Ferroli F43 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F43 fault code mean?
The F43 code means the boiler has triggered its heat exchanger protection function. In plain terms, the boiler has detected that water is not flowing properly through the system — or that air is present in the circuit — and has shut itself down to prevent the heat exchanger from overheating and sustaining damage. F43 is classified as a blocking fault on Ferroli boilers, which means it may clear on its own once the underlying cause is resolved, unlike 'A'-prefix codes that always need a manual reset. On certain Ferroli models (including the DOMIcondens and Optimax HE Plus ranges), F43 can also indicate a faulty flow sensor rather than a pure circulation problem, so it is worth checking your specific model's documentation if the steps below do not resolve it.
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
- Air trapped in the heating circuit Common
Air locks in the radiators or pipework prevent water from circulating freely. The boiler senses the drop in flow and activates heat exchanger protection before any damage can occur. This is the single most common trigger for F43, especially after recent plumbing work or if radiators have been cold at the top.
- Low system pressure Common
If the system pressure has dropped below around 1 bar, there may not be enough water in the circuit to maintain adequate flow past the heat exchanger. The boiler interprets this as a circulation failure and locks out with F43.
- Sludge or magnetite build-up restricting circulation Common
Over time, corrosion inside the heating circuit produces a thick black sludge (magnetite). This can partially block pipework, restrict the pump, and reduce water flow to a level the boiler considers dangerous. Systems without a magnetic filter fitted are particularly susceptible.
- Seized or failing circulation pump Sometimes
If the pump has seized — often after a long summer shutdown — or is running but not generating sufficient flow, the heat exchanger will overheat and F43 will trigger. You may hear unusual noise from the pump area, or no noise at all when the boiler tries to fire.
- Faulty flow or temperature sensor Sometimes
On some Ferroli models F43 specifically points to a damaged flow sensor sending incorrect readings to the PCB. Even if circulation is perfectly normal, a defective sensor can falsely trigger heat exchanger protection. This is a component swap that requires a Gas Safe engineer.
- Blocked or partially closed system valves Rare
A lockshield valve left partially closed after maintenance, or a faulty zone valve stuck in the wrong position, can restrict flow enough to trigger F43 even when the pump is working correctly.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top it up using the filling loop (usually a braided or chrome tap beneath the boiler) until the gauge reads around 1.2 bar, then close the filling loop firmly. Do not overfill beyond 2 bar.
- Bleed all radiators in the property DIY safe
Starting with the ground floor radiators and working upward, use a radiator bleed key to open the bleed valve on each radiator until water (not air) trickles out steadily. Re-check the boiler pressure after bleeding and top up again if it has dropped below 1 bar. This removes air locks that are the most frequent cause of F43.
- Check that all radiator and system valves are fully open DIY safe
Walk around the property and confirm that all thermostatic radiator valves (TRVs) and lockshield valves are open. A valve accidentally left closed can cause enough flow restriction to trigger the fault. Also check that any isolation valves on the boiler pipework are fully open.
- Listen for the circulation pump DIY safe
With the boiler attempting to run, place your hand or ear near the pump body (usually on the left or right side of the boiler). You should feel a slight vibration and hear a gentle hum. Complete silence may indicate a seized pump. Do not attempt to open or work on the pump yourself — note your findings for the engineer.
- Reset the boiler (up to twice) DIY safe
Once you have carried out the checks above, press and hold the reset button until the display changes or the boiler begins its start-up sequence, then release. Allow the boiler a full ignition cycle to see if it runs normally. If F43 returns, you may reset once more. Do not reset more than two or three times in total — repeated forced resets when a genuine fault exists can damage the heat exchanger or fan.
- Thaw a frozen condensate pipe if applicable (winter months) DIY safe
In cold weather, a frozen condensate pipe can indirectly affect boiler operation. If the white plastic condensate pipe (usually running to an outside drain or internal soil stack) is frozen, thaw it gently with warm — not boiling — water poured along its length, or wrap it with a warm cloth. Once clear, reset the boiler.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If F43 returns after your checks and resets, a qualified Gas Safe engineer should attend. They can assess pump condition and replace it if seized, perform a powerflush if sludge is the cause, test or replace the flow or temperature sensor, inspect heat exchanger integrity, and fit a magnetic system filter to prevent recurrence. Do not continue operating the boiler repeatedly if the fault keeps reappearing, as this risks permanent heat exchanger damage.
Parts you may need
- Circulator pump (Grundfos or equivalent) · from £85
- Flow sensor / NTC thermistor · from £25
- Magnetic system filter (e.g. Adey MagnaClean) · from £65
- Radiator bleed key · from £3
- Filling loop assembly · from £18
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 £90–£350, depending on the underlying cause.
Frequently asked questions
Will F43 clear itself, or do I always need to reset the boiler?
F43 is a blocking fault on Ferroli boilers, which means it can clear automatically once the condition that triggered it — such as an air lock or low pressure — is resolved. However, in practice most people still need to press reset after topping up the pressure or bleeding the radiators, as the boiler will not always restart unaided. If the fault clears and stays clear, no further action is needed. If it keeps returning, there is an underlying mechanical issue that needs professional attention.
Could F43 mean my circulation pump needs replacing?
Yes, a seized or failing pump is one of the key causes of F43. If you hear no noise or vibration from the pump when the boiler tries to fire, that is a strong indication. A Gas Safe engineer can confirm this and replace the pump if necessary. Pump replacement including labour typically costs £150–£300 depending on the pump model and your location in the UK.
My Ferroli boiler shows F43 and there is black water in the system — what does that mean?
Black or very dark water is a sign of significant magnetite sludge build-up from internal corrosion. This sludge restricts flow and is a common cause of F43. A Gas Safe engineer may recommend a powerflush — a process that forces clean water through the circuit at high velocity to remove the sludge. A powerflush typically costs £300–£500 depending on the size of your system. Fitting a magnetic filter afterwards (such as an Adey MagnaClean) will catch any remaining particles and protect the boiler going forward.
Is F43 dangerous — do I need to stop using the boiler immediately?
F43 is a protective shutdown code, meaning the boiler has already taken action to protect itself. While the boiler is locked out it is not actively dangerous. However, you should not keep forcing resets without investigating the cause, as running the boiler repeatedly with poor or no water circulation can cause expensive and sometimes irreparable heat exchanger damage. Carry out the basic checks (pressure, bleeding, valves) and if the fault persists, arrange for a Gas Safe engineer to attend promptly rather than leaving it for weeks.