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Northward Market Extension 

For Passive Solar Water 

Heaters by Using Pipe 

Freeze Protection with 

Freeze-Tolerant Piping 

Preprint 

J. Burch 

National Renewable Energy Laboratory 

M. Heater, M. Brandemuhl, and M. Krarti 

University of Colorado

To be presented at Solar 2006 

Denver, Colorado 

July 8–13, 2006 

Conference Paper 

NREL/CP-550-39664 

May 2006 
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NORTHWARD MARKET EXTENSION FOR PASSIVE SOLAR WATER HEATERS BY 

USING PIPE FREEZE PROTECTION WITH FREEZE-TOLERANT PIPING 

Jay Burch 

National Renewable Energy Laboratory 

1617 Cole Blvd.; Golden, CO 80401-3393 

E-mail: jay_burch@nrel.gov 

 

Morgan Heater, Mike Brandemuhl, and Moncef Krarti 

University of Colorado 

Architectural, Civil, and Environmental Engineering 

Boulder, CO 80302 

ABSTRACT

Combining a freeze protection method with fail-safe, freezetolerant cross-linked polyethylene (PEX) piping may 

provide a means to significantly extend the market 

northward for passive solar water heaters. Stress-strain data 

on PEX piping materials indicate that the 3.5% hoop strain 

from uniform freezing is likely tolerable for many cycles, 

but data are not definitive because it is not clear where 

permanent deformation starts occurring. Four PEX piping 

systems were freeze-thaw-cycled ~450 times. One brand 

showed no freeze tolerance, whereas two other brands 

appear freeze-tolerant, with no bursts in lengths greater than 

7” or less than 2”. Two geometries were identified that 

promote nonuniform freezing and that should be avoided: 1) 

nonuniform insulation increasing from ends toward the 

middle of the pipe in longer sections; and 2) pipes of ~4” in 

length with metal connectors on both ends. All metallic 

fittings have survived, but several polymer fittings broke 

and should be avoided when freeze-tolerance is desired. 

Further testing and analysis is needed to better understand 

the length dependence of freeze bursting and effects of 

piping aging, and field testing should proceed cautiously. 

1. INTRODUCTION

Reducing costs and increasing reliability of solar water 

heaters (SWH) is believed necessary for a substantial SWH 

market to exist in the U.S. (1). One strategy is through 

expanded use of passive SWH (PSWH). PSWH include 

integral-collector-storage and thermosiphon, as shown in 

Fig. 1. PSWH eliminate the pumps, controller, sensors, and 

power needs of active systems. New types of polymer-based 

PSWH with ~50% cost reduction are nearing market entry 

(1). However, the market for PSWH has been limited by the 

risk of freeze-induced bursting of the supply and return lines 

(2). These pipes carry pressurized potable water through 

unconditioned attic space to the thermal storage on or under 

the roof, as in Fig. 1. Because of excellent high temperature 

resistance, copper pipes are almost always used. However, 

copper pipes can freeze and burst in the attic, a catastrophe 

with costs that can be much larger than the SWH’s savings 

over its lifetime and contributing to erosion of SWH’s 

reputation generally. Therefore, the market has rightfully 

been restricted to areas with near-zero freeze probability, as 

indicated on the left side of Fig. 2 (2). As a result, lowercost, more-reliable PSWH are at present considered 

unsuited for almost all the continental U.S., where hard 

freezes occur at least occasionally. The goal of this work is 

to establish that practical pipe freeze protection may be 

provided by combining a sufficiently-reliable primary freeze 

protection (PFP) method with a fail-safe, freeze-tolerant 

piping, enabling northward extension of the PSWH market. 

A primary freeze protection (PFP) mechanism keeps the 

piping unblocked; when the supply/return piping is frozen, 

the homeowner must bypass the PSWH to regain hot water. 

PFP failure must be very rare (e.g., << 1 per 100 systemlifetimes) for this approach to be acceptable to the market. 

PFP recently investigated include: 1) natural convection 

loops (NCL) of room air in ducts surrounding the pipes 

(3,4); 2) NCL in the supply/return pipes themselves (fed by 

tank heat or room air heat) (5,6); and 3) freeze protection 

valves (FPV’s) on the return line as in Fig. 4 (7). FPV seem 

to be the least expensive option and do not consume site 

thermal energy, but they do reject to drain the water used to 

warm the piping. Water consumption from FPV is relatively 

small until hard-freeze climates are reached (7). The righthand side of Fig. 1 indicates areas with consumption less 

than 1000-gallon/year consumed (less than household 

average monthly consumption), a potentially large market 

extension. However, any primary means of freeze protection 

can fail usually in multiple ways, and there must be a failsafe back-up for the primary means of freeze protection. 

