The Active + range of stoves utilise a pressure transducer and venturi (located in the combustion air inlet pipe) to monitor and calculate the correct volume of combustion air to be drawn when in operation.
The venturi assembly looks like this:
Venturi Assembly (demounted from combustion air inlet pipe)
If the venturi is ever demounted, from the inlet pipe, (e.g. for inspection / cleaning / replacement etc.) it is important that it is relocated correctly to prevent false airflow readings or alarms being raised. Once reinserted, its correct positioning can be determined by looking down the air inlet pipe. If the assembly is out of position, it will appear as below:
Air Flow venturi incorrectly positioned
If it is correctly inserted, it will appear thus:
Venturi in correct position