When called to clean the chimney and stove at my clients house in Mullingar, i once again came across my pet hate, rear flue exits, dangerously installed. My client took my advice to have it installed with flue off the top. This was a narrow chimney  breast, housing 2 flueways, one servicing the stove and the other, servicing the oil burner. When we opened up blockwork to link the rigid flue to the flexi flue we discovered both flueways side by side, not back to back, with both flues inside a large cavity. The gases, smoke, toxins, poisons, whatever you like to describe them as, all came into this cavity and could exit via whichever clay liner flueway it liked, or worse still, could exit into the other room. Built 38 years ago one wonders what lurking behind blocks and plaster.