1
Some polymeric piping is freeze-tolerant: the pipe is elastic 

and expands upon freezing without harm. Polybutylene pipe 

was shown to be freeze-tolerant in (8), freezing over 700 

cycles before the first burst. However, that pipe was 

removed from the market ~ 1990. Cross-linked polyethylene 

(PEX) is presently the dominant polymer potable water 

piping material used in the U.S. Some PEX suppliers’ and 

manufacturers’ literature states that their pipe is freeze 

tolerant; however, substantive supporting data or analyses 

were not available. A literature search on PEX freeze 

tolerance provided no substantive information on PEX 

freeze tolerance. The objective of this study is to indicate by 

analysis and by test whether PEX piping systems are freeze 

tolerant. A mechanical analysis based upon material 

properties is followed by experimental freeze-thaw cycling 

results on four brands of PEX pipes and their recommended 

connectors. 

Fig. 1. Schematic two passive SWH. The balance-of-system 

is very simple, reducing hardware cost and system failures. 

Insulated copper pipe

Freeze Protection Valve + 

freezable piping

Safe/Non-wasteful areas=

Limited by Pipe Freeze Limited by water consumption 

BUT: collector/store freeze? 

Untested, other affects?

Fig. 2. On the left: Freeze probability map for 3/4” copper 

pipe with 1” insulation, with zero-freeze-probability areas in 

green. PSWH with insulated metallic pipes should be 

installed only in the green areas. On the right: Water 

consumption map for FPV, with the areas consuming less 

than 1000 gal/year in green. 

Aux 

Tank

Mains Inlet

Supply piping

Return piping

Collector

To drain

Freeze Protection 

Valve located HERE!

House load

Storage

Fig. 3. An indirect thermosiphon system with unpressurized storage is shown, with a freeze protection valve 

mounted at the ceiling-line just before the return pipe enters 

conditioned space. 

The ultimate goal of this and related work is to extend 

PSWH markets using a PFP method with freeze-tolerant 

PEX piping as the fail-safe backup in case the primary 

method fails. However, this goal presents many difficulties, 

this small study raises several questions, and caution is 

appropriate. Failure rates for possible PFP methods are not 

well-known, yet these rates must be very low for market 

acceptance of this approach. Another important caution 

stems from the fact that polymeric piping weakens and is 

subject to burst at higher temperature. PEX supply/return 

tubing has not been allowed by rating organizations because 

burst-causing high temperatures (e.g. 150 °

C) can occur 

when fluid starts circulating through stagnating SWH. 

Hence, high system temperatures must be completely and 

reliably avoided before polymeric piping can be safely 

considered. 

2. MECHANICAL ANALYSIS

An analysis based upon measured mechanical properties of 

PEX pipe is useful to establish a materials basis for piping 

freeze tolerance. The analysis is based upon the concept of a 

“quasi-linear region” where strain is ~linear with stress and 

the material returns without deformation to the initial 

geometry when the stress is removed (4). Despite lack of a 

“strictly linear” region in polymers, strain in the quasi-linear 

region does not induce measurable permanent deformation. 

If strain remains in this region under freezing, it is plausible 

that the pipe would be able to withstand repeated freezethaw cycles without bursting. For this analysis, we assume 

that the blockages are established before any freezing occurs 

in the pipe (a conservative assumption) and that insulation 

and freezing are uniform (as in normal piping runs). 

Mechanical data were taken on two pipe types, a silane PEX 

and an irradiated PEX. Tensile measurements were taken 

with a standard tensile test apparatus (Instron Model 1122 

with 5500R load frame) using ASTM D 638-9B procedures 

2
(9). A temperature-controlled chamber surrounding the 

instrument jaws allowed property measurement as a 

function of temperature, including near 0 °

C (uses liquid 

nitrogen). Results are shown in Fig. 4 for one tested pipe at 

0 °

C. At strain below ~4%, the deformation appears ~linear, 

although curvature is evident from the outset. Permanent 

deformation is clearly evident beyond 10% strain, where the 

material begins “flowing,” and is also most likely occurring 

above 6%. It is possible that some deformation might occur 

below the 4% point. The pipe would still be able to take 

some number (hopefully large) of freeze cycles before the 

accumulation of the small deformations would cause 

sufficient wall-thinning to fail. Behavior in the silane PEX 

is very repeatable over 5 samples. Irradiated PEX showed 

similar behavior, but sample variability was much higher 

(4). This was also the only PEX pipe that burst in lengths 

between fitting greater than 7”. 

Freezing and pressure-buildup in pipes are relatively 

complex (10). As pipes freeze, an annulus of ice attaches on 

the pipe inner surface and freezes inward toward the pipe 

axis. However, there is no pressure buildup until two 

separated “ice blockages” occur because until that occurs 

water is pushed back into the mains or forward into the 

downstream house piping system (which has significant 

expansion capability). Subsequent to blockages, further 

freezing causes pressure to build up. Pathological cases of 

nonuniform freezing with a “piston action” can be created 

by tapering insulation to force the freeze between the 

blockages to occur nonuniformly from the blockages in, 

forcing water inward and creating potentially very high 

pressures and/or strains; such “pathological insulation” was 

done here as an example of what not to do. 

