Overview
The display showing F (or FP) together with the burner light on a Keston S30 system boiler means the appliance is in frost protection mode. This is a built-in safety feature: when the boiler/water temperature falls below about 5°C the boiler will automatically fire intermittently to keep the system warm enough to avoid frozen pipes. In normal cold weather this is expected behaviour and is not a fault by itself. Severity is low if the boiler is simply protecting against frost. You should only be concerned if the boiler repeatedly fails to light, goes into lockout, runs continuously without bringing the system temperature up, or if other fault codes appear. Basic checks and simple actions can be performed by a homeowner (see troubleshooting), but any work that involves opening the boiler, gas components, or electrical parts must be carried out by a Gas Safe registered engineer (or local equivalent). Call a qualified engineer if you smell gas, the boiler locks out, the burner never lights after several restarts, or you are unsure what to do.
Possible Cause: The boiler is operating in frost mode
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property immediately and call your gas emergency number. Do not attempt any checks on the boiler.
- Do not remove the boiler cover or attempt internal repairs unless you are a Gas Safe registered engineer.
- Turn off electrical supply at the isolator before touching external wiring or filling loop valves.
Initial checks a homeowner can do:
1. Check the display and controls: confirm the code shown is F or FP and the burner indicator is on. This normally indicates frost protection is active.
2. Ensure the boiler has mains power and the mode knob is set appropriately (BOILER ON or the correct winter setting). Check the programmer/timer and any room thermostat or cylinder thermostat are set to allow heating.
3. Check other gas appliances in the house to confirm there is a gas supply. If no gas appliances will light, check the gas meter/credit and contact the gas supplier.
4. Check the system pressure gauge on the front of the boiler. If pressure is very low (well below 1.0 bar), you may need to top up the system (see manufacturer instructions). Do this only if you are comfortable with the filling loop procedure and follow the boiler manual closely.
Specific diagnostic and simple fixes to try:
1. If outside temperature is below freezing and the boiler shows F/FP but otherwise operates normally (fires occasionally, then goes to standby), you can leave it: it is doing its job. Make sure the mains power to the boiler is left ON so frost protection can operate.
2. Check for a frozen condensate pipe (common in cold spells): follow the condensate pipe from the boiler to its outside termination. If you suspect ice blockage, thaw the frozen section by pouring warm (not boiling) water over it or use a warm cloth. Do not use sharp objects or hot air guns. Once thawed, flush by running the boiler if safe to do so. Insulate the exposed condensate pipe or route it to a less-exposed position to prevent recurrence.
3. If the boiler is cycling rapidly, won’t reach temperature, or trips to lockout after a few ignition attempts (codes such as L2 or other fault codes appear), press the Restart button once as instructed in the manual. If it fails again, note any additional code shown and contact an engineer.
4. If system pressure is low, with the boiler turned off and cold, use the filling loop to bring pressure up to the level recommended in your manual (commonly around 1.0–1.5 bar for many systems). Close the filling valves and re-check after bleeding radiators. Consult the boiler manual for the exact safe procedure for your model.
When to call a professional:
- If the boiler will not ignite after several restart attempts or goes into lockout.
- If additional fault codes appear alongside or instead of F (for example L2, F1, F3, or PCB/thermistor related codes).
- If you cannot restore normal pressure, if you find a leak, or if you are not confident performing the condensate thaw or filling loop steps.
- Whenever work would involve opening the casing, replacing components, or any gas or sealed-system interventions.
Final notes:
- F/FP with the burner light is normally a protective, low-severity mode and often requires no more than leaving the boiler powered and insulated pipes/condensate.
- If you are unsure, or if symptoms persist (lockouts, repeated ignition failures, or strange noises), arrange a visit from a Gas Safe registered engineer or the manufacturer service team. Do not attempt internal repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston S30 System Gas Boiler.