Overview
E40 on a Keston C36 combi means the boiler has detected water pressure that is too high in the sealed heating system. The boiler monitors system pressure and will go into fault to protect components such as the heat exchanger, pump seals and pipe joints when pressure exceeds safe limits. On the C36 the normal cold system pressure target is about 1.0–1.5 bar and the absolute maximum operating pressure is 2 bar, so anything above the normal range can trigger E40. Common causes include: the system being overfilled (filling loop valves left open), a failed expansion vessel (lost pre-charge so the system cannot accept thermal expansion), or a failed/blocked pressure relief valve or automatic air vent failing to release excess pressure. If the pressure is only high when the boiler heats up (a large rise during firing) it points more to an expansion vessel or PRV/air vent problem. If the pressure is constantly high even when cold it is often simple overfilling or an open filling loop. Severity: this is a potentially damaging condition if ignored because sustained overpressure can cause leaks or damage to boiler components. Some corrective actions are straightforward for a competent homeowner (e.g. draining a little water to reduce pressure, checking the filling loop), but diagnosing or replacing expansion vessels, PRVs or any internal gas/heating components must be done by a Gas Safe registered engineer.
Possible Cause: Water pressure too high
Troubleshooting Steps
Safety precautions:
1) If the boiler is displaying E40 and you see water escaping from the boiler discharge pipe, or from the safety/PRV pipe, isolate gas and electricity to the boiler and call a Gas Safe engineer immediately. Water from the PRV may be hot—avoid contact and protect floors.
2) Turn the boiler off and allow the system to cool before you release any water; hot water and steam can cause burns. Have towels/containers ready to catch water.
3) Don’t open or try to repair gas components, the expansion vessel, pressure relief valve or internal parts yourself. Any internal work on the boiler must be done by a Gas Safe registered engineer.
Initial checks a homeowner can do:
1) Check the pressure gauge on the front of the boiler when the system is cold. Note the reading (bar). If it is above 1.5–1.6 bar when cold, the system is over-pressurised.
2) Check the external filling loop under the boiler. Confirm both filling loop valves are fully closed and that the braided hose (if fitted) is not left open or leaking. If the filling loop has a removable connector, ensure it is fitted correctly.
3) Look for visible leaks around radiators, pipework, boiler base and the PRV discharge pipe. If you find any active leaks, isolate gas/electric and call a Gas Safe engineer.
4) Check whether the pressure is always high when cold, or whether it rises significantly only when the boiler fires. To test this: note cold pressure, run the heating for a short period, then check pressure again (be careful—let the boiler run a short time then switch off and let gauge stabilise). A rise of 0.2–0.3 bar when hot is normal; a rise of 0.5 bar or more suggests expansion vessel or PRV issue.
Specific diagnostic and fix steps you can safely perform:
1) If the filling loop is open or you suspect overfilling: ensure the filling loop valve(s) are fully closed. If you have just filled the system, you may have put too much water in. Close the loop.
2) Reduce pressure by draining water from the system: a) Preferred simple method for most homes: bleed a radiator using a radiator key. Start with the radiator nearest the boiler. Protect floors, have a container and cloth ready, open the bleed valve slowly until water (not just air) flows and the boiler pressure gauge falls to around 1.0–1.3 bar cold, then close the valve. b) Alternative: use the system drain cock (if you know its location) to release water until the pressure is in the safe range. Turn drain cock slowly and recheck gauge.
3) After reducing pressure to about 1.0–1.3 bar cold, restart the boiler and observe. If pressure stays stable, reset the boiler (press the reset button) and monitor normal operation.
4) If the pressure falls below the target after draining, re-pressurise using the filling loop following the boiler manual instructions. Only add enough water to reach about 1.0–1.3 bar when cold, then close the filling loop completely.
When to call a professional (must call):
1) If you cannot reduce the pressure, if pressure continues to rise above safe limits during operation, or if the gauge exceeds the boiler maximum (around 2 bar) call a Gas Safe registered engineer. A rising pressure on heating typically indicates a failed expansion vessel or a faulty pressure relief/air vent which require specialist repair/replacement.
2) If you see continuous discharge (water dripping or flowing) from the PRV discharge pipe, or water present under or around the boiler, switch the boiler off, isolate gas/electric if safe to do so, and call an engineer. The PRV or discharge pipe is a safety device—do not block it.
3) If the boiler shows repeated pressure-related faults, or E40 returns after you reduce pressure, there may be an internal fault (expansion vessel pre-charge loss, stuck PRV, sensor fault) that needs a Gas Safe engineer.
4) If you are unsure at any stage, or uncomfortable performing the bleed/drain steps, contact a Gas Safe engineer rather than risk incorrect action.
Final steps and notes:
1) After any correction, reset the boiler following the manufacturer’s instructions and observe for normal operation. Monitor the pressure for several heating cycles to ensure stability.
2) Keep a note of what you did and when, and report persistent or recurring faults to the engineer. Don’t attempt to tamper with internal components, gas connections, or replace sealed parts yourself.
3) If a professional visit is required, tell the engineer: the E40 code, your cold pressure reading, whether pressure rises on heating and whether you saw any discharge or leaks—this information speeds diagnosis.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.