Non-condensing boilers are conventional water heaters that are fueled by natural gases. They operate without condensing the fuel gases inside the pressure vessel. Non-condensing boilers cost less than condensing boilers, do not have a limited heating capacity and are mostly used for commercial purposes.
Non-condensing boilers are not as safe as condensing boilers. They get the air needed for combustion from indoors, are not sealed and have a greater risk of debris getting sucked into the boiler. Non-condensing boilers have higher fuel and running costs. They use expensive venting materials, burn more energy, have higher carbon dioxide emissions and do not operate as efficiently as condensing boilers.