Cold-water sandwich
The brief slug of cold water a tankless heater sends when you reopen a tap moments after closing it.
The cold-water sandwich is the signature quirk of tankless water heaters: you get hot water, then a short burst of cold, then hot again — a 'sandwich' of cold between two layers of hot. It happens when you turn a tap off and back on within a minute or two. Warm water already in the pipes arrives first, then the slug of cooler water that was sitting in the heat exchanger while the burner relit, then the freshly heated water. It's harmless but annoying, most noticeable in showers and back-to-back kitchen tasks. A small buffer tank or a [recirculation setup](/guides/tankless-vs-tank-water-heater-cost) smooths it out, and many modern units minimize it in firmware. A storage tank never does this because it always has a full reservoir of pre-heated water ready to go.