ASHRAE Journal - July 2019 - 49

COLUMN ENGINEER'S NOTEBOOK

to the large reduction of ﬁeld joints required in a preinsulated steel piping system.

Transition to Building Service Piping
The pre-insulated PEX system can penetrate the building exterior wall or ﬂoor prior to connecting to other systems, such as mechanical room equipment or existing
metal piping inside the building. Plastic pipes expand
and contract dramatically with changes in temperature.
To avoid damage to the pre-insulated PEX system and
the connecting pipe components, the PEX system should
be properly anchored to the wall or ﬂoor.

Temperature Control Limit
Maximum water temperature control is critical when
using thermoplastic materials with heating hot water
systems. The pre-insulated PEX systems typically come
with a 25-year warranty on the PEX pipe as long as the
system operates within the temperature pressure rating
limits shown in Table 3. Hot water boilers have its operating control set for the desired hot water supply temperature. The limit control is set at a higher temperature
than the operating control and typically requires a manual reset. If a boiler reaches its limit control setpoint,
it usually means the operating control is not working.
Since boilers can operate above their operating setpoints
it is recommended to install controls to ensure the maximum allowable service temperature is not exceeded for
the pressure rating. This is typically not an issue if the
boiler limit control is set below the temperature pressure rating limit for the installed system.

Commissioning
PEX, like other piping materials, should be thoroughly
ﬂushed prior to testing to prevent damage to valves and
other ﬁttings from any material left in the pipe during construction. A minimum ﬂush velocity of 3 fps
(0.9 m/s) is recommended to move the material in the
pipe. PEX leak testing is usually done with water at a
pressure speciﬁed by the designer, typically 1.5 times
the normal operating pressure but not higher than the
temperature pressure rating limits shown in Table 3. It
is worth noting that this is a ﬁeld leak testing procedure
since PEX pipe is a lower modulus visco-elastic material
that dilates in diameter (creep-strains) when subjected
to higher stress during hydrotest. This means that for a
ﬁxed volume of clean ﬁll water, the hydrostatic pressure

will decline slightly during the ﬁrst 30 minutes, as the
PE molecular chains stretch and align under high stress.
During the test period, water is added to keep the pressure constant since the thermoplastic pipe will expand
with pressure. Normally, if the pressure remains within
5% of the target value for 90 minutes, the leakage test
passes.

Concluding Remarks
Based on decades of experience with direct-buried
heating hot water systems, the author has found that
piping materials that will not corrode and that prevent
joint leakage will typically provide reliable service for
close to 50 years. PEX has been slowly growing in popularity primarily due to high copper prices and increasing popularity of lower temperature underground
heating hot water systems. It is ﬂexible and therefore
requires fewer ﬁttings and allows for longer continuous
pipe runs. PEX has an overall good track record and is
cheaper, easier and faster to install than steel and copper. It also is much more resistant to mineral build-up
(scale formation) than steel and copper.
PEX is another pipe material the design engineer
should be familiar with when evaluating direct-buried
piping systems. An understanding of PEX material characteristics is an inherent part of the design process for
applying this material to underground piping systems.
With such an understanding, the piping designer can
use the properties of the material to design for optimum
performance for the intended service.

References

1. Peterson, K. 2017. "HDPE pipe for corrosion- and leak-free
operation." ASHRAE Journal 59(7).
2. Peterson, K. 2014. "Underground piping systems." ASHRAE
Journal 56(9).
3. NACE International. 2019. "Corrosion - A Natural But
Controllable Process" https://www.nace.org/resources/
general-resources/corrosion-basics.
4. ASTM Standard D2837(2013), Standard Test Method for Obtaining
Hydrostatic Design Basis for Thermoplastic Pipe Products.
5. PPI. 2018. "PEX Pipe Design Manual (HDB-Based) for Water,
Oil, Gas & Industrial Applications." Plastics Pipe Institute.
6. PPI. 2010. "TR3, Policies and Procedures for Developing
Hydrostatic Design Basis (HDB), Hydrostatic Design Stresses (HDS),
Pressure Design Basis (PDB), Strength Design Basis (SDB), and
Minimum Required Strength (MRS) Ratings for Thermoplastic
Piping Materials or Pipe." Plastics Pipe Institute.
7. PPI. 2017. "TN-52, Guide to High-Temperature Applications of
Non-Potable PEX Pipe and Tubing Systems." Plastics Pipe Institute.
8. ASTM Standard F876(2017), Standard Speciﬁcation for Crosslinked
Polyethylene (PEX) Tubing.
J U LY 2 0 19

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ASHRAE Journal - July 2019

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