Tankless capacity is expressed as hot-water flow at a particular temperature rise. A unit that suits one shower may struggle when a shower, sink, and appliance run together. Good sizing begins with a realistic peak-use picture.
FIELD NOTE
Describe The Busiest Ordinary Period
Write down which hot-water fixtures are likely to run at the same time, not every fixture that exists in the home. A morning with two showers and a dishwasher tells a more useful story than a count of bathrooms. Include a large tub or other unusual draw if it is used regularly. The installer can estimate the flow of those fixtures and check actual fixture ratings where available.
A practical plan may also involve changing when loads overlap. This can affect what capacity is sensible, but it should reflect how the household actually wants to live. Do not choose a smaller unit based on a schedule that no one will follow.
FIELD NOTE
Account For Temperature Rise
The heater must raise incoming cold water to the desired delivery temperature. Colder incoming water requires more heating for the same flow. Manufacturer capacity charts pair a flow rate with a specified temperature rise, so a single advertised gallons-per-minute number is incomplete.
Ask the installer what incoming-water assumption they used and how it affects capacity during the coolest part of the year. That makes comparisons between models meaningful. It also helps explain why a heater can provide continuous hot water yet still have a limit on simultaneous use.
FIELD NOTE
Review The Installation As A System
The proposed heater must also fit available gas or electrical capacity, venting needs where relevant, water connections, access, and manufacturer clearances. An upgrade from a tank can require changes outside the heater itself. Ask for those changes in the written scope rather than treating the equipment price as the total project.
Have the installer explain the final capacity choice in terms of your peak fixtures and temperature rise. Keep the model information and operating instructions afterward so that future service discussions begin with the actual installed unit.
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