Symptoms
- Hot water pulses between hot and lukewarm repeatedly
- Unit seems to ignite, run briefly, then shut off, then reignite
- Happens most at low flow rates, like a partially open faucet
- Full-flow use like a shower may not show the problem
Likely causes
- CommonFlow rate hovering near the unit's minimum activation thresholdTankless units need a minimum flow to stay on; flow dipping below it repeatedly causes cycling.
- CommonLow-flow fixture or partially clogged aerator restricting flow inconsistentlyA fixture flowing right at the edge of the minimum threshold can cause the unit to cycle.
- PossibleMixing valve or thermostatic faucet causing flow modulationCertain fixture designs adjust flow in ways that can trigger cycling on borderline units.
- PossibleScale buildup reducing effective flow through the heat exchangerBuildup can push actual flow below the minimum needed to sustain ignition.
What to do
- Try running the affected fixture at a slightly higher flow rate to see if that stops the cycling.
- Clean the faucet aerator or showerhead to rule out a restriction lowering flow below the threshold.
- Check your unit's specified minimum flow rate (often listed in gallons per minute) in the manual.
- Note whether cycling happens on multiple fixtures or just one, which helps isolate a fixture issue versus a unit issue.
- If in a hard water area and the unit hasn't been descaled recently, schedule a descaling service.
- If cycling persists across multiple fixtures at normal flow rates, the issue is likely with the unit itself.
Tools and parts
- Manufacturer's manual
When to call a pro
If descaling and checking fixtures doesn't resolve short cycling, a plumber familiar with tankless units can test the flow sensor and internal components.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.