Ferroli F21 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F21 fault code mean?
The F21 code on a Ferroli boiler is a blocking fault — meaning the boiler shuts down but may recover automatically once the underlying issue is resolved, without always needing a manual reset. Crucially, what F21 actually signifies depends on your specific Ferroli model. On the HE31C, F21 points to an ignition failure: the boiler attempted to light but could not establish or detect a stable flame. On the Optimax HE Plus range (such as the HE25S), F21 indicates excessive system pressure, typically above 3 bar, triggering a safety shutdown. Because F21 is not universally documented across all Ferroli ranges, always cross-reference your boiler's own user manual to confirm which interpretation applies to you. If you are unsure, treat the fault seriously and arrange for a Gas Safe registered engineer to diagnose 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
- Ignition electrode fouled or damaged (HE31C and similar) Common
Over time, carbon deposits, soot, or physical damage to the ignition electrode can prevent a strong enough spark to ignite the burner reliably. This is one of the most frequently reported causes of F21 on the HE31C. The electrode tip may also be misaligned relative to the burner, weakening the spark.
- Excessive system pressure — over 3 bar (Optimax HE Plus models) Common
On the Optimax HE Plus range, F21 triggers when the system pressure rises too high, usually exceeding 3 bar. This can occur if the filling loop has been left partially open, if the expansion vessel has failed, or if the pressure relief valve is not functioning correctly. Simply releasing pressure without investigating the root cause is not a long-term fix.
- Flame sensor contaminated with soot or oxidation Common
The flame sensor (also called the ionisation probe) detects whether a flame is present after ignition. A dirty or oxidised sensor surface can cause it to fail to register the flame even when combustion is occurring, resulting in a safety shutdown and F21 being displayed.
- Interrupted or low gas supply Sometimes
If the gas supply to the boiler is insufficient — due to a closed isolation valve, a meter issue, or a wider supply interruption — the burner cannot ignite. Other gas appliances in the home failing to light is a quick indicator that the supply itself may be the problem.
- Failed expansion vessel Sometimes
A waterlogged or under-pressurised expansion vessel causes system pressure to spike rapidly each time the boiler fires. On overpressure-variant models this will repeatedly trigger F21. The vessel's diaphragm may have perforated or the pre-charge pressure may have dropped too low.
- Wiring fault or loose connections to the control board Sometimes
Loose or corroded wiring between the ignition electrode, flame sensor, or pressure sensor and the PCB can cause intermittent or persistent F21 faults. Vibration over time can work connections loose, particularly on older boilers.
- Faulty gas valve Rare
If the gas valve fails to open properly, gas will not reach the burner and ignition will fail. Gas valve faults typically produce a persistent ignition failure after other causes have been ruled out.
- PCB (control board) failure Rare
The PCB coordinates ignition sequencing and monitors sensor signals. A faulty PCB may misread sensor data, fail to trigger the igniter at the right moment, or incorrectly report a pressure fault. PCB failure tends to occur after other components have been eliminated.
How to fix it
- Check your other gas appliances DIY safe
Before anything else, try lighting a gas hob or checking another gas appliance in your home. If nothing else works either, the problem is likely with your gas supply or meter — not the boiler. In that case, contact your gas supplier rather than a heating engineer, and do not attempt to reset the boiler repeatedly.
- Check the system pressure gauge (relevant to all models, essential for Optimax HE Plus) DIY safe
Look at the pressure gauge on the boiler's front panel. A healthy cold system should read between 1.0 and 1.5 bar. If it reads above 2.5–3 bar, the overpressure interpretation of F21 is likely. If it reads below 1.0 bar, low pressure may be contributing to ignition problems. Do not attempt to adjust gas-side components yourself.
- Bleed a radiator to reduce pressure if over 2.5 bar (Optimax HE Plus models) DIY safe
Using a radiator bleed key, slowly open the bleed valve on an upstairs radiator. Let a small amount of water out until the gauge drops to around 1.0–1.5 bar, then close the valve. Place a towel down first. This is a temporary measure — if pressure keeps rising, there is an underlying fault with the expansion vessel or pressure relief valve that a Gas Safe engineer must investigate.
