Error S.10

Safety precautions:

- If you smell gas, turn off the gas supply at the meter, do not operate electrical switches, open windows, leave the property and call your gas emergency number immediately.

- Before accessing any internal boiler components switch the boiler off at the external isolator and, if unsure, isolate electrical power at the fuse/consumer unit. Do not attempt to work on the gas or combustion parts of the boiler unless you are a Gas Safe qualified engineer.

- If you are not confident with plumbing or removing covers, stop and call a qualified engineer.

Initial checks a homeowner can do:

1. Confirm behaviour: note when S.10 appears. Does it only show when a hot tap or shower is on? If yes, this is normal status signalling hot water demand.

2. Try a simple reset: press the reset button on the boiler and observe. If S.10 clears and returns only when a tap is opened, likely normal operation.

3. Check hot water taps: ensure taps are fully closed and that there are no dripping taps or appliance valves causing a small continuous flow.

4. Check the mains cold water supply and isolation valve to the boiler: make sure the water supply to the house is on and any inlet isolation valves to the boiler are fully open.

5. Check boiler pressure: look at the pressure gauge — it should normally be around 1.0–1.5 bar when cold. Extremely low pressure can affect operation, although S.10 is specifically flow-related.

Specific diagnostic and DIY fix steps (only if you are competent with basic plumbing):

1. Observe and reproduce: Open a hot tap and watch the boiler display. S.10 should appear while the tap is open and the boiler should fire for hot water. Close the tap — the status should clear.

2. Clean the cold-water inlet strainer/filter: many combi boilers have a small mesh filter on the cold-water feed to the boiler that can become clogged with debris and cause erratic flow sensor behaviour. To do this:

- Turn off the cold-water inlet isolation valve to the boiler (and isolate electrical power to the boiler).

- Place a bowl/towel under the isolation valve to catch a small amount of water.

- Unscrew/remove the filter cap or union (follow your boiler manual for exact location and method).

- Remove and inspect the mesh screen; rinse under clean water and remove debris.

- Refit the screen and cap, open the isolation valve slowly and check for leaks.

- Restore power and test. If the strainer was dirty this can clear false flow readings.

3. Flush air/debris: with the cold inlet open, run a hot tap fully for 30–60 seconds to flush air from the pipework and to ensure there’s adequate flow to register on the pump/impeller.

4. Reset and retest: after cleaning/flush, reset the boiler and test several taps/showers to confirm normal behaviour.

When to call a professional:

- If S.10 appears continuously with no hot water usage (i.e. you are sure no taps/appliances are drawing hot water), or if the status prevents the boiler from supplying hot water properly, call a Gas Safe registered engineer.

- If you are not comfortable isolating the cold-water supply or removing/cleaning the inlet strainer, call a qualified engineer.

- If, after cleaning the inlet filter and flushing, the S.10 persists or the boiler behaves erratically, the impeller/flow sensor may be sticking or the wiring/PCB may be at fault. These components are internal and require a trained engineer to diagnose and replace safely.

What the engineer will likely check/do:

- Replicate the fault and monitor the flow sensor signal while operating hot outlets.

- Remove the front cover and inspect the flow/impeller assembly for debris, wear or seizure; clean or replace the sensor/impeller if needed.

- Check wiring/connectors to the flow sensor and associated electronics for corrosion or loose connections.

- Test the boiler controls and PCB inputs to confirm correct interpretation of the sensor signal.

- After repairs, run full operational tests to confirm hot water and heating modes work correctly.

Information to provide when you contact an engineer:

- Boiler model and serial number (from underside of front panel or data plate), exact display message (S.10), when it appears (only when tap open or all the time), and the checks you have already performed (reset, cleaned filter, pressure reading).

Do not attempt gas-side or sealed-system repairs yourself. Any replacement of safety devices, sensors or work on the combustion chamber and gas connections must be done by a Gas Safe qualified engineer.