Saunier Duval F.24 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.24 fault code mean?
The F.24 code indicates that your Saunier Duval boiler has detected little or no water flowing through the central heating circuit. To protect its internal components from overheating or running dry, the boiler locks out and shuts down, leaving you with no heating or hot water until the underlying problem is resolved. Because Saunier Duval is part of the Vaillant Group and shares the same control architecture, this code carries the same meaning across both brands. The boiler will need to be reset once the root cause has been addressed, though repeated resets without fixing the fault will not clear it permanently.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or seized circulation pump Common
The central heating pump is responsible for pushing water around the system. Over time, sludge build-up, wear, or prolonged inactivity can cause the pump to seize or run too weakly to maintain adequate flow. The boiler's flow sensor detects the drop in circulation and triggers the F.24 lockout. A grinding or humming noise from the pump area is a tell-tale sign.
- Sludge and magnetite build-up Common
Corrosion inside steel radiators and pipework gradually produces a black, gritty sludge known as magnetite. Over time this accumulates throughout the system, narrowing pipework and restricting water flow to the point where the boiler can no longer detect adequate circulation. Radiators that are cold at the bottom but warm at the top are a classic symptom of heavy sludge.
- Low system pressure Common
If the system pressure shown on the boiler's manometer has dropped below around 0.8 bar, there may not be enough water in the system to circulate properly. The ideal operating pressure for most Saunier Duval boilers is between 1.0 and 1.5 bar when cold. A gradual pressure drop usually points to a small leak or a failing pressure relief valve.
- Closed or partially closed isolation valves Sometimes
Isolation valves fitted to the pump or around the boiler are sometimes inadvertently left closed after a service, repair, or system flush. Even a partially closed valve can restrict flow enough to trigger F.24. This is a straightforward check but is one of the most frequently overlooked causes, particularly after any recent work on the system.
- Airlock in the pump or pipework Sometimes
Trapped air pockets in the circulation pump, the boiler heat exchanger, or the pipework can impede or completely block water flow. Airlocks most commonly occur after the system has been drained and refilled, or following a period of very low pressure. Bleeding radiators and the pump bleed screw (if accessible) can help release trapped air.
- Stuck or faulty diverter valve (combi models) Sometimes
On combi models such as the Themaclassic F24E, a diverter valve directs water between the heating circuit and the domestic hot water plate heat exchanger. If this valve sticks in an intermediate position, it can disrupt circulation in the heating circuit and confuse the flow sensor, leading to an F.24 fault — sometimes intermittently.
- Wiring fault or poor PCB connection Rare
A loose or corroded electrical connection between the circulation pump and the PCB can cause the control board to incorrectly detect a circulation failure even when the pump and water supply are perfectly healthy. Corrosion from a nearby leak is a common culprit. This is relatively rare but worth investigating if all other causes have been ruled out.
How to fix it
- Check the system pressure gauge on the boiler DIY safe
Look at the pressure gauge or manometer on the boiler front. If the reading is below 1.0 bar, the system is likely under-pressurised. Top up slowly using the filling loop until the needle reaches between 1.0 and 1.5 bar, then close the filling loop valves fully. Do not overfill beyond 2.0 bar.
- Inspect isolation valves around the boiler and pump DIY safe
Trace the pipework immediately around the boiler and pump. Isolation valves (typically with a flat-head slot on the body) should be turned so the slot runs parallel with the pipe, indicating they are fully open. If any are perpendicular to the pipe, they are closed — open them and then attempt a reset.
- Bleed your radiators to release trapped air DIY safe
Starting on the ground floor and working upward, use a radiator bleed key to open each radiator bleed valve just until water (not air) emerges, then close it again. Check the system pressure afterwards and top up if it has dropped. This helps eliminate airlocks that may be preventing adequate circulation.
- Reset the boiler (up to 2–3 times only) DIY safe
Once you have completed the checks above, press and hold the reset button on the boiler for 3–5 seconds, or follow the reset procedure in your user manual. Allow the boiler a few minutes to restart and attempt normal operation. If the F.24 code returns immediately or after a short time, stop resetting and move on — repeated resets without fixing the cause will not help and may mask a worsening problem.
- Listen and look for obvious pump problems DIY safe
With the boiler running (if it restarts), listen near the pump location for grinding, rattling, or complete silence when the pump should be running. Also check for any obvious signs of a nearby water leak, which could indicate why pressure has been dropping. Note your findings to share with an engineer — do not attempt to open or dismantle the pump yourself.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the fault persists after the homeowner checks above, a Gas Safe registered engineer is needed. They will test pump operation and flow rates, check the diverter valve on combi models, inspect wiring and PCB connections, and carry out a system flush or powerflush if sludge is identified. Pump replacement, valve repair, and any work involving the boiler's internal components must only be carried out by a qualified engineer.
Parts you may need
- Central heating circulation pump (e.g. Grundfos UPS2 15-50 or equivalent) · from £75
- Diverter valve assembly (combi models) · from £55
- Flow sensor / flow switch · from £30
- Magnetic system filter (e.g. Magnaclean Pro2 or equivalent) · from £95
- Pump wiring harness / connector · from £20
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.24 fault myself?
Some of the initial checks are perfectly safe for a homeowner — topping up system pressure via the filling loop, opening any closed isolation valves, bleeding radiators to remove airlocks, and resetting the boiler up to two or three times. However, if these steps do not clear the fault, the repair will almost certainly involve the pump, diverter valve, or internal wiring, all of which must be handled by a Gas Safe registered engineer. Attempting to dismantle or replace these components yourself is unsafe and could invalidate your boiler warranty.
How much does it typically cost to fix an F.24 fault in the UK?
For most UK homeowners, the repair falls between £150 and £400. A straightforward pump replacement — the most common fix — typically costs £150–£350 including parts and labour. If sludge is the root cause, a chemical flush for mild build-up runs around £100–£200, while a full powerflush (needed for heavily contaminated systems) costs roughly £350–£600 depending on the number of radiators. Adding a magnetic filter at the same time as a flush is advisable and typically adds £80–£150. In rare cases where a PCB fault is involved, costs can be higher — ask your engineer for a written quote before authorising any work.
Why does my F.24 fault keep coming back after resetting?
If the code clears after a reset but then returns within minutes, hours, or a few days, it means the underlying cause has not been resolved — the reset has simply allowed the boiler to attempt normal operation again. The most common reasons for recurring F.24 faults are a pump that is gradually seizing up, progressive sludge restriction, or a slow pressure leak that keeps bringing system pressure below the minimum threshold. Each reset without a fix adds wear to the boiler. Have a Gas Safe engineer investigate as soon as possible rather than continuing to reset.
Is the Saunier Duval F.24 the same as the Vaillant F.24?
Yes — Saunier Duval is part of the Vaillant Group and both brands use the same underlying control and fault-code system. An F.24 on either brand indicates a water circulation fault. The diagnostic approach and likely causes are identical, although the specific components may differ slightly between models. If you have a Vaillant engineer or a specialist familiar with either brand, they will recognise the fault immediately.