in the shape of a drain trap as a second safety member which is series connected with said condensate drainage device 6 fig5 is a side view of the condensate drainage device 6 together with the hydraulic connections 7 and 8 the device 6 consists indeed of a plastic material body made up of an upper portion 9 comprising the portion for connection with the inlet pipe of the burnt gas condensate mixture as well as of a lower portion 10 comprising the connection with the drain trap shaped pipe 8 a cylindrical float 4 provided with a cover 12 is arranged inside the outer body of the device 6 see fig6 said float being guided along its vertical stroke and being illustrated respectively in the open and closed positions in fig6 a housing 13 for a gasket 14 that closes the orifice in the lower portion of the body of the condensate drainage device is provided in the lower portion of the body 11 of the float itself when the boiler 1 is started there is no condensate so that the drain trap pipe 8 which is series connected to the condensate drainage device 6 is empty because of the pressure existing in the combustion chamber gases might flow through the drain trap itself till reaching the outside environment this is prevented from occuring as a result of the fact that the float mentioned.