate, drain, space, access, and permit implications before treating a capacity number as installable.
Use the right rating for each heater type
Storage heater sizing
Tankless heater sizing
First-hour rating estimates how much hot water a fully heated storage unit can deliver in the first hour of a defined draw pattern. It is more informative than nominal tank gallons alone. [1]
Performance tables show flow available at specific temperature rises. A headline maximum flow without the rise condition is incomplete. [2]
Recovery rate matters after and during depletion. Fuel input, heat transfer, and starting temperature affect how quickly storage is restored.
Simultaneous demand matters more than total daily gallons. A long one-shower draw can fit while two high-flow uses do not.
Household details that can change the answer
- A large soaking or whirlpool tub that needs a high volume in one continuous fill.
- Two or more showers that routinely overlap, especially with high-flow heads or body sprays.
- A recirculation system that adds heat loss or interacts with tankless activation.
- Long piping runs and uninsulated distribution that lose heat before water reaches the fixture.
- A household schedule that clusters demand rather than spreading it through the day.
- Planned additions, bathroom changes, or fixtures with meaningfully different flow.
- Available gas line, electric service, venting, airflow, drain, and physical service access.
Undersizing shows up as exhausted storage, slow recovery, or tankless temperature drop when predictable fixtures overlap. The solution is not automatically a hotter setpoint; higher outlet temperature creates scald risk and a tankless unit must add even more heat per gallon. [4]
Oversizing is not harmless insurance. A larger storage tank can carry more standby loss and occupy more space. A higher-input tankless unit may require larger gas piping, a different meter review, more electrical power, or another vent design. DOE selection guidance recommends comparing demand, energy, efficiency, size, and life-cycle factors together. [3] [1]
What the sizing calculation should show
- The fixtures assumed to run at the same time and their flow assumptions.
- The incoming-water temperature and desired delivered temperature used in the calculation.
- For a tank, the selected first-hour rating and recovery basis.
- For tankless, the selected model's output at the calculated temperature rise.
- Utility, vent, drain, access, and permit work required to install that capacity.
- How future changes or recirculation were included without inflating the project unnecessarily.