Our cast iron built-in fireplaces include two components of heating, namely convection and radiation. The radiation heating comes through the glass in the form of infrared waves, but for the convectional heating to work correctly and not only heat the space directly around the combustion chamber, but the entire room,, there is a component of the installation that needs to be done properly.
All the fireplace units have heat transfer fins on the outsides of their bodies. These are used to maximise surface area and efficiently transmit the heat from the fireplace enclosure to the surrounding air. In order for that to happen effectively, the built-in fireplace needs to have a minimum of 5-10 cm empty space around it on all sides, instead of the area around the fireplace being built up around the unit. There needs to be an air inlet opening underneath the fireplace where cool air can be drawn in. The cool air is heated and it rises due to convection principles. There also needs to be a top air vent where the heated convection air can be released to the room. Essentially under the surround a cavity is formed where convection takes place. It is extremely important for any gaps upwards towards the chimney to be blocked off to prevent the heated air (via convection) to go up the chimney as it does with conventional open fireplace.
In summary, with a correctly built surround, air is drawn in from the wood storage underneath the fireplace (or from bottom vent depending on how build is done) – then gets heated up around the sides of the fireplace and gets released from the top 1 or more vents. This ensures continuous circulation in the room and a much better heat distribution than radiant heating only.