Overview
E30 on a Keston C40/C55 combi means the boiler has detected an excessive temperature difference between the flow (outgoing) and return (incoming) water. In normal operation the flow will be hotter than the return, but if the difference is too large it indicates poor circulation through the heating circuit so the boiler is overheating the flow pipe while very little heat is being returned. Common causes are a failed or seized central heating pump, an airlock, closed or stuck valves, a blocked heat exchanger or magnetic filter, or incorrect pump direction/wiring or sensor issues. Severity: this fault is important because persistent poor circulation can cause the boiler to overheat and go into lockout, reduce system performance, damage components and produce noisy radiators or banging. Some basic checks are safe for a homeowner, but diagnosing and repairing pump faults, internal wiring, thermistors or gas-related parts should be left to a Gas Safe registered engineer. If the error repeats after simple checks or you see leaks, smells of gas, or unusual noises, arrange a professional visit promptly.
Possible Cause: Difference between flow and return temperatures too high – check circulation.
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property immediately and call the gas emergency number—do not try to operate the boiler.
- Do not remove the boiler casing or attempt internal electrical or gas repairs. Only a Gas Safe engineer should open the appliance.
- Turn off electrical supply at the isolator if you are uncomfortable performing basic checks.
Initial homeowner checks you can safely do:
1. Note the exact error code and any repeating pattern. Press the boiler reset button once and allow it to attempt a restart. If it clears and stays running, monitor for recurrence.
2. Check the boiler pressure gauge. If pressure is low (commonly below 1.0 bar), top up the system to about 1.0–1.5 bar following the boiler handbook. Low pressure can affect circulation.
3. Make sure central heating controls/thermostat and timers are calling for heat. Ensure any zone valves are fully open.
4. Feel the flow and return pipes at the boiler (careful, they may be hot). If the flow pipe is very hot and the return is much cooler (a large delta, e.g. >20 °C), this confirms poor circulation.
5. Bleed radiators to remove trapped air which can cause airlocks. Start from the lowest radiator and work upwards.
6. Check for obvious restrictions: if you have a magnetic dirt filter, check its indicator and condition (do not dismantle unless you know how—many filters have an isolation valve and drain which are safe to operate per manufacturer instructions).
7. Listen at the pump for vibration or humming. If it is silent when heat is called or sounds grinding, the pump may be seized or faulty.
More specific diagnostic steps (proceed only if comfortable and safe):
- Confirm pump direction: some faults arise if the pump is installed or wired backwards. This check and correction should be done by an engineer.
- If the boiler has external bypass or system bypass valves, ensure they are correctly set (incorrect settings can cause high delta T).
- If radiators heat unevenly after bleeding, or some stay cold, this suggests a circulation blockage, closed TRVs, or zone valve issue.
When to call a professional:
- If reset does not clear the fault or E30 returns repeatedly.
- If you suspect the pump is seized, noisy, or running but not circulating properly.
- If you cannot restore normal flow/return temperatures after bleeding radiators and checking valves.
- If you find water leaks, persistent low pressure, or electrical faults.
What the engineer will likely do:
A Gas Safe heating engineer will isolate the boiler, remove the casing, check pump operation and speed settings, inspect and test flow and return thermistors and wiring, verify pump direction and any zone valves, check and clean/replace magnetic filter or heat exchanger if blocked, and run more advanced tests (e.g. pressure and flow checks). They can safely replace faulty pumps, sensors, valves or carry out powerflushing to clear blockages.
Notes for the homeowner:
- Record the circumstances (when the fault happens, temperatures if visible, audible symptoms) and report these to the engineer – it speeds diagnosis.
- Do not attempt internal electrical/gas repairs; this is a safety risk and legally must be done by a Gas Safe engineer.
- If the boiler frequently locks out with E30, arrange a professional visit rather than relying on repeated resets.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C40 C55 Combi Gas Boiler.