Boiler
F13
The F13 error code on a Viessmann boiler indicates a fault with the domestic hot water (DHW) temperature sensor — specifically the flow temperature sensor on the secondary heat exchanger. It’s a moderately serious fault that will typically disable hot water and may affect heating until it’s resolved.
What does this error mean?
Your Viessmann boiler continuously monitors water temperature using sensors throughout the system. The F13 code means the boiler’s control board is receiving an abnormal signal from the DHW temperature sensor — either the reading is outside the expected range, or the sensor has failed entirely. The boiler locks out as a safety precaution to prevent overheating or unsafe operation.
Est. repair cost
£150–£300 for NTC sensor replacement (parts + labour); £300–£500+ 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 a line) for 3–5 seconds. If F13 clears and does not return within an hour of normal use, the fault may have been a temporary sensor glitch. Monitor closely.
Step 2 — Check the boiler manual: Confirm F13 is the DHW sensor fault on your specific Viessmann model (Vitodens 050-W, 100-W, 200-W, and similar combi boilers all carry this code, but always verify in your installation manual or the Viessmann app).
Step 3 — Inspect what you can safely see: With the boiler switched off and unplugged from the mains, visually check any visible wiring connections at the front of the boiler for obvious damage, burns, or disconnected plugs. Do not remove the boiler casing or touch any internal gas components.
Step 4 — Check your water pressure: Ensure the system pressure is between 1–1.5 bar on the pressure gauge. Low pressure can sometimes contribute to sensor misreadings. Top up via the filling loop if needed — this is safe for homeowners to do.
Step 5 — If the fault returns after reset, stop here and call a Gas Safe registered engineer. Replacing the NTC sensor or inspecting internal wiring requires the boiler casing to be removed, which must only be done by a qualified professional on a gas appliance.
📞 When to call a professional
If F13 returns after a reset, call a Gas Safe registered engineer — do not attempt to open the boiler casing or replace the sensor yourself, as this is illegal without Gas Safe registration and could invalidate your warranty. When you call, tell the engineer: ‘My Viessmann boiler is showing fault code F13 — I believe it’s the DHW temperature sensor.’ This helps them bring the right parts. If your boiler is also making unusual noises or you smell gas, call the National Gas Emergency line (0800 111 999) immediately.
⚖️ Repair or replace?
An NTC sensor replacement on a Viessmann boiler typically costs £150–£300 all-in, including parts and labour. If the PCB is at fault, expect £300–£500+. For boilers under 10 years old in otherwise good condition, repair is almost always worthwhile. If your boiler is over 12–15 years old and this isn’t the first fault, a new A-rated combi boiler (£1,800–£3,500 installed) is likely the smarter long-term investment.