The latest expansion of natural gas network in Greece has turned gas boilers into a popular new way for heating. There are two kinds of gas boilers, conventional and condensing. Condensing boilers retrieve a large portion of the heat of the exhaust gases through the use of a specially designed heat exchanger. Total coefficiency can surpass 1,10 while conventional boilers reach 0,90. Moreover, the automatically-controlled and variable-speed fan of the condensing boiler ensures perfect combustion and a high rate of efficiency throughout the power range of the boiler. Finally, the overall losses of the chimney and condensing boiler walls stand much lower than a conventional boiler, due to the lower temperature (45-50 °C) where the exhaust gases are compelled.