Protherm F.21 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.21 fault code mean?
The F.21 fault code on Protherm boilers (including the Gepard series) indicates a safety lockout triggered because one or more operating parameters have gone beyond the boiler's permitted limits. In plain terms, the boiler's control system has detected an unsafe condition — most commonly related to temperature or flow — and has shut everything down to protect the appliance and your home. This is a deliberate safety action, not a random glitch, so it should be investigated rather than simply reset and ignored. F.21 sits within the F.20–F.42 range of Protherm error codes, which cover more serious faults affecting both performance and safety. Nearby codes to be aware of: F.22 signals a low-water or pump lockout, F.23 indicates an excessive temperature difference between the flow and return pipes, F.30 relates to a lockout sensor open circuit, and F.31 to a lockout sensor short circuit — a faulty sensor can, in some cases, falsely trigger an F.21 condition even when the boiler itself is functioning correctly.
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
- Overheating due to restricted water flow Common
If water cannot circulate freely around the system, heat builds up in the heat exchanger until it breaches the boiler's safety threshold. This is the most common trigger for F.21 and is usually caused by one of the underlying issues listed below rather than being a fault in its own right.
- Seized or failing circulation pump Common
The circulation pump drives hot water from the boiler around your radiators and back again. If it seizes, runs too slowly, or fails altogether, heat has nowhere to go and accumulates inside the boiler. A pump that is making unusual noises — grinding, humming loudly, or running silently when it should be audible — is a strong indicator.
- Sludge or limescale blocking the heat exchanger Common
Over time, magnetite sludge and limescale can build up inside the heat exchanger, narrowing the waterways and preventing efficient heat transfer. The boiler then overheats locally, triggering the lockout. Hard-water areas of the UK are particularly prone to this.
- Airlock in the heating system Sometimes
A pocket of trapped air in the pump or pipework can partially or fully stop water circulation, mimicking the effect of a pump failure. The pump may sound as though it is running but moving little or no water. Unusual gurgling from the boiler or pump housing can point to this cause.
- Faulty overheat or lockout sensor Sometimes
If the sensor that monitors the boiler's high-limit temperature develops a fault — reading incorrectly high or intermittently — the control board may trigger an F.21 lockout even when actual temperatures are within safe limits. Related Protherm codes F.30 and F.31 cover sensor open-circuit and short-circuit faults respectively, so an engineer will check these in tandem.
- Electrical supply voltage irregularities Rare
Voltage surges or significant dips on the mains supply can cause sensors and the PCB to behave erratically, producing readings that the control board interprets as a threshold breach. If the fault clears on reset but returns regularly, unstable mains voltage is worth investigating.
How to fix it
- Reset the boiler once or twice DIY safe
Switch the boiler off using its controls, wait two to three minutes, then switch it back on. A one-off lockout caused by a momentary spike or minor glitch may clear this way. If the F.21 code returns promptly or persists after two resets, do not keep resetting — repeated resets on a genuine fault can mask the underlying problem and delay a proper fix.
- Check your gas supply is on DIY safe
Confirm the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Check whether other gas appliances in the property — such as your hob or gas fire — are working normally. If there is a wider gas supply problem, contact your gas supplier or call the National Gas Emergency Service on 0800 111 999.
- Check and top up system pressure if low DIY safe
Look at the boiler's pressure gauge. For most Protherm boilers, normal cold-system pressure is between 1.0 and 1.5 bar. If it reads below 1 bar, use the filling loop (typically a braided flexi hose with a valve beneath the boiler) to bring the pressure up to around 1.2–1.5 bar, then close the filling loop valve. Note that low pressure alone is more associated with F.22, but it is worth confirming as part of a basic check.
- Listen and observe the pump for airlock symptoms DIY safe
With the boiler attempting to run, listen carefully near the pump housing for gurgling, rushing air sounds, or an unusually quiet pump that should be audible. If the pump has an accessible manual bleed screw on its front face, you can carefully loosen it a quarter-turn with a flat-head screwdriver — once the hissing stops and a small amount of water appears, retighten it. Only do this if you can clearly identify the bleed point and have a cloth ready to catch any drips. If you are unsure, leave this step to an engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the F.21 fault persists or returns after basic checks, a Gas Safe registered engineer is needed. They will carry out a full diagnostic, checking the circulation pump (for seizure, correct speed, and output), the heat exchanger (for sludge or scale blockage), the overheat and lockout sensors (including checks related to F.30 and F.31 sensor codes), and the PCB for signs of voltage damage. Depending on findings, they may recommend a power flush to clear the system, component replacement, or — if the heat exchanger or PCB is the culprit on an older boiler — potentially a full boiler replacement if the repair cost is disproportionate to the appliance's value.
Parts you may need
- Circulation pump (Protherm-compatible) · from £95
- Overheat/lockout thermostat sensor · from £35
- PCB/control board · from £200
- Heat exchanger · from £320
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–£420, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting a Protherm F.21 fault and using the boiler?
No. One or two resets to see whether the fault clears is reasonable, but repeatedly resetting a genuine lockout is not safe. F.21 is a deliberate safety shutdown — the boiler has detected that something is outside its safe operating limits. Continuing to reset it without finding the cause risks damaging the heat exchanger or, in a worst case, creating an unsafe situation. If the fault returns after two resets, stop and call a Gas Safe registered engineer.
How much does it typically cost to fix a Protherm F.21 fault in the UK?
For the most common repairs — circulation pump replacement or a power flush to clear sludge — most homeowners pay in the region of £150 to £420 including parts, labour, and a call-out fee. A straightforward pump swap tends to sit toward the lower end; a power flush of a larger system or sensor replacement toward the middle. More expensive components such as a PCB (around £250–£450 fitted) or a heat exchanger (£400–£650+ fitted) can push costs higher, and in those cases it is worth comparing the repair quote against the cost of a new boiler, especially if your Protherm is over ten years old.
What is the difference between Protherm F.21, F.22, and F.23?
These three codes all indicate a safety lockout but point to different triggers. F.21 means general threshold values have been exceeded — typically an overtemperature or overheating condition. F.22 specifically flags a low-water pressure or pump-related lockout. F.23 points to an excessive difference between the flow (outlet) and return (inlet) water temperatures, which often suggests poor circulation. Knowing which code your display shows helps an engineer narrow down the diagnosis much faster.
Could the F.21 fault be a false alarm caused by a faulty sensor?
Yes, though it is less common than a genuine circulation or overheating issue. The overheat/lockout sensor can develop a fault — either an open circuit (related to Protherm code F.30) or a short circuit (F.31) — causing it to report a threshold breach when temperatures are actually normal. A Gas Safe engineer will test the sensor's resistance values with a multimeter and compare them against Protherm's specifications to confirm whether the sensor is at fault or whether the lockout is genuine.