Saunier Duval F23 Fault Code: Water Circulation Fault
The F23 code appears when your Saunier Duval boiler detects that the temperature difference between the flow pipe (carrying hot water out of the boiler) and the return pipe (bringing cooled water back) has exceeded 35°C. This gap tells the boiler that water is not moving around the heating circuit properly — instead of flowing freely, it is stagnating near the heat exchanger, overheating, and triggering a safety shutdown. The boiler locks out to prevent damage until the underlying circulation problem is resolved.
General guidance only — not a substitute for professional advice. 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.
What does the Saunier Duval F23 fault code mean?
The F23 code appears when your Saunier Duval boiler detects that the temperature difference between the flow pipe (carrying hot water out of the boiler) and the return pipe (bringing cooled water back) has exceeded 35°C. This gap tells the boiler that water is not moving around the heating circuit properly — instead of flowing freely, it is stagnating near the heat exchanger, overheating, and triggering a safety shutdown. The boiler locks out to prevent damage until the underlying circulation problem is resolved.
Common causes
- Faulty or seized circulation pump Common
The pump is responsible for pushing water around your heating circuit. If its shaft has seized, it is running at the wrong speed, or it has failed electrically, water barely moves and the return pipe stays far cooler than the flow pipe. This is the single most frequent cause of an F23 lockout.
- Blocked system filter or heavy sludge build-up Common
Saunier Duval boilers typically include a filter on the return circuit. Over time it collects rust particles, limescale, and black iron oxide sludge. A heavily clogged filter creates significant flow resistance, starving the heat exchanger of circulating water even when the pump itself is working fine.
- Low system pressure Common
If the pressure gauge reads below approximately 1.0 bar, there is insufficient water in the circuit. The reduced volume circulates poorly, the heat exchanger overheats rapidly, and the boiler triggers the F23 shutdown. This can be caused by a slow leak, a recently bled radiator, or a faulty pressure relief valve.
- Airlocks in the heating circuit Sometimes
Pockets of trapped air in radiators or pipework can block or slow water flow just as effectively as a physical obstruction. Airlocks often develop after a system has been drained or after radiators have been bled without topping up the pressure afterwards.
- Closed isolation valves or stuck thermostatic radiator valves Sometimes
If one or more isolation valves on the flow or return pipework are shut, or if all thermostatic radiator valves are closed at the same time, water has nowhere to travel. The boiler heats it in place and quickly detects the extreme temperature differential.
- Faulty NTC thermistor (temperature sensor) Sometimes
The boiler relies on NTC thermistors to measure flow and return temperatures accurately. A sensor that has drifted out of calibration or developed an internal fault can report a false temperature gap, triggering the F23 code even when circulation is perfectly normal.
- Physical blockage in the pipework Rare
A severe sludge deposit or a foreign object lodged in the pipework can restrict flow to a point where the pump cannot overcome the resistance. This is less common than filter blockage but tends to require a full power flush to clear.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, use the filling loop (usually a braided hose or lever valve beneath the boiler) to top up the pressure until it sits around 1.2–1.5 bar, then close the filling loop valve fully. If you are unsure how to locate the filling loop, consult your boiler's user guide.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in the home — such as a gas hob — are working normally. If no gas appliances are functioning, the issue is with your gas supply rather than the boiler. Contact your gas supplier or call the National Gas Emergency line on 0800 111 999 if you suspect a gas leak.
- Check all isolation valves and radiator valves are open DIY safe
Walk around your home and make sure every thermostatic radiator valve (TRV) is not fully closed. Also check that the isolation valves on the flow and return pipes close to the boiler are in the open position (handle in line with the pipe). Opening at least some radiator valves ensures the pump has a circuit to push water through.
- Bleed the radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve (the small square fitting at the top corner) just until you hear hissing air escape. Close it as soon as a steady trickle of water appears. Work your way back towards the boiler, bleeding each radiator in turn. After bleeding, recheck system pressure and top up if needed, as bleeding can cause it to drop.
