Smith Cast Iron Boilers (booth 4332) offers the 28HE Series high-efficiency boilers with the highest combustion and thermal efficiencies possible with forced draft firing. The 28HE Series offers thermal and combustion efficiencies of up to 86% for oil and 83% for gas. Series 28HE boilers are available in 15 basic sizes, with gross output ratings from 931 to 4,622 mbh and can be used in either water or steam systems with light oil, gas or gas/light oil. Smith Cast Iron Boilers.