Boiler
F15
The F15 error code on an Alpha boiler indicates a fault with the domestic hot water (DHW) temperature sensor — it’s a fairly common fault that will prevent your boiler from heating your hot water properly and requires prompt attention from a Gas Safe registered engineer.
What does this error mean?
The F15 code means your Alpha boiler’s control board has detected an abnormal reading from the domestic hot water NTC (temperature) sensor. This sensor monitors the temperature of water flowing to your taps, and if the signal falls outside expected parameters — due to a faulty, damaged, or disconnected sensor — the boiler locks out as a safety precaution. You may still have central heating, but hot water from taps and showers is likely to be absent or inconsistent.
Est. repair cost
£80–£180 for a Gas Safe engineer call-out, diagnosis, and DHW NTC sensor replacement (parts typically £15–£40; labour £60–£140 depending on region and engineer)
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame or ‘R’) for 3–5 seconds. If the F15 clears and the boiler runs normally, monitor it closely for 24 hours. A one-off fault can sometimes self-resolve.
Step 2 — Check the boiler manual: Confirm F15 refers to the DHW sensor on your specific Alpha model (e.g. Alpha E-Tec, CD, or InTec range), as codes can vary slightly between models.
Step 3 — Do NOT attempt any further DIY: All further diagnosis and repair involves accessing internal gas boiler components, including electrical wiring connected to a live appliance. This must only be carried out by a Gas Safe registered engineer. Do not open the boiler casing.
Step 4 — Book a Gas Safe engineer: Contact a Gas Safe registered heating engineer and tell them your boiler model, that you have a persistent F15 fault code, and that you suspect a DHW NTC sensor issue. This helps them bring the right parts.
📞 When to call a professional
If the F15 code returns after a reset — or does not clear at all — you must call a Gas Safe registered engineer; do not attempt to investigate further yourself. Tell the engineer your exact Alpha boiler model and that the fault code is F15, indicating a DHW temperature sensor fault. If your boiler is showing additional symptoms such as strange noises, a smell of gas, or visible water leaks, treat this as urgent and call immediately.
⚖️ Repair or replace?
Replacing a DHW NTC sensor on an Alpha boiler is a straightforward job for an engineer and is typically cost-effective. If your boiler is under 8 years old, repair is almost always the right choice. For boilers over 10–12 years old — particularly if you’ve had multiple faults — it’s worth getting a replacement quote alongside the repair cost, as a new boiler may offer better long-term value and energy efficiency.