User Guide

Plumbing Design Assistance Manual l Chapter 5 – System design and layout l 59
Thermal expansion
and contraction
All piping systems expand
and contract at different
rates depending on material
characteristics. PEX piping
systems exhibit a higher
expansion and contraction rate
when subjected to changes
in temperature as compared
to metallic piping systems.
Because of its lower modulus
of elasticity, PEX is less rigid
than metallic piping and
develops less force than
metallic pipe when exposed to
temperature changes.
For this reason, Uponor PEX
can be easily restrained by
installing PEX-a Pipe Support.
With proper strapping, PEX-a
Pipe Support can lessen the
need for numerous directional
changes and expansion loops
as the pipe will absorb much
of the expansion stresses.
Allowing for controlled
expansion and contraction
in multiple parts of a piping
system is an accepted means
of preventing added stresses
in other parts of the system
that could compromise system
performance, damage the
structural integrity of the
piping components, or
damage the structure that
support the piping.
Including design elements
to address changes in pipe
length will redistribute the
stresses that would otherwise
result in uncontrolled thermal
movement. According to
Plastics Pipe Institute (PPI)
TR-21 Thermal Expansion and
Contraction in Plastics Piping
Systems, the design and
installation of a plastic piping
system often requires special
attention to thermal expansion
and contraction.
Free-body coefcient
The free-body coefcient of
expansion is a calculated,
theoretical value in which all of
the surfaces would be free of
friction and contain no end or
branch connections. The free-
body coefcient of expansion
for PEX-a piping is 1.1"/10˚F
ΔT/100 ft. (27.94mm/5.56°C
ΔT/30.48m). For a given
change in temperature, the
theoretical expansion and
contraction can be easily
calculated for a given length
of piping. In practice, the free-
body condition for installed
piping is unlikely as all
surfaces would need to be free
of friction.
Installation of piping in the
eld requires supports,
strapping and bracing that
inherently apply friction to
the pipe surfaces, therefore,
restraining pipe movement.
In addition, it takes much
more time for PEX-a piping to
reach temperature equilibrium
due to its coefcient of
thermal conductivity and
wall thickness. Thermal
expansion in PEX-a piping
is more likely to occur in
the middle temperature
range than at the extremes
produced by operating water
temperatures. Therefore the
free-body coefcient is a
very conservative means of
calculating pipe movements in
installed conditions.
Test validation
Uponor has tested a variety
of suspended PEX piping
assemblies to determine
which type of expansion
solution works best for most
applications. The results
strongly support the use
of PEX-a Pipe Support
in conjunction with xed
anchor points positioned
every 65' for hot-water and
every 150' for cold-water
plumbing systems. Pipe
runs installed in this manner
experience 89 percent to 93
percent reduction in thermal
expansion. The two scenarios
outlined in Table 5-15 detail
the installation of these pipe
runs. The strut and strut
clamps experienced the most
reduction in expansion (93
percent) because of the snug
grip and increased friction
from pipe clamps yielding an
effective expansion rate of
0.08"/10˚F ΔT/100 ft; clevis or
loop type hangers offered an
89 percent reduction yielding
an effective expansion rate
of 0.12"/10˚F ΔT/100 ft. The
thermal expansion rate for
copper is 0.11"/10˚F ΔT/100 ft
so using PEX-a Pipe Support
with Uponor PEX makes the
piping system perform very
similarly to copper with respect
to thermal expansion.
The highest force measured
at xed anchor points during
these tests was 173 lbs.,
which is much less than the
guided force of metallic piping.
Properly braced xed anchor
points and commonly specied
drop-in anchors, beam
clamps or similar fastening
components for metallic piping
are of sufcient strength to
support the PEX install.
Given the exible nature of
PEX pipe, a slight snaking
pattern may be experienced
even when PEX-a Pipe
Support is utilized to control
expansion. This is considered
normal and does not affect
the integrity of system. Pipe
insulation can help to minimize
the aesthetics of a slight snake
in the pipe run.
Effects of thermal expansion with PEX-a Pipe Support
Scenario
Water
temperature
(°F)
Delta T (°F)
Support
spacing
Anchor
spacing
Linear
expansion
Expansion
rate (inches
per 100' per
10°F ΔT)
Reduction
vs. bare PEX
Strut and
strut clamps
180 105 96" 65' ½" 0.08 93%
Loops and clevis 175 100 96" 65' ¾" 0.12 89%
Table 5-15: Uponor PEX thermal expansion test results