The flue pipe on a gas geyser works in the same way that a chimney pipe does on a fireplace. The flue pipe will draw the CO2 gasses produced by the gas burners out of the geyser, along with excess heat. Without a flue pipe, the CO2 gasses settle over the gas burners & literally smother the flames.
All the geysers that are B Type or the RS20DH must have a flue and where you are doing an outdoor installation with a flat roof weather box then a flue must be installed. In high wind areas a weather box or will help prevent the flames from being blown out.
