Boiler
F15
The F15 error code on a Ferroli boiler indicates a fault with the domestic hot water (DHW) flow sensor or NTC temperature sensor circuit — it’s a lockout fault that stops your boiler firing until the issue is resolved.
What does this error mean?
The F15 code means your Ferroli boiler has detected an anomaly in the domestic hot water NTC (temperature) sensor — either an open circuit, short circuit, or a reading outside the expected range. The boiler’s control board cannot trust the temperature data it’s receiving, so it shuts down as a safety precaution. This affects hot water supply and, depending on your boiler model, may also disable central heating.
Est. repair cost
£80–£180 for NTC sensor replacement; £200–£400+ if PCB replacement is needed
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame and cross symbol) for 3–5 seconds. If F15 clears and the boiler fires normally, monitor it for 24 hours to see if the fault returns.
Step 2 — Check your boiler pressure: Low system pressure (below 1 bar on the gauge) can cause sensor misreadings. If pressure is below 1 bar, top up via the filling loop until it reads 1.2–1.5 bar, then attempt another reset.
Step 3 — Check for obvious wiring issues (visual only): If you can safely access the boiler casing without removing any panels, visually inspect whether any visible wiring near the sensor looks burnt, pinched, or disconnected. Do not touch or disconnect any components.
Step 4 — Do not attempt further DIY: The NTC sensor, wiring harness, and PCB are internal gas appliance components. Replacing or testing these requires a Gas Safe registered engineer. Stop here and call a professional.
Step 5 — Turn off the boiler if it keeps locking out: If the fault returns repeatedly after resetting, switch the boiler off at the programmer or isolator switch to avoid further stress on the system while you wait for an engineer.
📞 When to call a professional
Call a Gas Safe registered engineer if the F15 fault returns after one or two resets, or if you cannot clear it at all. Tell them: ‘My Ferroli boiler is displaying error code F15 — I believe it’s related to the DHW NTC temperature sensor.’ This helps them arrive with the correct part. Do not attempt to replace the sensor or inspect internal wiring yourself — this is illegal without Gas Safe registration and potentially dangerous.
⚖️ Repair or replace?
Replacing a faulty NTC sensor on a Ferroli boiler typically costs £80–£180 all-in with a Gas Safe engineer, including parts and labour. If the PCB is at fault, expect £200–£400+. If your Ferroli boiler is over 10 years old and requiring repeated repairs, a new A-rated combi boiler (£1,800–£2,800 installed) is likely more cost-effective than continued repairs on ageing parts.