Ferroli F08 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F08 fault code mean?
The F08 code on a Ferroli boiler means the overtemperature protection circuit has triggered because the heating water in the heat exchanger has reached an excessively high temperature — specifically, if the water temperature hits 100°C or above and stays there for five or more seconds, the boiler logs this event. F08 is classified as a blocking fault, which means it is recorded in the boiler's event register but does not necessarily stop the boiler from running immediately. However, if the underlying problem is not addressed and F08 occurs repeatedly, it can escalate into a full lockout displayed as A08 (or A03 in some circumstances). The companion code A08 is the lockout version of the same fault and requires a manual reset to clear. Both codes point to the same root cause: the heat exchanger is getting too hot, usually because water is not circulating properly around the system.
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
- Circulation pump failure or blockage Common
The pump is responsible for keeping heated water moving around the system. If it seizes, runs too slowly, or becomes blocked with sludge, water stagnates inside the heat exchanger and temperature climbs rapidly. This is the single most frequent trigger for F08 and A08 on Ferroli boilers.
- Air trapped in the heating circuit Common
Air pockets disrupt the flow of water through the system, creating areas of poor circulation. When the heat exchanger cannot shed heat efficiently because of an air lock, temperatures rise and the overtemperature sensor trips. Bleeding radiators often resolves this, particularly after a pressure top-up or recent plumbing work.
- Low system pressure Common
If the water pressure drops below around 0.8–1.0 bar, there is simply less water mass available to absorb heat from the burner. This reduces cooling of the heat exchanger and can cause localised overheating. The gauge on the front of the boiler will confirm if this is a factor.
- Faulty or damaged heating flow temperature sensor Sometimes
The NTC sensor that monitors the flow temperature can develop a fault, become corroded, or give a false high reading. When the boiler's control board receives an inaccurate temperature signal suggesting overheating, it logs F08 even if actual circulation is fine. A Gas Safe engineer can test sensor resistance to confirm this.
- Sludge or limescale restricting flow Sometimes
Years of corrosion inside older steel radiators produces magnetite sludge, and hard-water areas accumulate limescale in the heat exchanger. Both narrow the internal waterways, restrict flow, and force temperatures higher. Noisy pipework or cold radiator bottoms are tell-tale signs alongside a recurring F08.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when the system is cold. If the needle is below 1.0 bar, use the filling loop (usually located beneath the boiler) to top the system up to around 1.2 bar. Close the filling loop valves fully once the correct pressure is reached.
- Bleed the radiators to release trapped air DIY safe
Work through each radiator in the house using a radiator bleed key, starting on the ground floor. Open the bleed valve slowly — a hissing sound confirms air is escaping. Close the valve as soon as water appears. Re-check boiler pressure afterwards and top up again if it has dropped below 1.0 bar.
- Listen and feel for the circulation pump DIY safe
With the boiler running (or attempting to run), place your hand carefully on the pump body — usually a cylindrical component on the pipework near the boiler. A working pump produces a faint hum and very slight vibration. Complete silence or an unusual grinding noise suggests the pump may have failed or seized and will need professional attention.
- Reset the boiler DIY safe
On most Ferroli models there is a reset button on the control panel, often labelled 'R' or marked with a reset symbol. Press and hold it for one to two seconds. Allow the boiler a few minutes to complete its startup sequence. If the fault clears and the boiler runs normally, monitor it closely. Do not reset more than two or three times — if F08 or A08 returns quickly, a genuine fault is present and forcing further restarts risks damaging the heat exchanger.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If F08 or A08 keeps reappearing after the checks above, a qualified engineer needs to investigate further. They will test the heating flow sensor resistance, inspect the pump for correct speed and output, check for sludge or limescale restriction in the heat exchanger, and may recommend a power flush if the system is heavily contaminated. Do not continue operating the boiler repeatedly if the code keeps returning, as sustained overheating can cause lasting damage to the heat exchanger.
Parts you may need
- NTC heating flow temperature sensor · from £25
- Central heating circulation pump · from £80
- Pump head (motor only) · from £45
- Central heating inhibitor (1 litre) · from £15
- Radiator bleed key · from £3
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 £100–£320, depending on the underlying cause.
Frequently asked questions
What is the difference between Ferroli F08 and A08?
Both codes relate to the same overtemperature problem in the heating circuit, but they behave differently. F08 is a blocking fault — it is logged in the boiler's internal event register but may not visibly stop the boiler straight away. A08 is the lockout version: the boiler shuts down completely and will not restart until someone presses the reset button. If your boiler is showing A08, it means the overtemperature condition has been serious enough to trigger a full lockout. Repeated F08 events that go unresolved are a common route to an A08 lockout.
Can I fix Ferroli F08 myself, or do I need an engineer?
There are a few checks any homeowner can safely carry out: topping up system pressure if it is below 1.0 bar, bleeding the radiators to release air, and performing one or two resets. If these steps clear the fault and it does not return, you may not need an engineer immediately. However, if F08 or A08 keeps coming back, the likely causes — a failing pump, a faulty temperature sensor, or a sludge-restricted heat exchanger — all require a Gas Safe registered engineer to diagnose and repair safely.
How much does it typically cost to fix a Ferroli F08 fault in the UK?
For the most common repairs — replacing a temperature sensor or swapping a circulation pump — most households pay somewhere between £100 and £320 including parts and labour. A power flush to clear sludge from the system typically costs £300–£500 depending on the number of radiators. In rare cases where the heat exchanger itself has been damaged by prolonged overheating, costs can be considerably higher, and on an older boiler it may be more economical to replace the boiler entirely.
Why does my Ferroli boiler keep showing F08 even after I reset it?
If the code returns within a short time of resetting, there is an active underlying fault that the reset is not resolving — it is just temporarily clearing the alarm. The most likely culprits are a pump that is struggling or has partially seized, a section of the system blocked by sludge or limescale, trapped air that has not been fully bled, or a temperature sensor giving inaccurate readings. Continuing to reset without addressing the root cause can eventually damage the heat exchanger, so it is worth having an engineer investigate if the fault recurs more than two or three times.