Record the household pattern
Identify whether all fixtures are affected or only a shower or sink. Note whether the concern occurs after a normal heating period, during overlapping use, or after hot water has been drawn.
Give the installer the model, installation date, current external setting if visible, and changes already made. Do not open covers or adjust internal controls.
Ask the provider to make safe temperature measurements. Do not expose skin to increasingly hot water as a test.
Compare equipment with delivery
Have the qualified water-heater provider establish actual heater performance under the exact model's instructions. Is the heater reaching its intended operating temperature and supplying appropriate hot water?
If it is, have an appropriate plumbing professional assess relevant mixing devices, fixture controls, and household delivery. Ask whether a device is intentionally limiting temperature or whether a fault or installation condition needs correction.
Do not bypass a mixing or anti-scald device to obtain hotter water. Request a supported correction that preserves needed safeguards.
Resolve the new-installation question
Ask for the commissioning record and intended setting. Have the installer explain what delivery was verified at completion and whether the present result differs.
If a part is recommended, request the fault finding. If a different operating setting is recommended, request its reason and household scald safeguards. A larger tank is not automatically the answer to a temperature mismatch.
Agree on the result
Request a written explanation identifying whether the issue belongs to the heater, installation, or fixture/plumbing work. Make provider responsibility and quote inclusion clear.
Your next step is to ask the installer for a heater-to-fixture temperature comparison. Keep settings steady while the cause is investigated, then verify the correction at the affected fixtures.