Error 109

Safety precautions:

- If you see water leaking from the boiler, from the PRV discharge pipe, or any signs of burst pipework, switch off the boiler electrical supply at the isolator or consumer unit and turn off the gas supply if you are trained to do so. Do not block or cap any PRV discharge pipe.

- Do not attempt internal repairs on gas valves, the PRV assembly, or the expansion vessel unless you are a Gas Safe registered engineer.

Initial checks a homeowner can do:

1. Check the pressure gauge on the boiler when the system is cold. Note the reading. Pmax faults typically occur above the manufacturer’s maximum (often around 3 bar but check the sticker/manual for your model).

2. Look for water at the PRV discharge pipe (usually a small plastic pipe leading to the exterior or a waste drain). Evidence of recent discharge indicates overpressure events.

3. Locate the filling loop (a flexible braided hose or isolating taps usually under the boiler). Ensure both filling loop valves are fully closed. If the filling loop is left open the system can be overfilled.

4. Check radiators and pipework for visible leaks.

Diagnostic and basic fix steps:

1. If the pressure is only slightly above the safe limit and there is no visible leak, you can reduce pressure by bleeding a radiator. Choose a low radiator near the boiler, use a bleed key, and release water until the boiler pressure on the gauge drops to the normal working range (commonly 1.0–1.5 bar cold). Have a container and cloth ready and close the bleed screw once the pressure is correct.

2. If bleeding radiators does not reduce pressure or pressure quickly rises back when the boiler heats, do not continue to operate the boiler. This suggests an internal fault (expansion vessel or stuck fill valve).

3. If you find the filling loop is open, close the taps and then check pressure again. If pressure remains high, proceed to bleeding radiators or call an engineer.

4. If the PRV is discharging water or you can see constant drips from the discharge pipe, the PRV is likely operating or leaking and needs an engineer to replace or test it. Do not plug or restrict the discharge pipe.

5. If the pressure is high when cold and rises further when the boiler runs, suspect a failed expansion vessel (lost air charge). Testing and recharging the expansion vessel requires specialist tools and should be done by a professional.

Reset and monitoring:

- Only attempt a boiler reset after you have corrected the pressure (reduced to within the normal range) and there are no signs of active leaks. If the fault clears but returns, leave the boiler off and call a Gas Safe engineer.

When to call a professional:

- If you see water coming from the PRV discharge pipe.

- If closing the filling loop and bleeding radiators does not stop the pressure rising or the fault keeps recurring.

- If you suspect the expansion vessel is faulty or if internal valves need replacing or testing.

- If you are not comfortable performing the checks above.

Important: do not attempt to repair or replace the PRV, expansion vessel, gas valves or internal boiler components yourself. Contact a Gas Safe registered heating engineer to diagnose and repair persistent overpressure faults and to certify the work.