Viessmann boilers
Viessmann is a German heating company. It was founded in 1917. Today it’s part of Carrier, an American firm, and it trades as Viessmann Climate Solutions. In the UK, its gas boilers all carry the Vitodens name.
The Vitodens range
Each Vitodens boiler has a number and a letter after the name, such as 100-W or 222-F. Viessmann’s UK website lists these gas boilers:
- Vitodens classic, a compact combi from 24 to 28 kW;
- Vitodens 025-W, 050-W, 100-W and 111-W, all combis;
- Vitodens 200-W, which also comes in large sizes for commercial use;
- Vitodens 222-F, another combi;
- Vitodens 100-W Heat Only, for homes with a hot water cylinder.
The full model name is on the front of the boiler. The kW size is on the data label.
How Viessmann boilers show a fault
A Viessmann boiler shows a short fault code on its screen, such as 10, 18 or A4. Viessmann has a fault code page on its UK website. Most of the codes on it are for engineers. Viessmann also says a code can mean different things on different boilers and controls. So the same code on a 100-W and a 200-W may not mean the same thing.
Some codes can be cleared with a simple reset. Others need an engineer. If you’re not sure, Viessmann says to contact a Viessmann trained installer.
Two codes come up a lot on boilers with an outdoor sensor. That’s a small box on an outside wall. It tells the boiler how cold it is, so the boiler can adjust the heat. Codes 10 and 18 mean the sensor or its cable has failed. The boiler then runs as if it’s 0°C outside. Your heating still works, but it may run hotter than it needs to.
Pressure and frozen pipes
Viessmann says the pressure should be around 1.25 bar. The gauge is on the front of the boiler. If it’s too low, top it up with the filling loop. If it’s too high, bleed a little water out. See how to top up low boiler pressure.
Viessmann also says the condensate pipe is the part most likely to freeze, where it leaves the house. You can thaw it slowly with warm water. Our guide shows how to thaw a frozen condensate pipe.
No hot water?
Viessmann suggests some checks before you book a repair:
- Is there a fault code on the screen?
- Has a fuse tripped? If so, reset the boiler’s timer afterwards.
- Is the pressure right?
- Are there signs of a leak, such as damp patches or water on the pipes?
If none of these helps, call a Gas Safe engineer.
Viessmann gas boiler ranges
These are the gas boiler ranges on the Viessmann website as of 21/10/2026.
- Vitodens classic Combi
- Vitodens 025-W Combi
- Vitodens 050-W Combi
- Vitodens 100-W Combi
- Vitodens 111-W Combi
- Vitodens 200-W
- Vitodens 222-F Combi
- Vitodens 100-W Heat Only
Common Viessmann fault codes
Codes can differ between models and ages of boiler. Always check the user manual for your exact model.
| Code | What it means | What you can do |
|---|---|---|
10 | The outdoor temperature sensor has a short circuit. The boiler runs as if it's 0°C outside. | Your heating should still work. Book a Gas Safe engineer to replace the sensor. |
18 | The outdoor temperature sensor cable is broken. The boiler runs as if it's 0°C outside. | Your heating should still work. Book a Gas Safe engineer to check the sensor and its cable. |
A4 | On the Vitodens 222-F, there's air in the pump or not enough water flow. The boiler starts a venting cycle by itself. | Check the pressure gauge and top up if it's low. If the code keeps coming back, call an engineer. |
Viessmann manuals
Viessmann puts its user guides and installation manuals on its own website. You can find Viessmann manuals here. You'll need your model name. The GC number helps too. Our guide shows you how to find your boiler's model and GC number.
Stay safe. You can reset a boiler, top up its pressure and thaw a frozen condensate pipe yourself. Anything inside the case is a job for a Gas Safe registered engineer.
If you smell gas, leave the building and call 0800 111 999.