When a blocked pipe freezes, pressure builds up and the 

pipe expands in both the circumferential (hoop) and the 

axial direction. Elementary mechanical analyses of a thinwalled, closed cylinder in the linear domain where stress (σ) 

is proportional to strain (ε) shows that these variables are 

related as 

σaxial = 1⁄2σhoop ⇒ εaxial=1⁄2εhoop ⇒ 

(Lice-Lwater)/Lwater=1⁄2(rice-rwater)/rwater (1) 

The volume ratio of the ice to the water can be expressed as 

Vice/Vwater = ρwater/ρice = (rice /rwater)

2

*(Lice/Lwater) (2) 

with ρw/ρi =1.091. Substituting Lice/Lwater from (1) into (2) 

yields 

(rice/rwater)

3

 + (rice/rwater)

2

 – 2ρwater/ρice = 0 (3) 

which results in (rice/rwater) = 1.035. Thus, εhoop =3.5%, and 

εaxial = 1.75. The calculated hoop strains compared very well 

with data on 10 pipe sections, where hoop strain averaged 

3.48% (4). These strains appear to be in the quasi-linear 

region, indicating that the pipe could probably be freezethaw cycled a number of times. However, note that this 

strain is uncomfortably close to where the curvature rapidly 

increases and where significant permanent deformation 

might start occurring. Thus, any concentrations of stress due 

to nonuniformities of freezing should be avoided. Although 

uniform freezing appears safe, nonuniform freezing can 

induce higher strain and permanent deformation. This 

speculation appears to be supported by observed freezebursts in shorter piping lengths, which promote nonuniform 

freezing. 

Fig 4. Stress versus strain for 5 samples of silane PEX. A 

quasi-linear region without significant permanent 

deformation exists below ~3-4% strain. The boundary is 

hard to establish. Slippage during the initial cool-down to 0 

°

C is evident at initial strain. 

3. FREEEZE-CYCLING EXPERIMENT

A freeze-cycling experiment was set up using a computercontrolled freezer with internal heaters and fans, and a 

temperature-controlled circulator, as shown in Fig. 5. Four 

brands of PEX piping were used, as shown in Table 1 and 

Fig. 6. All commonly-used cross-linking methods are 

included, including chemical (peroxide and silane) and 

physical (irradiation) methods. In addition, a multi-layer 

PEX-Aluminum-PEX pipe (PAX) was tested Both 1/2” and 

3/4” diameters were used, along with recommended fittings 

(tees, elbows, and connectors, and reducers). Piping ran 

between inlet and outlet manifolds in the freezer with 36 

ports. For each pipe type and diameter there were three 

straight runs (one uniformly insulated) and one “U-bend” 

run with vertical runs. Tables 1 and 2 give the number of 

pipes (by length) and fittings installed of each type, with the 

details of mounting in (4). In addition, pathological cases 

with insulation tapered from ends to the middle were 

installed to exemplify a configuration to be avoided. These 

cases consisted of a pipe of length 4’ to 16’with uninsulated brass connectors at the ends (to establish two 

3
freeze blockages early in the freeze cycle), and with 

insulation increasing toward the middle, forcing a piston 

action as freeze progressed toward the middle of the pipe. 

Fig. 5. Schematic diagram of pipe freeze experiment. The 

circulator keeps pressurized water in the piping while 

limiting water inventory. Fans maintain temperature 

uniformity, and a heater thaws the pipes out. 

Fig. 6. Four types of PEX piping tested (top), along with 

some of the fittings tested (bottom). 

At the beginning of a cycle, the pipes are warm and the 

freezer is turned on. Temperatures decay to ~0 °

C, staying at 

that temperature until the latent energy is exhausted and the 

pipe is fully frozen. The temperature of the ice plug then 

decays toward freezer air temperature (~-40 °

C). When all 

pipes are totally frozen, the freezer is turned off, and the 

heater is turned on to thaw the pipes. After thawing, the 

heaters are turned off, and warm water (~40 °

C) circulates 

for 1/2 hour through the pipes (to stress the pipes and 

fittings via thermal expansion) before starting the next 

cycle. The sensor under the middle of the heaviest tapered 

insulation piece controlled the staging of the cycles. All 

pipes were assumed frozen when this sensor indicated -15 

°

C, and were considered thawed when it indicated +30 °C. 

Cycles take 8-16 hours (depending on insulation). Fig. 7 

shows an example cycle. 

