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PE, ASHRAE Presidential Fellow, LEED AP BD+C, BEAP, MCIBSE
Design Considerations For Underground Hot Water Piping
ASHRAE Journal, July 2019
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It is important to understand available pipe materials when direct-burying pipe underground. Pre-insulated crosslinked polyethylene-based pipe (PEX) has gained acceptance as a cost-effective alternative to ferrous pipe in direct-bury applications for certain heating hot water systems. These systems can also be used on cooling systems but there are many other non-ferrous cost-effective solutions as discussed in a previous column. While steel pipe can deliver unmatched temperature and pressure service levels, properly designed PEX pipe can deliver corrosion-free and leak-free operation. There are some unique challenges requiring special design considerations due to PEX piping’s visco-elastic nature, very low material allowable stress, large coefficient of thermal expansion, and relatively thick pipe walls. This month I will review these considerations in order to design and specify underground hydronic heating hot water PEX piping systems.
Underground Piping Objectives
In the past, most direct buried heating hot water systems used either carbon steel, ductile iron (DI) or copper pipe. These systems and modes of failure were presented in a previous ASHRAE Journal column.2 Ductile iron is typically joined using gasketed push-on joints. These joints are restrained with either concrete thrust blocks or external bolted-on mechanical couplings. The author has found monthly and annual allowable leakage through push-on joints on large distribution systems can be substantial and will result in loss of treated water. Excessive makeup water can also lead to loss of adequate levels of corrosion inhibitor in the circulating water. One objective is to design and install distribution piping that will not leak.