- Top up system pressure if below 1.0 bar (all models) DIY safe
If the pressure gauge reads below 1.0 bar, locate the filling loop (usually a braided hose or lever beneath the boiler) and slowly open it until the gauge reads around 1.2–1.5 bar, then close it firmly. Do not overfill. If pressure keeps dropping, there may be a leak in the system that needs professional attention.
- Reset the boiler (once or twice only) DIY safe
Once you have checked gas supply and pressure, press and hold the reset button — often marked with an 'R' or a flame symbol — for around five seconds. Allow the boiler a full start-up sequence before assessing whether the fault has cleared. If the F21 code returns within a short time, do not continue resetting. Repeated forced restarts can damage the heat exchanger and fan.
- Have a Gas Safe engineer inspect the ignition electrode and flame sensor Gas Safe engineer
If F21 persists and you have an HE31C or similar model, a Gas Safe engineer should remove the burner assembly and inspect the ignition electrode for damage, carbon fouling, or misalignment, and clean or replace the ionisation probe. This work involves accessing gas-carrying components and must not be attempted by a homeowner.
- Have a Gas Safe engineer investigate the expansion vessel and pressure relief valve Gas Safe engineer
If pressure keeps rising after bleeding, a Gas Safe engineer needs to test the expansion vessel's pre-charge pressure and inspect the diaphragm. A failed vessel must be re-pressurised or replaced. The pressure relief valve should also be checked; a valve that has been weeping or has lifted repeatedly may need replacing.
- Have a Gas Safe engineer check wiring, the gas valve, and the PCB if the fault persists Gas Safe engineer
If simpler causes have been eliminated, the engineer should check the wiring harness connections to the PCB, test the gas valve for correct operation, and assess whether the control board itself has developed a fault. Both the gas valve and PCB are engineer-only components. Arrange this through a Gas Safe registered professional and ask for a written quote before work proceeds.
Parts you may need
- Ignition electrode · from £25
- Ionisation / flame sensor probe · from £20
- Expansion vessel (replacement) · from £55
- Gas valve · from £130
- PCB / control board · from £150
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–£300, depending on the underlying cause.
Frequently asked questions
Why does the F21 code mean different things on different Ferroli boilers?
Ferroli assigns fault codes at the model range level rather than across the entire brand, and F21 is not universally documented. The HE31C uses it to flag ignition failure, while the Optimax HE Plus range uses it for overpressure. Always check your specific model's manual — the code printed on the boiler's data plate will confirm your exact model. If you cannot locate the manual, Ferroli's UK technical helpline or a Gas Safe engineer can identify the correct interpretation for your unit.
Is it safe to keep resetting a Ferroli F21 fault?
Resetting once or twice is reasonable if the fault is a one-off occurrence. However, if F21 returns after each reset, there is a real mechanical or safety issue that resetting will not solve. Forcing the boiler to keep attempting ignition when there is a gas supply or electrode fault can damage the heat exchanger. If the code returns more than twice in quick succession, stop resetting and call a Gas Safe engineer.
My Ferroli Optimax HE Plus keeps showing F21 even after I bleed the radiators — why?
Bleeding a radiator only provides temporary pressure relief. If pressure keeps climbing back up to 3 bar or above, the most likely culprit is a failed expansion vessel — its internal diaphragm may have perforated, meaning it can no longer absorb the expansion of heated water. A Gas Safe engineer can test the vessel's pre-charge pressure with a simple gauge. Replacement typically costs £120–£220 including labour, which is far cheaper than ignoring the fault and allowing the pressure relief valve to weep repeatedly or fail altogether.
How much does it cost to fix a Ferroli F21 fault in the UK?
For most homeowners, the repair bill sits in the £90–£300 range. A diagnostic visit plus sensor cleaning or electrode replacement is typically at the lower end (£90–£150). Expansion vessel replacement usually runs £120–£220. Fan or PCB replacement, if required, tends to cost £180–£300. Gas valve replacement is at the higher end, typically £250–£350. If your Ferroli is more than ten years old and facing a major component failure, it is worth getting a new boiler quote alongside the repair quote — repair costs approaching a third of a replacement price usually favour replacement.