In this paper is presented the analyzer`s results of the work of the boilers with the burning gases steam condensation. At the gases cooling till 30...40 ° C, the condensate mass is 1.1 - 1.3 kg/m3 natural gas which increases the boiler`s efficiency with 12 - 14%. At the installation of the boilers inside of the heated buildings the efficiency with reference to the fuel inferior heating value constitutes 106 - 108%, with the superior heating value - about 12% less.