Error E78

Safety precautions:

1) If you smell gas, stop using the boiler, leave the property immediately and call your gas emergency service — do not attempt any checks. 2) Before doing any visual checks inside the boiler casing: turn the boiler off at the programmer/control panel and isolate the electrical supply at the mains. 3) Do not attempt to repair or replace gas valves, burners, electrodes, or internal wiring unless you are a Gas Safe registered engineer.

Initial checks a homeowner can do:

1) Look at the boiler pressure gauge. Normal cold system pressure is usually around 1.0–1.5 bar (check your manual for the exact recommended range). If the gauge reads below 1.0 bar the system is under-pressurised. If it is well above the recommended range (eg over 2.5–3 bar) the system may be over-pressurised. 2) Inspect visible pipework, radiators and the boiler for signs of leaks or damp patches. 3) Listen for unusual pump noises or gurgling in radiators which can indicate air or poor circulation. 4) Check that the external boiler flue and condensate discharge (if visible) are not blocked or frozen in cold weather.

Basic remedial steps you can try:

1) Top up system pressure: if pressure is low, use the filling loop/homeowners’ filling valve to add water until the pressure is in the correct range (usually around 1.0–1.5 bar). Close and secure the filling loop once done. Do not leave the filling loop open. 2) Bleed radiators to remove trapped air, then re-check and re-top if pressure drops below the required level. 3) Reset the boiler: after correcting pressure, reset the boiler using the reset procedure in the handbook (typically press and hold the reset button for 5–10 seconds). Wait for the boiler to attempt restart. 4) If the pressure rises very quickly or keeps dropping over a few days, that indicates a leak, faulty pressure relief valve (PRV), or a leak in the system — do not continue to top up repeatedly; call an engineer.

If E78 remains after the above checks or if pressure and circulation look normal:

1) Do not repeatedly keep resetting the boiler — repeated lockouts can hide rising faults and may cause further damage. 2) Contact a Gas Safe registered engineer. Tell them the boiler model, that it is showing E78, what you have already checked (pressure reading, topping up, bleeding radiators), and whether the pressure is stable or falling. The engineer will check the water-pressure sensor, its wiring and connector, the flow sensor/pump and the boiler PCB and may need to replace the sensor or perform calibration or electronic diagnostics.

When to call a professional immediately:

1) Persistent E78 after basic checks and reset. 2) Repeated pressure loss, visible leaks, or water dripping from the boiler or radiators. 3) Any signs of overheating, repeated lockouts, or unusual smells. 4) Any work involving internal wiring, sensor replacement, pump removal, gas components or PCB diagnostics.

Summary reminder: you can safely check the pressure, top up, bleed radiators and try a reset. Anything beyond those steps — especially opening the boiler, working on wiring, sensors, pump or gas components — must be done by a Gas Safe registered engineer.