A hot water tap runs hot briefly, turns cold, then hot again. The likely cause to describe is:

The pattern in the complaint identifies the failure. Hot, then cold, then hot again is a sequence, and sequences point to how a tank stratifies and delivers water rather than to anything at the fixture. A dip tube carries incoming cold water to the bottom of the tank so it does not mix with the hot water at the top where the outlet draws. When the dip tube cracks or breaks off short, cold inlet water discharges near the top and short-circuits straight to the outlet, producing a slug of cold in the middle of a draw. An undersized or overloaded heater produces a related pattern by exhausting its stored hot water mid-draw. Both explanations are consistent with the observation, which is why the report describes the likely mechanism and recommends evaluation rather than naming a single part.

A cracked or broken dip tube lets incoming cold water mix at the top of the tank and reach the outlet mid-draw, and a heater undersized for demand produces a similar interruption. Both are consistent with hot-cold-hot, so the report describes the likely cause and recommends evaluation.

A storage water heater stratifies, with hot water drawn from the top and cold entering through a dip tube that discharges near the bottom. Dip tube failure and undersizing both interrupt the delivery of hot water during a sustained draw.

Hot, Cold, Hot Means the Tube. Cold arriving in the middle of a draw got in at the top — the dip tube is not carrying it down.

Test more than one fixture before describing the cause. A pattern present everywhere is a source problem; a pattern at one tap is local.

• •Attributing a temperature pattern to air in the lines
• •Blaming a single fixture's aerator for a whole-house symptom
• •Naming one cause when the observation supports two