Error E18

Safety precautions:

1. If the boiler is showing E18, switch off the boiler at the programmer and isolate electrical supply at the fuse/consumer unit before doing any work. Do not open the boiler casing or attempt internal repairs unless you are a qualified gas-safe engineer.

2. Allow the boiler and system to cool before touching pipework or radiators. Beware hot surfaces and hot water.

3. If you smell gas or suspect a gas leak, leave the property immediately and call the gas emergency number and a Gas Safe engineer.

Initial checks a homeowner can do (no boiler cover removal):

1. Note the error code, take a photo of the display and any status lights. Try a single reset using the boiler reset button/controls and observe whether the fault returns. If it resets and runs briefly then returns to E18, do the checks below.

2. Check system water pressure on the boiler gauge. Aim for roughly 1–1.5 bar (check your manual for the exact recommended pressure). If pressure is very low, repressurise using the filling loop per the manual and re-check for leaks. Low pressure can affect circulation.

3. Check that all heating zone valves and radiator isolation valves nearest the boiler are open (motorized zone valves should be in the correct position when a call for heat is made). Ensure the programmer/room thermostat is calling for heat when you test.

4. Listen at the boiler for the central heating pump when the CH is running. Feel the pump casing/pipes (carefully) for vibration or warmth – a completely silent, cold or unmoving pump may be seized or unpowered.

5. Bleed radiators (starting at high points/nearest the boiler) to remove trapped air which can cause airlocks and poor circulation.

6. Check for obvious signs of leaks or blocked condensate drains and for radiator thermostatic valves fully open when testing.

Specific diagnostic and basic corrective steps:

1. Reset and observe: After basic checks and any bleeding/repressurising, reset the boiler and run central heating. If circulation is restored the error may clear. Monitor flow and return by feeling flow pipe (hot) and return pipe (cooler) close to the boiler. A large temperature difference with little flow indicates poor circulation.

2. Confirm pump operation and settings: If the pump has a speed selector, try adjusting to a higher speed and re-test. If the pump appears not to run electrically but you hear a hum, it may be seized. If comfortable and competent with electrical isolation, you can check that the pump is getting supply, but do not open the boiler or disturb gas/electrical internals—leave this to an engineer.

3. Check for blocked heat exchanger/sludge: If some radiators are cold or you have a history of dirty water/old system, sludge or scale can block circulation. This typically requires a powerflush or chemical clean performed by a professional.

4. Check diverter/zone valves and thermostatic valves: A stuck diverter or zone valve can prevent flow through the heat circuit and lead to local overheating. Operate controls to force the motorised valves open and check if that restores circulation.

5. If the fault consistently returns and you have eliminated valves, airlocks, low pressure and radiator bleeding, do not attempt internal component replacement. Components that commonly need professional attention are the boiler pump, flow/return thermistors, control box, diverter valve, or internal heat exchanger faults.

When to call a professional and what to tell them:

1. Call a Gas Safe registered engineer if the E18 reoccurs after the above basic checks, if the pump appears faulty or seized, if you suspect a blocked heat exchanger, or anytime you would otherwise need to remove the boiler casing or work on gas/electrical parts.

2. When booking, give the engineer the boiler model (Keston C40/C55), the E18 code, what you’ve already checked (pressure, bleeding, pump noise, valves), and whether the boiler locked out or showed any other codes. Provide the photo of the display if you took one.

Final notes:

- Do not attempt to open or repair internal boiler components (pump replacement, thermistors, control box, heat exchanger) unless you are a qualified Gas Safe engineer. Internal repairs involve gas and electrical hazards.

- If you notice persistent pressure loss, leaks, or multiple fault codes, isolate the appliance and get a professional out promptly. Recording the sequence of events and any conditions that make the fault appear will help the engineer diagnose and repair more quickly.