Protherm F.24 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.24 fault code mean?
The F.24 fault code on Protherm boilers indicates that the boiler's PCB has detected an abnormally rapid rise in water temperature — specifically faster than 10°C per second — within the heating circuit. This tells the boiler's safety logic that heat is not being carried away quickly enough, almost always because water isn't flowing through the system at the rate it should be. Rather than allow the heat exchanger to overheat, the boiler locks out and displays F.24. You'll lose both heating and hot water until the underlying circulation problem is resolved. Protherm boilers share the Vaillant Group control platform, so this code carries the same meaning across the related boiler range.
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
- Faulty or seized circulation pump Common
The pump is responsible for pushing water around the heating circuit. If it has seized internally, is clogged with debris, or its motor has failed, water flow drops dramatically and the heat exchanger temperature spikes. This is the single most common trigger for F.24 on Protherm boilers.
- Low system pressure Common
When the system pressure drops below around 0.8–1 bar, there simply isn't enough water volume in the circuit to absorb heat properly. A slow leak, a recently bled radiator, or a faulty pressure relief valve can all cause pressure to fall to a level that starves the heat exchanger.
- Airlocks in the heating circuit Common
Pockets of trapped air in radiators, pipework, or the pump itself can interrupt or slow water flow without any obvious external sign. After draining work or if the system has run low on water, airlocks are a frequent culprit.
- Closed or partially closed isolation valves Sometimes
The isolation valves beneath the boiler must be fully open for water to circulate freely. If a recent service left one or both valves in the closed position, the flow restriction is immediate and severe enough to trigger F.24.
- Sludge or limescale blockage Sometimes
Years of use can allow iron oxide sludge and limescale to accumulate in the heat exchanger or pipework, narrowing the waterway and restricting flow. Older systems without a magnetic filter are particularly susceptible.
- Incorrect pump speed setting Sometimes
Protherm boilers can have the pump speed configured during commissioning. If the speed is set too low for the size of the property and heating circuit, the pump simply cannot move enough water to keep pace with the burner's output.
- Faulty flow or temperature sensor or PCB wiring fault Rare
In rarer cases, the flow or NTC temperature sensor is giving the PCB inaccurate readings, causing F.24 to trigger even though circulation itself is adequate. A wiring fault between the pump and PCB can produce the same false alarm.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's fascia. It should read between 1 and 1.5 bar when the system is cold. If it's below 1 bar, top up using the filling loop (the braided or chrome valve beneath the boiler) until it reaches around 1.2 bar, then close the loop. Refer to the boiler manual if you're unsure where the filling loop is.
- Inspect the isolation valves under the boiler DIY safe
Locate the service valves on the flow and return pipes beneath the boiler. The slot on the valve head should run parallel to the pipe — that means open. If any slot is at a right angle to the pipe, it's closed. Use a flat-bladed screwdriver to turn it so the slot aligns with the pipe, opening the valve.
- Bleed the radiators to release trapped air DIY safe
Work from the radiator furthest from the boiler and move back towards it. Use a radiator bleed key to open the bleed valve (usually in the top corner of the radiator) by a quarter turn. Hold a cloth underneath. When the hissing stops and a steady dribble of water appears, close the valve. Check system pressure again afterwards and re-pressurise if needed.
- Reset the boiler DIY safe
Once you've completed the checks above, press and hold the reset button (consult your manual for its location — on most Protherm models it's labelled with a flame or reset icon) for around 3 seconds. Allow the boiler to go through its ignition sequence. If F.24 returns promptly, do not keep resetting; move on to calling an engineer.
- Check that the gas supply is functioning DIY safe
Confirm other gas appliances in the property (hob, gas fire) are working normally. If they aren't, contact your gas supplier as there may be a wider supply interruption.
- Call a Gas Safe registered engineer to inspect the circulation pump Gas Safe engineer
If the steps above don't clear the fault, the most likely cause is a pump that has seized, is running at the wrong speed, or has an internal blockage. A Gas Safe engineer will test pump operation, check for debris or airlock within the pump body, adjust speed settings, and replace the pump if necessary.
- Have the engineer check sensors, wiring, and the PCB Gas Safe engineer
A Gas Safe engineer can use diagnostic tools to read the boiler's live data, test the NTC flow and return temperature sensors, and inspect the wiring loom between the pump and PCB. Faulty sensors or corroded connectors can produce a false F.24 reading and require component-level repair.
- Arrange a power flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of heavy sludge or limescale in the heat exchanger or pipework, a professional power flush will clear the system. Afterwards, fitting a magnetic system filter (such as a Magnaclean) will slow future build-up. This work must be carried out by a qualified heating engineer.
Parts you may need
- Circulation pump (compatible replacement) · from £110
- NTC flow/return temperature sensor · from £25
- Pump wiring harness/connector · from £20
- Magnetic system filter (e.g. 22mm inline filter) · from £55
- Pressure relief valve · 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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I reset the Protherm F.24 code myself, and will it clear the fault?
Yes, a reset is always worth trying after you've carried out the basic checks — checking pressure, opening isolation valves, and bleeding radiators. However, a reset only clears the lockout signal; it doesn't fix the underlying problem. If F.24 comes back within a short time, stop resetting and call a Gas Safe engineer. Repeatedly resetting without resolving the cause can mask a worsening fault.
How much does it cost to fix a Protherm F.24 fault in the UK?
For most people, the repair involves a pump service or pump replacement, which typically costs between £150 and £380 all-in (parts and labour). If the fix turns out to be something straightforward — an airlock, a closed valve, or low pressure — you may only pay a call-out and diagnostic fee of around £60–£100. In rarer cases where a PCB replacement is needed, costs can rise to around £400–£550, at which point it's worth comparing the repair cost against a new boiler quote.
Why does my Protherm boiler keep showing F.24 after resetting?
A recurring F.24 almost always means the root cause hasn't been fixed. The most common reasons for a repeat fault are a pump that is progressively seizing, a slow system leak keeping pressure low, or a persistent airlock. Each reset temporarily allows the boiler to fire, but the same circulation problem triggers the lockout again. A Gas Safe engineer can use live diagnostic data to pinpoint exactly where the flow is restricted.
Will bleeding my radiators fix the Protherm F.24 error?
It can, if trapped air is the cause of reduced circulation. Bleeding releases air pockets that slow water movement and cause the heat exchanger temperature to spike. After bleeding, remember to re-pressurise the system to 1–1.5 bar and then reset the boiler. If F.24 clears and stays clear, airlocks were likely the issue. If the error returns, the problem lies elsewhere — most probably with the pump or heat exchanger — and you'll need a Gas Safe engineer.