Boiler
F13
The F13 error code on an Alpha boiler indicates a flow temperature sensor fault — specifically that the boiler is detecting an abnormal or out-of-range reading from the NTC flow thermistor. It’s a moderately serious fault that will lock out your boiler until it’s resolved.
What does this error mean?
Your Alpha boiler constantly monitors the temperature of water leaving the heat exchanger via a flow NTC (Negative Temperature Coefficient) sensor. When the F13 code appears, the boiler’s PCB has detected that the sensor is sending a signal outside the expected resistance range — either because the sensor itself has failed, the wiring is damaged, or there’s a genuine overheating issue triggering an abnormal reading. The boiler shuts down as a safety precaution to prevent heat exchanger damage.
Est. repair cost
£80–£150 for NTC sensor replacement (parts + labour); £200–£400 if the PCB requires replacement
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button for 3–5 seconds (refer to your Alpha boiler manual for the exact button location on your model). Wait 2 minutes and check if the boiler fires up normally. If the F13 code returns immediately, do not continue resetting — further resets will not fix an underlying fault.
Step 2 — Check for a power issue: Turn the boiler off at the mains, wait 30 seconds, then switch it back on. A one-off voltage spike can occasionally trigger sensor error codes. If the fault clears and stays clear, monitor it for 24 hours.
Step 3 — Inspect the boiler casing for obvious signs: With the boiler switched OFF at the mains, check around the boiler for any visible water leaks near the sensor wiring area. Water ingress onto electrical connections can cause erratic sensor readings. Do not open the boiler casing yourself.
Step 4 — Call a Gas Safe registered engineer: If the code persists after a reset, this fault requires a qualified engineer. Do not attempt to open the boiler casing, touch the NTC sensor, or handle any internal wiring — this is a gas appliance and internal work is illegal for unqualified individuals under UK Gas Safety Regulations.
Step 5 — Tell the engineer it’s an F13 fault: An engineer will use a multimeter to test the NTC thermistor resistance (typically 10kΩ at 25°C for Alpha sensors), inspect the wiring loom, and test the PCB signal input. This allows them to come prepared with the likely replacement part.
📞 When to call a professional
Call a Gas Safe registered engineer as soon as a reset fails to clear the F13 code. Do not repeatedly reset the boiler — if there is a genuine overheating issue, continued attempts could cause heat exchanger damage. Tell the engineer: ‘My Alpha boiler is showing error code F13 — I believe it’s a flow NTC sensor fault.’ This will help them arrive with the correct part and resolve the issue in a single visit.
⚖️ Repair or replace?
An NTC thermistor replacement on an Alpha boiler is relatively inexpensive — the part itself costs £10–£30, and a Gas Safe engineer should complete the job in under an hour. If the fault lies with the PCB, costs rise to £200–£400 including labour. For boilers over 10–12 years old, weigh repair costs against a new boiler (£1,500–£2,500 installed); if repairs exceed 30% of replacement cost, replacing is the smarter long-term investment.