Does anyone know about a boiler flue going through a chimney void? Is it illegal? We have a combi boiler mounted on the gable wall with the flue going straight out through the wall. This was done about 4 years ago. We are trying to rent out the flat and have had a gas safety inspection done which has failed due to the overflow pipe being to long and not turned inwards to the wall. Is this correct? And he thinks the flue from the boiler is going straight through the chimney void to the outside wall. Any advice would be much appreciated!