Keston E18 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E18 fault code mean?
The E18 code on a Keston boiler signals that the water temperature in the flow pipe has climbed beyond safe operating limits — a condition known as flow overheat. When this happens, the boiler shuts itself down and enters lockout mode as a protective measure. The root cause is almost always related to poor water circulation: if hot water cannot move freely through the system, heat builds up rapidly in the heat exchanger and triggers the overheat sensor. Note that E25 (flow temperature rising too quickly) is a closely related code and points to the same family of circulation faults. Keston boilers use a twin-flue design with a modulating pump, which means not every Gas Safe engineer will be familiar with them — it is worth specifically seeking out someone with Keston experience.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Seized or failing circulation pump Common
The pump is responsible for pushing water around the system. If it has seized, is running too slowly, or has failed outright, water stagnates in the heat exchanger and overheats rapidly. This is the single most frequent cause of E18 on Keston boilers.
- Sludge or scale blocking the heat exchanger Common
Over time, iron oxide sludge and limescale accumulate inside the heat exchanger, restricting water flow. If you can hear a rumbling or kettling sound — similar to a kettle boiling — this is a strong indicator. Restricted flow causes localised overheating and triggers the E18 lockout.
- Air lock in the heating system Sometimes
Trapped air in the pipework or heat exchanger prevents water from circulating freely. Even a partial airlock can create a hot spot that trips the overheat thermostat. Bleeding radiators may help, but a persistent airlock often requires an engineer to locate and clear it properly.
- Faulty overheat thermostat or flow sensor Rare
If the thermostat or sensor that monitors flow temperature is giving inaccurate readings, it may trigger the E18 fault even when the actual water temperature is within normal range. This is less common but can occur on older units or after a genuine overheat event has damaged the component.
- PCB fault Rare
In rare cases the printed circuit board misinterprets sensor data or fails to control the pump correctly, causing an apparent or real overheat condition. This is a last-resort diagnosis once all other causes have been ruled out.
How to fix it
- Turn the boiler off at the mains and wait 30 seconds before switching it back on DIY safe
A brief power cycle can clear minor electronic glitches. Wait a full 30 seconds to allow the control board to fully reset before restoring power.
- Press and hold the reset button (marked with a spanner symbol) for 3–5 seconds DIY safe
After the power cycle, attempt a manual reset. Do not reset more than two or three times — repeated resets without resolving the underlying fault can cause further damage. Note that simply switching the boiler on and off does not perform a reset; you must press the reset button.
- Bleed your radiators to release any trapped air DIY safe
Turn off the boiler and allow it to cool. Use a radiator bleed key to open each bleed valve until water (not air) flows out steadily, then close it. Start upstairs and work downwards. After bleeding, check the system pressure is still between 1 and 1.5 bar on the pressure gauge.
- Top up system pressure via the filling loop if it has dropped below 1 bar DIY safe
Locate the filling loop — usually a braided hose with one or two valves beneath the boiler. Slowly open the valve(s) until the pressure gauge reads approximately 1–1.5 bar, then close the valve(s) securely. Low pressure on its own is unlikely to cause E18, but correcting it before an engineer visits saves time.
- Call a Gas Safe registered engineer with Keston experience Gas Safe engineer
If the fault returns after a reset, or if it did not clear at all, a qualified engineer must attend. They will inspect the circulation pump for seizure or wear, check the heat exchanger for sludge or scale, verify there are no persistent airlocks, and test the overheat thermostat and flow sensor. If sludge is found, a power flush or chemical clean is typically needed before any new components are fitted. Because Keston boilers use a twin-flue arrangement and a modulating pump, ask specifically for an engineer who has worked on Keston appliances — you can find one via keston.co.uk or by calling the Keston technical helpline.
Parts you may need
- Circulation pump (Keston-compatible) · from £120
- Overheat thermostat / high-limit sensor · from £35
- Flow temperature sensor (NTC thermistor) · from £25
- Magnetic system filter (e.g. Magnaclean) · from £75
- PCB (printed circuit board) · from £320
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear the E18 fault?
You can try resetting once or twice, but if the fault keeps returning it means the underlying problem — most likely a pump or circulation issue — has not been resolved. Repeatedly resetting a boiler with a genuine overheat fault puts stress on the heat exchanger and other components. Arrange an engineer visit if the fault reappears after a couple of resets.
What is the difference between E18 and E25 on a Keston boiler?
Both codes point to a water circulation problem. E18 means the flow temperature has actually exceeded the safe limit, triggering a full lockout. E25 means the flow temperature rose too quickly — a warning that circulation is restricted before it reaches the critical threshold. If you see E25 frequently, treat it seriously: it often precedes an E18 lockout and the causes and fixes are the same.
How much does it cost to fix a Keston E18 fault?
Most people pay between £150 and £450 depending on the cause. A straightforward pump replacement typically costs £150–£350 including labour. If the heat exchanger is heavily sludged, a power flush adds roughly £150–£300 on top. In rare cases where the PCB needs replacing, costs can exceed £500 — at that point it is worth discussing with your engineer whether repair or boiler replacement makes more financial sense, especially on an older unit.
Do I need a specialist engineer for a Keston boiler?
Ideally, yes. Keston boilers have a twin-flue design and a modulating pump that differ from standard condensing boilers, so not every Gas Safe engineer will be familiar with them. Ask explicitly whether the engineer has Keston experience before booking. You can also use the installer finder on keston.co.uk or contact the Keston technical helpline, who can recommend a suitably experienced local engineer.