ASHRAE Journal - July 2019 - 45

COLUMN ENGINEER'S NOTEBOOK
Kent W. Peterson

Polyethylene-Based Pipe (PEX)

Design Considerations
For Underground Hot
Water Piping
BY KENT W. PETERSON, P.E., PRESIDENTIAL MEMBER/FELLOW ASHRAE

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.1 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
ournal 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.
Pre-insulated carbon steel and copper piping are also
commonly used in direct-buried heating hot water
systems. The piping has unmatched temperature and
pressure ratings but is subject to both internal and
external corrosion. According to NACE International,3
corrosion of ferrous metals buried underground is a
naturally occurring process and is the leading cause of
underground piping system failures. Pre-insulated and
jacketed steel pipe is protected from soil corrosion by
providing a continuous physical and vapor barrier over
the insulation. Insulation joint installation errors or
Kent W. Peterson, P.E., is chief engineer/COO at P2S in Long Beach, Calif. He is former
chair of Standard 189.1.
J U LY 2 0 19

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ASHRAE Journal - July 2019

Table of Contents for the Digital Edition of ASHRAE Journal - July 2019

Contents
ASHRAE Journal - July 2019 - Intro
ASHRAE Journal - July 2019 - Cover1
ASHRAE Journal - July 2019 - Cover2
ASHRAE Journal - July 2019 - 1
ASHRAE Journal - July 2019 - Contents
ASHRAE Journal - July 2019 - 3
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ASHRAE Journal - July 2019 - Cover3
ASHRAE Journal - July 2019 - Cover4
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