What are the features of fluid devices? Why must they be preferred?

Fluid devices are systems, in which required combustion air is taken from the environment and waste gases are thrown out through a proper flue in central heating boilers, geysers or natural gas stoves. Constant fresh combustion air is required in the environment of fluid devices. Therefore you must have ventilation culverts open to atmosphere and keep these culverts open all the time. The flue, which the waste gas is connected to, must have proper sizes and good draught conditions.