Viessmann [A9] fault codewhat it means and how to fix it

A9 on a Vitodens indicates the condensate drain is blocked or cannot flow away. A condensing boiler produces a genuinely surprising volume of water — several litres a day in cold weather — and it is mildly acidic, typically around pH 3 to 4. It has to go somewhere. When the drain blocks, condensate backs up inside the appliance, and Viessmann shut the boiler down before it reaches anything it should not. Viessmann literature also associates A9 with a group of codes where the appliance cools down because sensor inputs are not being read correctly, so on some models it can present alongside sensor-related behaviour. The most common cause by far, and the one you can address yourself, is the external condensate pipe freezing. Beyond that the internal trap can silt up over years of service, and that one does need an engineer since it sits inside the case.

1. 1

   Go outside and find the condensate pipe — plastic, usually white or grey, running from the appliance towards a drain. Follow its whole length rather than just the section near the boiler.

2. 2

   Feel along it for a rigid, ice-cold section. Bends, the lowest point of a horizontal run, and the open end are where ice forms first.

3. 3

   Pour hand-hot water slowly along the frozen section, working from the open end back towards the appliance so the meltwater has somewhere to escape.

   Never boiling water and never a heat gun or naked flame. Cold plastic cracks readily and a split condensate pipe is a far worse position to be in.
4. 4

   Check where the pipe discharges. If it goes into an outside gully, clear any leaves or debris blocking it.

5. 5

   Press the reset button and call for heat. If it runs, lag the external pipe before the next cold spell — and if it freezes every year, ask about upsizing or rerouting it rather than accepting it as normal.