These heaters function both by the heat produced by burning the gas and via the infrared radiation emitted by the ceramic surfaces of the burner.

The hot air generated by the burner rises to create a natural convection flow, which combines with the infrared radiant heating effect. The heat is both diffused into the room and radiated onto the relevant area in a few moments, with light also emitted. 

No flue is required for the combustion gases.