Vaillant F.15 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.15 fault code mean?
The F.15 code appears on Vaillant boilers when the appliance cannot confirm that water is circulating properly around the system. This typically points to one of three root issues: the system has insufficient water pressure, the circulation pump is not functioning correctly, or the electrical connection to the pump is faulty. When the boiler detects this condition it locks out as a safety measure to prevent overheating or internal damage. F.15 sits within the F.10–F.15 group of pump and sensor-related lockout codes on Vaillant appliances, all of which indicate the boiler cannot safely continue operating without engineer investigation.
General guidance only — not a substitute for professional advice. 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
- Low system pressure Common
Vaillant boilers require system pressure of roughly 1.0–1.5 bar when cold to circulate water effectively. If the pressure has dropped below this range — due to a small leak, a recently bled radiator, or natural loss over time — the boiler will shut down rather than risk running dry. This is one of the most frequent triggers for F.15 and is worth checking first.
- Seized or faulty circulation pump Common
If the boiler has sat unused through the warmer months, the pump impeller can seize up due to limescale or corrosion. Even if the motor still draws power, a seized or sluggish pump cannot circulate water adequately, causing the boiler to lock out. A pump can also simply fail electrically or mechanically after years of use.
- Air trapped in the system or pump Common
An airlock in the pump or pipework prevents water from flowing freely. This can happen after radiators have been bled, after a drain-down, or when pressure has been very low and air has entered the system. The boiler detects poor flow and triggers F.15 as a protective response.
- Faulty pump wiring or connector Sometimes
The cable harness or plug connecting the pump to the PCB can become corroded, loose, or damaged over time. Even with a perfectly functional pump, a broken or intermittent electrical connection will prevent the boiler from confirming pump operation, resulting in an F.15 lockout.
- Sludge or blockage restricting circulation Sometimes
A build-up of magnetite sludge or debris in the pipework and heat exchanger can restrict flow to the point where the boiler detects an anomaly. This is more common in older systems that have never been power-flushed or treated with inhibitor, and often causes repeated lockouts rather than a one-off event.
- Faulty NTC flow thermistor Rare
The NTC (negative temperature coefficient) thermistor monitors water temperature in the flow circuit. If it has come loose from its pocket, developed a fault, or the wiring harness has an interruption, the boiler may misread conditions and generate an F.15 code. The thermistor is a relatively low-cost part but requires engineer diagnosis to confirm.
How to fix it
- Check the system pressure gauge on the boiler DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 0.8 bar, low pressure is the likely culprit and you can top it up yourself using the filling loop (see next step).
- Top up system pressure using the filling loop DIY safe
Locate the filling loop — usually a flexible braided hose with one or two valves beneath or near the boiler. Slowly open the valve(s) and watch the pressure gauge rise. Stop when the needle reaches approximately 1.2–1.3 bar, then close the valve(s) fully. Never overfill beyond 1.5 bar. If you are unsure how to do this safely, call an engineer instead.
- Bleed your radiators to release trapped air DIY safe
If you suspect an airlock, use a radiator bleed key to release air from each radiator starting at the ground floor. Hold a cloth beneath the bleed valve, open it slowly until water dribbles out steadily, then close it. After bleeding all radiators, recheck the boiler pressure and top up again if it has dropped.
- Reset the boiler once DIY safe
Once pressure is correct and radiators have been bled, press and hold the reset button (or turn the dial to the reset position, depending on your model) for around three seconds, then release. Wait two to three minutes for the boiler to attempt a restart. If F.15 reappears immediately or after a short time, do not reset again — further resets will not resolve a genuine mechanical fault and can cause additional wear.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in the home — such as a gas hob or fire — are working normally. If nothing gas-powered is working, contact your gas supplier or call the National Gas Emergency Service on 0800 111 999. Do not attempt to investigate the gas supply to the boiler itself.
- Have a Gas Safe engineer test and inspect the pump Gas Safe engineer
A registered engineer will check whether the pump is running, measure its speed and current draw, listen for mechanical resistance or unusual noise, and inspect the wiring loom and connector for damage or corrosion. If the pump has seized, they may be able to free it; if it has failed, they will replace it with a compatible unit.
- Have the engineer inspect for sludge and consider a power flush Gas Safe engineer
If the engineer identifies restricted flow caused by magnetite or debris, they will advise on flushing the system. A full power flush uses specialist equipment to clean the entire circuit and is usually combined with the addition of a corrosion inhibitor to prevent recurrence. This is not a DIY task.
- Have the engineer check the NTC thermistor and wiring harness Gas Safe engineer
The engineer will test the resistance of the flow NTC sensor against the manufacturer's specification and inspect the cable harness for breaks or poor connections. A faulty thermistor is inexpensive to replace; a damaged harness requires careful rewiring. Either way, this must be carried out by a competent engineer with access to Vaillant wiring diagrams.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If the pressure was already correct and the boiler continues to show F.15 after a single reset, the fault is mechanical or electrical and beyond safe DIY intervention. Contact a Gas Safe registered engineer — you can verify registrations at gassaferegister.co.uk. Under the Gas Safety (Installation and Use) Regulations 1998, work on gas appliances must be carried out by a registered professional.
Parts you may need
- Circulation pump (Grundfos or Wilo, Vaillant-compatible) · from £85
- NTC flow thermistor · from £20
- Pump wiring harness / connector · from £25
- Magnetic system filter (e.g. Adey MagnaClean) · from £55
- Central heating inhibitor (e.g. Fernox F1, 1 litre) · from £14
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 £120–£400, depending on the underlying cause.
Frequently asked questions
Can I fix a Vaillant F.15 fault myself?
The only things a homeowner can safely do are check and top up the system pressure using the filling loop, bleed radiators to release trapped air, and attempt a single boiler reset. If any of these steps resolves the fault, great — but if F.15 returns, you will need a Gas Safe registered engineer. Anything involving the pump itself, its wiring, the NTC sensor, or internal components must be handled by a professional.
How much does it cost to repair a Vaillant F.15 fault?
Most people with this fault pay somewhere between £120 and £400 all in. A straightforward fix such as clearing an airlock or re-pressurising the system tends to cost £100–£150 including the call-out. A pump replacement — the most common repair — typically runs £250–£400 depending on the pump model and your location. NTC thermistor replacement is usually at the lower end since the part itself costs under £30. In rare cases where the PCB is implicated, costs can rise considerably higher, but this is not the typical outcome for F.15.
Why does my Vaillant boiler keep showing F.15 after I reset it?
A fault code that keeps returning after a reset almost always means there is an underlying mechanical or electrical issue that the reset is not fixing. For F.15 this usually means the pump is genuinely failing, seized, or has a wiring problem, or that the system has a persistent airlock or pressure loss caused by a small leak. Repeated resets without resolving the root cause can stress components. Book an engineer rather than continuing to reset the boiler.
Could F.15 be caused by sludge in my heating system?
Yes, though it is not the most common cause. If your heating system has never been flushed or treated with inhibitor, a build-up of magnetite sludge can restrict water flow enough to trigger pump-related lockouts including F.15. Signs that sludge may be involved include some radiators being cold at the bottom, the pump making grinding noises, or the same fault appearing repeatedly even after the pump has been checked. An engineer can assess flow rates and advise whether a power flush is worthwhile — typically costing £300–£600 for the whole system.