Cold Water Sandwich Effect on a Tankless Water Heater

You're in the shower, the water is hot, then for a few seconds it turns cold before going hot again — that gap is the "cold water sandwich," and on most tankless installations it's a normal side effect of how the units work, not a malfunction.

Most likely causes, ranked

• It's inherent to how tankless units cycle. A tankless heater only fires while water is actively flowing past its minimum threshold. Between two hot draws — say, someone starts a shower, pauses to turn off the tap, then turns it back on — the burner shuts off during the pause, and the water already sitting in the hot-water pipe between the unit and the fixture cools during that gap. When flow resumes, that cooled slug of water reaches the fixture before freshly heated water catches up behind it.
• Long or wide pipe runs between the unit and the fixture. The more water sitting in the pipe, the bigger the cold slug and the longer it takes to flush through — this is why the effect is often worse at the farthest fixture from the unit.
• No recirculation system or buffer tank. Installations without a small buffer tank or hot water recirculation loop have nothing to smooth over the restart gap while the burner re-ignites.
• Reignition lag at the minimum flow threshold. Most tankless units need flow above roughly 0.4 GPM to fire — see Rheem P1 for how this threshold is documented. If flow briefly dips below that during a fixture change or a second fixture opening elsewhere in the house, the unit can shut off and take a moment to relight.
• Multiple fixtures cycling on and off in quick succession. Switching between a shower valve and a nearby sink, or two people using hot water close together, can trigger the same on/off/on cycle that produces the cold slug.

Check this first

1. Note whether it happens at every fixture or mainly at the one farthest from the unit — a pattern tied to distance points at pipe volume, not a unit fault.
2. Time roughly how long the cold gap lasts and how it lines up with any pause-and-restart in water use (turning a valve off then back on, a second fixture opening).
3. Check whether the home has a hot water recirculation pump or buffer tank installed, and if so, whether it's actually running.
4. Avoid quickly toggling a shower valve on and off — leaving it running through a brief pause, if the situation allows, avoids retriggering the restart cycle.

Which fault code are you seeing?

This symptom usually isn't tied to an error code at all — it's expected tankless behavior. If the display does show a code around the same time, look it up directly: code lookup, or jump to a brand: Rinnai, Navien, Rheem, AO Smith.