Overview
E109 on a Baxi 600 Combi is a pre-circulation fault. The boiler is reporting that it could not detect normal water circulation during its pre-start check, so it will not light the burner. This is a safety behaviour to prevent the boiler from running with little or no water flow (a dry fire), which can damage the heat exchanger and other components. The fault can be caused by simple, temporary issues — low system pressure, air trapped in the system, closed isolation valves or a zone valve stuck closed — or by more serious faults such as a seized or failed circulation pump, a blocked pipe or magnetic filter, or an electrical/sensor fault. Because some causes are straightforward to fix at home and others need a trained Gas Safe engineer, you should do basic safe checks first but call a professional if the problem persists or you are unsure.
Possible Cause: Pre-circulation fault.
Troubleshooting Steps
Safety first
- If you smell gas, get everyone out of the building, do not operate electrical switches or the boiler, and call the gas emergency number immediately.
- If working near the boiler, switch the boiler off and isolate electrical power at the fuse switch before touching any plumbing parts. If you are not comfortable with any step, stop and call a Gas Safe engineer.
Initial checks a homeowner can do
1. Check the boiler pressure gauge. Aim for roughly 1.0–1.5 bar for a combi system when cold. If pressure is below 0.8–1.0 bar, topping up is often required.
2. Check that all radiator thermostatic valves and any isolation valves on the boiler flow and return pipes are open.
3. Ensure central heating mode is selected on the boiler and that the room thermostat is calling for heat.
4. Listen for the circulation pump when the boiler attempts to start: you should hear or feel it running for a moment when the boiler tries to fire.
5. Try a single boiler reset after checks: press and hold the reset button for 5–10 seconds and allow the boiler to attempt a restart. Do not repeatedly reset if the code returns immediately.
How to top up the system pressure (only if you are comfortable doing this)
- Locate the filling loop (usually a braided hose or dedicated valve beneath the boiler). With the boiler off, slowly open the valve(s) on the filling loop until the pressure gauge reaches about 1.0–1.5 bar. Close the valve(s) fully and remove or secure the filling loop as appropriate. If you are unsure where this is, consult the boiler manual or call an engineer.
Bleeding air from radiators
- Air in the system can stop circulation. Bleed radiators starting with the radiator nearest the boiler and working outwards. Use a radiator key, catch any water in a cloth or small container, and repressurise if pressure falls below 1.0 bar.
Basic diagnostics after initial checks
- If pressure is corrected and radiators bled, reset the boiler once. If the boiler starts and circulation is restored, monitor the system. If pressure drops again or E109 returns, do not keep resetting.
- If you hear the pump running when the boiler attempts to start but radiators stay cold, suspect a blockage, closed valve, stuck diverter/zone valve, or blocked magnetic filter. These often require a trained engineer because isolating and cleaning filters or removing blockages usually involves draining the system.
- If the pump is silent or makes unusual noises or the boiler shows no sign of trying to circulate, the pump may be electrically faulty or seized, or there may be an electrical/PCB fault. Do not attempt internal electrical repairs — call a Gas Safe engineer.
When to call a professional
- Call a Gas Safe registered engineer if: topping up pressure, bleeding radiators and a single reset do not clear the fault; the fault recurs frequently; the pump appears seized or dead; you suspect a blocked magnetic filter or heat exchanger; or you are not confident performing the above checks.
- When you call, tell the engineer the model (Baxi 600 Combi), the error code E109, the boiler pressure reading, what you have already tried (pressure topped up, radiators bled, reset), and any noises or other symptoms.
Do not
- Do not repeatedly reset the boiler if the E109 reappears — repeated resets can cause further lockouts or damage.
- Do not attempt gas, burner, PCB or detailed pump repairs unless you are a qualified Gas Safe engineer.
Following these steps will cover the common, safe checks a homeowner can do. If the fault remains after these checks, book a Gas Safe engineer to diagnose and repair the underlying circulation, pump, valve, filter or sensor fault.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 600 Combi Gas Boiler.