Error S.41

Safety precautions:

- If water is discharging from the PRV drain pipe, protect floors and furniture and locate the drain outlet. Do not stand directly under escaping hot water.

- Turn off the boiler electrically using the isolator switch and turn the heating and hot water controls to off. Do not attempt internal repairs; gas and sealed system work must be done by a qualified engineer.

- Allow the system to cool before doing any bleeding or pressure checks to avoid scalding.

Initial checks a homeowner can do:

1. Check the display pressure reading. S.41 indicates >2.9 bar. Note the exact pressure reading if visible.

2. Look under the boiler for the filling loop and its two isolation valves. Confirm both filling valves are fully closed (valve heads usually perpendicular to the pipe when closed). If either valve is partly open, close it.

3. Inspect the PRV discharge pipe (usually a small plastic pipe running to outside or a drain). If it is wet or dripping there may have been a recent discharge.

4. If the boiler was recently serviced or an engineer worked on it, confirm the engineer fully closed the filling loop and set the system correctly.

Steps to reduce pressure and test temporarily:

1. With the system off and cool, read the pressure. If it is above about 2.9 bar you need to reduce it before restarting the boiler.

2. Bleed one or more radiators using a radiator key to release water until the pressure on the boiler display falls into the normal cold range (about 1.0 to 1.5 bar for Vaillant boilers). Collect discharged water in a container and close the bleed valve immediately when pressure is correct.

3. Alternatively, if you have access to a drain-off point or a drain valve on the system, you can use that to lower pressure. Only use this if you understand how to close the drain valve afterwards.

4. Once pressure is within the normal range, switch the boiler back on and observe. If pressure remains stable and S.41 clears, monitor over the next heating cycle.

Further diagnostic checks (may require professional help):

1. If pressure rises again each time the heating runs or after each cycle, that strongly suggests the expansion vessel has lost its pre-charge or the vessel membrane is damaged. A gradual rise can also indicate a stuck or faulty automatic filling valve letting mains water into the system.

2. Check if the filling loop is definitely fully closed. If closed and pressure is still rising, listen for water flow from the area of the filling loop; continuous ingress indicates a faulty valve or internal feed.

3. Inspect the PRV for evidence of recent discharge (wetness, rust, mineral deposits) and check whether the PRV is now dribbling or continuously letting water out. A PRV that has discharged may need replacement.

4. Expansion vessel check: locate the expansion vessel (usually inside the boiler casing or nearby) and its Schrader valve. Measuring the vessel pre-charge requires isolating the system and partially draining it so the vessel is not under water pressure; this is an advanced procedure and should be done by a Gas Safe engineer. Typical pre-charge is around 0.8–1.0 bar for domestic systems.

When to call a professional:

- If you cannot lower the pressure to the normal range by bleeding radiators, or the pressure repeatedly climbs each heating cycle.

- If the filling loop valves appear closed but pressure still rises, or you can hear/see mains water entering the system.

- If the PRV has discharged and is leaking or the PRV shows mineral deposits or is wet.

- For any work on the expansion vessel, PRV replacement, automatic filling valve replacement, internal boiler component repairs, or gas-related work.

Contact a Gas Safe registered heating engineer to carry out a full inspection and repairs. Tell them the boiler showed S.41 and describe whether the PRV discharged, whether the filling loop was confirmed closed, and whether pressure increases with each heating cycle. These details help the engineer target the likely causes (expansion vessel, filling valve, PRV). Do not attempt internal repairs on the boiler yourself.