Protherm F.25 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.25 fault code mean?
The F.25 fault code on Protherm boilers is a safety lockout that can have two related meanings depending on your model. On chimney-flue (open-flue or natural-draught) models such as the Gepard MOV and Panther, it signals that the flue gas or draft thermostat has detected combustion products escaping into the room and has cut the boiler off to prevent poisoning. On all Gepard and Panther models it can also indicate that the boiler's flow temperature has risen dangerously quickly — typically five degrees per second or more — often because system pressure is dropping at the same time. In either case the boiler locks out and will not restart until the root cause is resolved. Do not keep resetting the boiler and ignoring this code; both triggers represent genuine safety hazards.
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
- Blocked, restricted, or leaking chimney flue Common
On open-flue models the most frequent trigger is poor or disrupted draft. A blocked chimney — caused by debris, a bird's nest, cobwebs, or ice at the terminal — prevents combustion gases from leaving the building. The flue gas thermostat detects the resulting backdraft or elevated temperature and opens the safety circuit, producing F.25. Depressurised or poorly jointed flue connections have the same effect.
- Rapid flow temperature rise due to low system pressure Common
If the system pressure in the heating circuit drops below the boiler's minimum threshold, water flow slows and the heat exchanger heats up very quickly. The control board detects the abnormally fast temperature rise (roughly 5 °C per second) and issues F.25 as a lockout. Check the pressure gauge on the boiler — if it reads below about 1.0 bar when cold, low pressure is likely contributing.
- Circulation pump failure or restricted water flow Common
A pump that is seized, running slowly, or blocked by sludge cannot move water around the system fast enough. Heat builds up in the heat exchanger, the flow temperature spikes rapidly, and F.25 is triggered. Air locks, heavy sludge deposits, or a partially closed isolation valve can produce the same effect without the pump itself being faulty.
- Faulty flow or return NTC temperature sensor Sometimes
Protherm boilers use NTC sensors to measure flow and return water temperatures. If one of these sensors has drifted out of calibration or developed an open circuit, the boiler's control board can receive a false reading that appears to show an extreme temperature rise, causing F.25 to be logged even when the actual temperatures are normal.
- Flue gas thermostat or high-limit thermostat failure Sometimes
The flue gas thermostat is the component that physically detects combustion products in the room on open-flue models. If the thermostat itself has developed a break in its circuit or has lost calibration, it can open and produce F.25 without there actually being a flue problem. Similarly, the boiler's emergency high-limit thermostat can fail in the open position and lock the boiler out.
- PCB (control board) fault or power surge Rare
Voltage spikes or an ageing control board can cause the boiler to misread sensor signals or trigger safety shutdowns incorrectly. This is a less common cause of F.25 but should be considered if all other components test satisfactory.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia. When the system is cold it should read between 1.0 and 1.5 bar. If it is below 1.0 bar, top it up through the filling loop until it reaches about 1.2–1.3 bar, then attempt a single reset. If pressure is normal, move to the next step.
- Reset the boiler (no more than twice) DIY safe
Press and hold the reset button as described in your user manual — usually for 3 seconds. If the boiler fires and runs normally, monitor it over the next hour. If F.25 returns after one or two resets, stop resetting and investigate further. Repeated resets without fixing the cause can mask a dangerous fault.
- Inspect the flue terminal for obvious blockages DIY safe
Go outside and visually check the flue outlet at the wall or roof. Look for ice, leaves, a bird's nest, or any physical obstruction covering the terminal. Do not touch or modify any flue pipe — simply report what you see to the engineer. On open-flue (chimney) models, also ensure no furniture or objects are blocking the air vent in the room.
- Bleed your radiators to clear air locks DIY safe
If you suspect air is restricting circulation, use a radiator bleed key to release air from each radiator, starting at the lowest and furthest from the boiler. Have a cloth ready. Once only water comes out, close the bleed valve. Re-check system pressure afterwards and top up if needed, as bleeding often causes a small pressure drop.
