Most open fires lose much of the heat produced by allowing it to be removed up the chimney flue - a 20% to 25% efficiency would be the norm. By comparison, as the Joule Inset is a Convector Stove, the room heating efficiency would normally be in excess of 65%. Air, which is drawn around the sides and back of the appliance is heated and projected into the room through outlets at the top of the unit above the door.
The convection process is very much dependant on a good connection to the flue, and this is why it is recommended that the chimney is lined. A good and dedicated flue connection procedure ensures that the chimney draught will only draw on the appliance, a factor taken into account during the approval testing process.