F5 error on a Viessmann boiler: what it means and what to do
The F5 error on a Viessmann boiler indicates a fault in the flue gas exhaust system. Specifically, it means that the air pressure switch does not open when the burner starts or does not close when the fan reaches operating speed. The main causes are a faulty fan, a defective pressure switch, or a blocked flue. The boiler goes into safety lockout and produces neither hot water nor heating.
Condensing boiler
Viessmann wall-mounted condensing boiler series Vitodens 050-W
Price range: €1.821,46 through €2.174,04 IVA inclusaCondensing boiler
Viessmann wall-mounted condensing boiler series Vitodens 100-W
Price range: €2.551,02 through €3.192,74 IVA inclusa
Viessmann wall-mounted condensing boiler series Vitodens 100-E
€2.071,56 IVA inclusaMain causes of the F5 lockout
When the F5 code appears on your Viessmann boiler display, the electronic control board has detected that combustion fumes are not being expelled correctly. Here are the three most common culprits:
- Faulty or blocked fan
The fan’s job is to push the exhaust gases out through the flue. If it is dirty, mechanically blocked, or if the motor is burnt out, the pressure switch does not detect airflow and triggers the F5 lockout.
- Symptom: the boiler attempts to start, you hear a humming noise, but the fan does not spin.
- Faulty air pressure switch
The pressure switch is the sensor that “detects” whether the fan is working. Sometimes the fan runs perfectly, but the pressure switch is stuck (contacts stuck closed) or broken, sending a false signal to the control board.
- Symptom: the fan spins at full speed, but after a few seconds the F5 error appears.
- Blocked flue or air intake pipe
If the exhaust flue pipe (or the combustion air intake pipe) is blocked by bird nests, ice, or debris, air cannot circulate.
- Symptom: the error often occurs on very windy or very cold days.
What to do when the F5 error appears: step-by-step guide
Before calling for service, you can safely carry out a few visual checks.
WARNING! Never attempt to open the sealed combustion chamber or dismantle gas components unless you are a certified technician.
- Reset the boiler: press the “Reset” button (often marked with an “R”). If it was a temporary error caused by a gust of wind blowing back into the flue, the boiler will restart. If the error immediately returns, the problem is hardware-related.
- Check the exhaust terminal: go outside and inspect the flue terminal. Make sure it is not covered by leaves, nests, or ice.
- Listen to the noise: when you reset the boiler and request hot water, do you hear the fan running?
- If it does NOT run: likely a fan or control board fault.
- If it runs but then locks out: likely a faulty pressure switch or dirty venturi tubes.
When should you call a technician?
If resetting does not solve the issue, intervention by a qualified technician is required. The F5 error almost always involves replacing a component (fan or pressure switch) or performing a deep cleaning of the heat exchanger and venturi system—operations that require proper certification.
FAQ: Frequently Asked Questions
No. The boiler enters a total safety lockout to prevent exhaust fumes (potentially dangerous) from re-entering the room or the boiler from overheating.
Generally no. Low water pressure is usually indicated by errors such as F4 or specific symbols on the pressure gauge. The F5 error is specifically related to air and flue gases. However, always check that the pressure gauge reads between 1.0 and 1.5 bar to rule out secondary issues.
It indicates a fault in the flue gas exhaust system. The air pressure switch does not detect correct fan operation, so the boiler locks out for safety because it cannot expel combustion gases outside.
Press and hold the physical “R” or RESET button for about 3 seconds. On touchscreen models, tap the reset warning displayed on the screen. If the error reappears immediately, do not keep trying—call a technician.
