Flow temperature overheat
The L1 error code on Keston boilers indicates flow temperature overheat.
Keston L1 means: flow temperature overheat. The usual causes are circulation pump failure or air lock in the system, blocked or restricted heat exchanger (sludge or limescale) or system valves closed restricting water flow. You can safely reset the boiler once — if L1 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does L1 Mean
The L1 code is shown by the boiler’s control board when it detects flow temperature overheat. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows L1 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 1 technical sources.
| Cause |
|---|
| Circulation pump failure or air lock in the system |
| blocked or restricted heat exchanger (sludge or limescale) |
| system valves closed restricting water flow |
| faulty thermistor giving incorrect temperature reading |
| thermostat wiring fault |
| insufficient water flow through the boiler |
Safety Warning
L1 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
Check the circulation pump — verify it is running and water is flowing (feel the pipes either side of the pump for temperature difference)
Check system valves — ensure all isolation valves and zone valves are fully open
Check the system for air locks — bleed radiators and check for gurgling noises
Inspect the heat exchanger — check for limescale build-up or sludge
Test the flow and return thermistors — compare the temperature reading with an external thermometer
Check pump speed setting — increase if necessary
If the system has a filter (magnetic or inline), check if it needs cleaning
How to Fix It
Bleed the system of air
flush the heating system if sludge is present (consider a power flush)
replace the pump if it is not circulating
descale or replace the heat exchanger if blocked with limescale
open any closed valves
replace faulty thermistors
Refer to the Keston service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Keston models that can display fault code L1:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Keston models | — | Possible |
Frequently Asked Questions
Bleed the system of air. According to the technical documentation, this is the first check on the list for L1.
L1 is not classified as a high-severity fault, so the boiler can be reset once. If L1 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.