Overview
E19 on a Keston C40/C55 combi means the boiler has detected the return-water temperature is higher than the maximum allowed. In other words the water coming back into the boiler from the heating circuit is too hot relative to what the boiler expects, and the boiler has tripped the fault to protect itself. Common reasons include poor or blocked circulation (pump not moving water properly, airlocks, closed valves or blocked radiator/heat exchanger), a stuck or incorrectly wired zone/diverter valve, or in some cases a faulty return thermistor giving an incorrect temperature reading. Severity is moderate to high for the appliance: the boiler will usually lock out to avoid overheating and potential damage. It is not usually an immediate danger to the household if the boiler shuts down, but you should not ignore repeated occurrences because sustained overheating can damage the heat exchanger or other components. Some simple checks can be done by a competent homeowner, but diagnosing and especially repairing pump, valve, sensor or internal boiler faults should be left to a Gas Safe registered engineer.
Possible Cause: Maximum return temperature exceeded – check circulation
Troubleshooting Steps
Safety precautions:
1. Do not attempt to remove the boiler casing or work on gas components unless you are a Gas Safe registered engineer. Working inside the boiler is dangerous and illegal for unqualified people.
2. If the boiler is locked out, press reset once and observe. If the fault returns immediately or within a short time, turn the boiler off at the isolator and call an engineer.
3. Isolate electric supply to the boiler if you need to work near exposed pipework or controls, but do not isolate the gas unless instructed by an engineer.
Initial checks a homeowner can do:
1. Note exactly when E19 appears (on heating only, during hot water demand, after reset, continuously). This info helps the engineer.
2. Press the boiler reset once and watch: does the boiler run for a short time and then display E19 again? If it is persistent, stop trying resets.
3. Check the central heating controls: is the room thermostat calling for heat? Are programmer/timers correct? Make sure two heating controls aren't fighting each other (e.g. one set to OFF, another to ON).
4. Listen and feel for the circulation pump: with care, feel the pump body and pipes (don’t touch hot parts). The pump should hum or vibrate slightly when the boiler is calling for heat. If the pump is completely cold/still when the boiler is trying to fire, circulation may be failing.
5. Check radiator thermostatic valves and lockshields: ensure radiator TRVs are not fully closed and that lockshield valves are open enough to allow flow. Bleed radiators to remove any airlocks that could restrict circulation.
6. Check boiler pressure gauge: although E19 is temperature-related, very low pressure or repeated pressure loss can indicate leaks or circulation problems. If pressure is very low follow the manual to repressurise only if you are confident doing so.
Specific diagnostic and basic fix steps (do not open the boiler):
1. Reset test: after a single reset, observe flow and return behaviour. If E19 does not recur, it might have been transient; monitor over the next heating cycles.
2. Radiator bleeding: bleed radiators starting with the highest in the system to remove airlocks. After bleeding, check system pressure and top up to the correct level if required (follow boiler manual). Restart and see if E19 returns.
3. Check for blocked or closed valves: locate and verify any isolating valves on the pump, manifold or boiler flow/return are fully open. If there are external pumps or zone valves, ensure they are powered and moving (you may hear them or see actuators moving). If a valve is closed or stuck it can cause high return temps.
4. Pump speed: some pumps have a speed selector. If you can safely access it (and only if you’re comfortable), try increasing the pump setting by one notch to improve circulation, then monitor the boiler for recurrence. If unsure, do not change pump wiring or internals.
5. Check for system blockages or buildup: sludge, limescale or a blocked heat exchanger will restrict flow and raise return temperatures. Signs include cold radiators, noisy pump, or large flow/return temperature difference. These issues usually require a professional powerflush or heat-exchanger cleaning.
6. Thermistor/sensor suspicion: a failing return sensor can report falsely high temperatures. If other checks suggest circulation is fine (radiators warm, pump running) but the boiler still reports E19, a sensor fault is possible and will need an engineer to test and replace components.
When to call a professional:
1. Call a Gas Safe registered engineer if E19 persists after the basic checks above, if the fault returns immediately after reset, if you suspect pump failure, zone/diverter valve failure, heat-exchanger blockage, or a faulty thermistor. These components either require electrical/gas-safe work or specialist diagnostic tools.
2. If the boiler locks out repeatedly, leave it switched off and isolated at the mains until the engineer arrives. Repeated lockouts can damage the boiler and are a sign an engineer should intervene.
Final notes:
1. Provide the engineer with details you gathered: when the fault happens, whether hot water or heating was running, whether pump sounded to be running, if radiator bleeding changed anything, and any other observed behaviour.
2. Do not attempt internal repairs on the boiler, open gas valves, or alter wiring. Those are engineer-only jobs.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C40 C55 Combi Gas Boiler.