Tankless sizing: flow rate and temperature rise

Ask for the performance point, not just the headline

A tankless recommendation should connect simultaneous demand with the temperature increase the equipment must provide. The same unit’s usable output changes with operating conditions. Ask the installer to show which conditions support the proposed selection.

Write a simple schedule of overlapping uses: morning showers, a bath being filled, kitchen use and laundry. Record what actually overlaps rather than adding every fixture in the building as though it runs continuously.

Make the units clear

Ask whether flow is shown in litres per minute or gallons per minute, and which gallon definition is used. Compare like units throughout the quote. Temperature rise is the difference between incoming and delivered water temperatures; it is not the thermostat number alone.

For illustration, two simultaneous uses of 6 litres per minute represent 12 litres per minute of combined fixture flow. That arithmetic is not a heater-sizing result: mixed-water temperatures, incoming conditions and equipment performance still need professional assessment.

What to ask before accepting the quote

Request the model’s performance table, the intended peak scenario and the assumptions used. Ask how the system behaves if demand exceeds the recommendation, and whether the proposed utility and venting work supports it.

Keep the sizing basis with the quote. It provides a useful reference if the household later adds a suite, bathroom or larger bath.

Common questions

Sources & further reading

• NRCan water heater guide

Check the selected manufacturer’s current instructions. Program eligibility and installation requirements can change.

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Make this useful for your project

Build a short installation brief with the details and questions to bring to an assessment.