Symptoms
- Water starts hot, briefly turns cold, then goes hot again
- Happens most when turning the tap on and off quickly, like pausing a shower
- More noticeable with long pipe runs between the heater and the fixture
- Unit itself shows no error code
Likely causes
- CommonResidual warm water in the pipe being pushed out before the unit reignitesWhen flow briefly stops and restarts, there's a delay before the burner relights, during which leftover cooling water reaches the tap.
- CommonLong distance between the tankless unit and the fixtureLonger pipe runs hold more water that can cool during the pause, making the effect more noticeable.
- PossibleLow minimum flow rate triggering repeated on/off cyclingSome fixtures or low-flow showerheads can cause the unit to cycle on and off more than expected.
What to do
- Recognize that a brief cold sandwich, typically a few seconds, is a known characteristic of tankless units rather than a malfunction.
- Avoid pausing and restarting the water flow quickly if this bothers you, since a continuous flow avoids triggering the effect.
- Check if your unit has a 'recirculation' or 'buffer tank' option that can reduce or eliminate the delay.
- If it's especially severe, confirm the showerhead or fixture isn't restricting flow below the unit's minimum activation rate.
- Consider a small buffer tank or recirculation pump if the effect is disruptive and your household uses a lot of on/off water use.
- If the cold gap lasts much longer than a few seconds, that's beyond typical behavior and worth having checked.
Tools and parts
- Manufacturer's manual
When to call a pro
If the cold water period is unusually long or severe, a plumber can check whether a buffer tank, recirculation setup, or unit sizing would help.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.