Hello, I am new to this forum and I am a bit unsure of protocol but I have a question about an Esse sovereign (cooker and Central Heating). Its an Oil burner, as basic as it gets, its been serviced repeatly (I think the engineer is confused as us!), chimney drawing perfectly, oil flow at the correct level, air flow good, but continues to burn with a yellow flame...any suggestions? as its only achieving a luke warm heat on any setting and its a bit chilly here! Help most appreciated!!