The research aims to design and construct a new mixed vertical boiler (fire tube – water tube) with three gas passes. The strength of this technological innovation is in the best use of the thermic transmission receiving fluid (hot water, steam, thermal oil), this due to its multipurpose function of three steps using alternative fuels (Diesel, Liquid Petroleum Gas LPG, natural gas), by improving the thermal efficiency of the boiler its temperature is reduced with gases at low temperatures, which in turn also reduce environmental pollution. The methodology focuses on calculating the transfer area with the calculation method that will allow dimensioning the boiler, considering the calculation of losses and the fluid speed, with two defined procedures, the first for fire tube and water tube boilers. And another alternative. The results obtained allowed optimizing the thermal efficiency level, achieving very significant thermal efficiency results: With LPG 92.4% for hot water and 92.42% to generate steam in the same way with natural gas 90.25% for hot water and 90.24% to generate steam as well with Diesel 2; 89.21% for hot water and 89.31% to generate steam.