ASHRAE Journal - July 2019 - 48

COLUMN ENGINEER'S NOTEBOOK

fittings, which are not subject to corrosion.

PHOTO 1 Typical underground PEX connection vault. Photo courtesy of Uponor.

Selection and Design
PEX Design and Installation Considerations
PEX pipe diameter is typically specified by copper tubing size (CTS). This means PEX pipe will have a significantly smaller inside pipe diameter than nominal pipe
size (NPS) steel pipe; therefore, it is sometimes oversized
to achieve the desired inside pipe diameter. For example,
3 in. CTS (76 mm) SDR 9 PEX pipe has an inside diameter
of 2.398 in. (61 mm), whereas A53 carbon steel pipe would
have an inside diameter of 3.068 in. (78 mm). When sizing
PEX piping, both the much lower pipe roughness value
and the decreased inner diameter of nominal pipe sizes
should be considered. In some cases, depending on butterfly valve dimensions used in the system, flange adaptors with tapered ID spacers may be required for the
valve to open and close freely. In most cases, ball valves
can be used for isolation due to the smaller pipe sizes.
PEX has a unique benefit of being shipped pre-insulated in lengths of 300 to 600 ft (91 to 182 m) coils to
minimize field joints in trenches. In the author's experience, underground field joints on pre-insulated steel
piping tend to be the single greatest risk of corrosion
failure. These field joints and fittings are required every
20 ft (6 m) in addition to directional and tee fittings. In
many cases, the long lengths and flexibility of PEX tubing eliminates the need for underground field joints
between buildings and the flexibility allows the pipe to
bend where normally 45- or 90-degree fittings would be
used with rigid piping. The flexibility in the PEX system
does not require trenches to be dug straight and level
and also makes avoiding underground obstacles easier
since the pipe can be installed to snake around the
obstacles. If an underground field joint is required for
tees, they should be provided in an accessible connection vault similar to Photo 1 and not direct buried.
When determining the minimum depth to bury the
pipe, the compressive wall stress on the pipe from
exterior forces should be kept less than the allowable
compressive stress of the PEX material. Generally,
the American Association of State Highway and
Transportation Officials (AASHTO) H-20 truck loading is
met using SDR 9 pre-insulated pipe with 2 ft (0.6 m) of
cover. Burying the pipe below the frost line can prevent
heaving and improve thermal performance. As with
most thermoplastic piping systems, do not install the
48

ASHRAE JOURNAL

ashrae.org

J U LY 2 0 19

pre-insulated PEX system in soil or groundwater conditions, which are thought or known to be contaminated
with fuels, organic compound, or solvents, as these
substances could permeate the pipe and contaminate
the water or damage the integrity of the piping system.
A minimum of 4 in. (100 mm) of sand should surround
the pre-insulated PEX system in the trench.
Expansion loops that allow linear expansion and contraction for rigid piping are generally not required when
using pre-insulated PEX piping. The unique nature of
the service pipe encasement and the expansion absorption built into the pre-insulated PEX systems virtually
eliminates the effects of linear expansion as long as the
points of connections to buildings are anchored.
Once buried and backfilled, PEX piping cannot be
located via metal detectors. Therefore, it is suggested
PEX piping, as with all non-ferrous piping, should be
buried with a 12 gauge (2.05 mm) tracer wire attached
thereto, all the way back to an accessible manhole or
indoor space at each end. This will allow a small electrical current to be used to assist a locator in finding
the pipe route at some date in the future. A cautionary
"underground buried piping" vinyl warning tape should
also be located in the backfill above the pipe to warn
future unsuspecting excavators.
Installed pre-insulated PEX piping systems are less
expensive than installed pre-insulated A53 standard
steel piping systems. The author has seen 20 to 35% construction cost savings when comparing pre-insulated
PEX to pre-insulated steel piping systems for connections between buildings or heating distribution building branches. Most of these cost savings are a result of
substantially less labor and shorter installation time due

https://www.ashrae.org/

ASHRAE Journal - July 2019

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