After a brief pause in water use, a tankless shower can deliver warm water left in the piping, then a short cool stretch, then freshly heated water. That sequence is sometimes called a cold water sandwich. A shower that stays cold or repeatedly cycles while running calls for a different check.
Record when the cold stretch starts
Note whether it happens after briefly turning the shower off, when another fixture closes, or during a continuous shower. Compare with another hot tap and record any error code without opening the heater.
Help the technician reproduce the pattern
Share the model and describe any recirculation equipment or recent changes to fixtures. The technician can check whether the heater is firing during the event and whether flow at the fixture is enough to keep it running.
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