- Have a Gas Safe engineer inspect and clean the flue and chimney Gas Safe engineer
A registered engineer will carry out a full flue integrity check, measure draft, inspect all joints and seals, and clear any internal blockage. On open-flue models this is the most critical step. Flue work on a gas appliance is a legal requirement for Gas Safe engineers only.
- Have the engineer test the circulation pump and system flow Gas Safe engineer
The engineer will check whether the pump is running at the correct speed, measure flow rates, and look for sludge or scale blockages in the circuit. A powerflush may be recommended if the system is heavily contaminated. Pump replacement or the fitting of a magnetic system filter may be advised.
- Have the engineer test the NTC sensors and high-limit thermostat Gas Safe engineer
Using a multimeter and temperature reference, the engineer will verify that the flow and return NTC sensors are reading accurately across their full range. The flue gas thermostat and high-limit thermostat will also be checked for continuity. Any faulty components will be replaced with genuine Protherm-compatible parts.
- Have the engineer assess the PCB if all other components are satisfactory Gas Safe engineer
If sensors, flue, pump, and pressure are all within specification but F.25 persists, the control board may be producing erroneous lockout signals. PCB diagnosis and replacement must be carried out by a Gas Safe engineer with the appropriate electronic skills.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the basic homeowner checks above have not resolved F.25, or if the fault returns after a reset, contact a Gas Safe registered engineer. This is a legal requirement for any work beyond the simple checks above, and the safety risks associated with F.25 — flue gas ingress and overheating — mean it should not be left unresolved. You can find registered engineers at GasSafeRegister.co.uk.
Parts you may need
- NTC flow temperature sensor (Protherm compatible) · from £25
- NTC return temperature sensor (Protherm compatible) · from £25
- Flue gas thermostat / draft thermostat · from £35
- High-limit safety thermostat · from £30
- Circulation pump (Protherm Gepard / Panther compatible) · from £95
- PCB / control board (Protherm Gepard / Panther) · from £180
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 £130–£420, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.25 fault dangerous — can I keep using the boiler?
You should not continue running the boiler while F.25 is active. On open-flue models the code can indicate that combustion gases are entering the living space, which is a carbon monoxide risk. On all models it can also mean the heat exchanger is overheating dangerously. If your CO alarm activates, leave the property and call the Gas Emergency Helpline on 0800 111 999. Otherwise, leave the boiler off until a Gas Safe engineer has diagnosed and cleared the fault.
Why does my Protherm Gepard keep showing F.25 after I reset it?
A reset clears the lockout signal but does nothing to fix the underlying cause. If F.25 returns repeatedly, the boiler's control board is detecting the same unsafe condition each time — whether that is a flue problem, a failing pump, low system pressure, or a faulty sensor. Resetting more than two or three times without a proper diagnosis is inadvisable and can mask a worsening fault. Book a Gas Safe engineer to carry out a full investigation.
Can low boiler pressure cause the F.25 error on a Protherm?
Yes. If the system pressure drops below roughly 1.0 bar, water circulation slows and the heat exchanger temperature rises very rapidly. The boiler's control board interprets this fast temperature rise as a dangerous condition and issues F.25. Top the pressure up to around 1.2 bar using the filling loop, then reset once. If pressure keeps dropping, there is likely a leak somewhere in the system that also needs professional attention.
How much does it typically cost to fix a Protherm F.25 fault in the UK?
Most F.25 repairs fall between about £130 and £420 including parts and labour. Simple fixes such as replacing a faulty NTC sensor or flue thermostat tend to sit at the lower end of that range. A pump replacement or a powerflush of a sludged system will push costs toward the top of the range. A PCB replacement is a less common outcome but can cost £280–£450 fitted — if your engineer recommends this on an older boiler, it is worth comparing that cost against a new boiler installation (typically £1,800–£2,800 supplied and fitted).