Water temperature fluctuates between hot and cold during a shower on a Marey tankless heater.

Safety first: No need to open the unit initially; most checks are external.

Steps:

• Open tap fully: Ensure the hot water tap is fully open. Avoid partially opening single-handle mixers; instead, open hot fully and mix with cold at the shower valve if needed.
• Remove low-flow devices: Temporarily remove water-saving showerheads or aerators to increase flow and test.
• Check set temperature: Set the Marey heater to a moderate temperature (e.g., 115–120°F / 46–49°C) and avoid mixing too much cold at the fixture.
• Descale unit: If you have hard water, perform a descaling flush on the heat exchanger to restore proper heat transfer.
• Inspect sensors (advanced): If fluctuations persist, the outlet temperature sensor may be faulty and should be inspected or replaced.

When to call a technician: If temperature swings are severe despite good flow and descaling, or if sensor/control issues are suspected.