- Reset the boiler DIY safe
Once you have carried out the pressure check, valve checks, and radiator bleeding, press and hold the reset button (usually marked with a flame or 'Reset' symbol) for around three seconds. Allow the boiler to go through its startup sequence. Do not reset more than two or three times — repeated resets without fixing the root cause can cause additional wear.
- Inspect the system filter Gas Safe engineer
A Gas Safe engineer should inspect and clean the magnetic system filter on the return circuit. This involves isolating the filter, removing it, and flushing out the captured sludge and debris. A heavily contaminated filter strongly suggests wider system sludge, and the engineer may recommend a power flush of the entire circuit.
- Have the circulation pump tested and replaced if needed Gas Safe engineer
A Gas Safe engineer will check whether the pump is receiving power, running at the correct speed, and actually moving water. A seized or failed pump must be replaced. Pump replacement typically takes two to three hours and is the most common repair required for an F23 fault.
- Have the NTC thermistors tested Gas Safe engineer
Using a multimeter, a Gas Safe engineer can check the resistance values of the flow and return temperature sensors against the manufacturer's specification. A sensor reading outside the expected range will be replaced — these are inexpensive parts, so if there is any doubt, replacement is straightforward.
- Consider a power flush if sludge is widespread Gas Safe engineer
If the system filter is heavily blocked and radiators are cold at the bottom or slow to heat, a Gas Safe or heating engineer may recommend a power flush. This process uses specialist equipment to force a cleansing solution through all the pipework and radiators at high velocity, clearing accumulated sludge and scale.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler continues to display F23 after you have carried out the homeowner checks above, or if the fault returns shortly after resetting, do not keep resetting it. A Gas Safe registered engineer should attend to diagnose and repair the underlying fault. You can find registered engineers at GasSafeRegister.co.uk.
Parts you may need
- Central heating circulation pump (e.g. Grundfos UPS2 or Wilo equivalent) · from £95
- Magnetic system filter (e.g. Adey MagnaClean or Fernox TF1) · from £60
- NTC thermistor / flow or return temperature sensor · from £12
- Filling loop assembly (if faulty) · from £18
- Three-way diverter valve · from £55
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F23 fault myself?
There are a few checks you can safely do yourself: topping up the system pressure via the filling loop, bleeding the radiators to remove trapped air, making sure all radiator and isolation valves are open, and resetting the boiler once or twice. However, if these steps do not clear the fault, the most likely culprits — a failed circulation pump, a clogged system filter, or a defective temperature sensor — all require a Gas Safe registered engineer to diagnose and repair safely.
How much does it cost to fix an F23 fault on a Saunier Duval boiler?
The cost depends on the cause. A circulation pump replacement — the most common repair — typically costs £150–£300 including parts and labour, rising to around £350–£400 in London and the South East. An NTC thermistor replacement is considerably cheaper at roughly £80–£150 all-in because the part itself costs very little. If widespread sludge is the problem and a power flush is needed, budget £300–£500 on top of any component replacements. In extreme cases involving a cracked heat exchanger, costs can exceed £600 — but this is uncommon for an F23 code specifically.
Why does my Saunier Duval keep showing F23 after I reset it?
A recurring F23 means the root cause has not been resolved. The most likely explanations are a circulation pump that is failing progressively, a system filter that is so heavily blocked it restricts flow almost immediately after the boiler starts, or a slow water leak that is dropping system pressure repeatedly. If the fault returns within minutes or hours of a reset, stop resetting and call a Gas Safe engineer — repeated lockouts can sometimes indicate a more serious issue such as a partially open valve or a developing leak somewhere in the system.
Is the F23 code dangerous?
The F23 itself is a safety response rather than a dangerous condition — the boiler has correctly detected a circulation problem and shut itself down to prevent overheating damage. That said, you should not continue resetting the boiler repeatedly without addressing the cause, as persistent overheating can eventually damage the heat exchanger. There is no gas leak associated with this fault, but if at any point you smell gas, stop what you are doing, open windows, and call the National Gas Emergency Service on 0800 111 999 immediately.