 Fig. 7. Typical freeze cycle. The slowest-decaying 

temperature (top line at hrs. 19, 24) is beneath 2” of 

insulation, freezes/thaws last, and controls the cycle. 

4. FREEZE-TEST RESULTS

As of paper submission, there were 450 completed freezethaw cycles on the piping systems in the freezer. We plan to 

continue cycling until 1000 cycles are reached. Results are 

shown in Tables 1 and 2 for pipes and fittings, respectively. 

In general, the bulk of the pipes and fitting were able to 

withstand the freeze cycles, with exceptions discussed 

below. 

TABLE 1: PIPES: #INSTALLED/#BURST

Brand Name/Type l < 1” 1”< l < 5” l > 7” 

hePEXplus/silane PEX 120/0 17/9 23/0 

QestPex®/peroxide PEX 0/0 6/1 17/0 

Durapex/Irradiated PEX 0/0 9/0 19/3 

Kitec XPA/PEX-AL-PEX Not freeze-tolerant 

4.1 Pipes and Fittings that Broke

PAX pipes and fittings both broke within 10 cycles, often 

less, demonstrating that Kitec pipes and fitting are not 

freeze-tolerant. PAX is apparently not suitable for use with 

water where there can be freezing events, and is not 

discussed further. 

We also found that any of the pipes with “pathologicallytapered” insulation installed over them would eventually 

break. Surprisingly, it took 20-50 cycles to break the pipes, 

4
depending on the length of the pipe and the details of the 

insulation layering. Although no sane person would install 

pipe this way, it is a simple, clear demonstration of the 

perils of nonuniform freezing and piston action. 

TABLE 2: CONNECTORS: #INSTALLED/#BROKEN

Name/Material Fittings 

ProPex®/PolySulphone1 20/2 

ProPex/Brass1 25/0 

Sioux Chief®/Copper2 101/0 

Kitec Compression/Brass Not freeze-tolerant 

1) ProPex fitting are used only with the silane PEX. Sioux 

Chief fittings can also be used with this pipe. 

2) This fitting is recommended with silane PEX and 

irradiated PEX. 

Surprisingly, pipes of length 3-4” tended to burst, if there 

were brass or metal connectors on each end of the short 

section. There were nine such bursts with the silane PEX, 

one case with the peroxide PEX, and no cases of this with 

irradiated PEX. In no case did any of the pipes burst when 

polysulphone connectors were used, rather than metallic. 

For the silane PEX, the average cycles to break was sizedependent, taking ~35 cycles for 1/2” pieces and ~95 cycles 

for 3/4” pieces. One would expect that with the metal 

connectors (as opposed to the polymer connectors) freezing 

will occur first at the connector location, creating piping 

blockages at these locations. It can be inferred that a piston 

action follows that is strong enough to burst the pipe in 30-

100 cycles, if it is about 3.8” long. Samples of the burst 

short pipes are shown in Fig. 8. Note that the pipes show a 

small bulge near the location of the burst, consistent with 

permanent strain before break. There is a reasonable 

explanation for why shorter or longer pipes did not burst, 

although future work must verify the hypotheses: 1) very 

short pipes: for very short segments (e.g., <2”) with brass 

connectors, the pipe is so short that after blockage is 

established there is not much water left to freeze “inward to 

the middle”, ameliorating pressure buildup; 2) long pipes: 

the piston action apparently did not extend far enough down 

the pipe to cause abnormally high strains to build up, with 

most freezing subsequent to blockage occurring relatively 

uniformly. 

Three long pieces of the irradiated PEX broke. This seems 

somewhat anomalous, given that no other long pipes failed 

and that the stress-strain data indicates that uniformlyfreezing long pipes should not burst until many cycles, if at 

all. It may be that nonuniform mechanical properties were 

an issue with this pipe type, based upon: a) the observation 

that the six irradiation PEX samples tested mechanically 

showed ~20% scatter in results; and b) the fact that 

irradiation-based cross-linking has been associated in the 

literature with nonuniform properties (4). 

Fig. 8. Pipe burst failures in pipes with lengths about 3.8”. 

All of the brass compression fittings broke within 10 cycles, 

and are not suitable for use where freezing can occur. 

Although the polysulphone ProPex fittings seemed to help 

avoid rapid buildup of blockages, there were several of 

these plastic fittings that broke. It would seem prudent to 

not use these fittings if the piping system is designed to be 

freeze-tolerant. 

4.2 Pipes and Fittings that Didn’t Burst

Longer lengths of pipes (with exception of the irradiated 

PEX as noted above) all have survived freezing of over 450 

cycles without burst. Three quarters of these pipes were not 

insulated, and the remaining 1/4 were uniformly insulated. 

This result indicates that the two chemically-cross-linked 

piping brands are freeze tolerant if they are installed in 

sections longer than some minimum (perhaps 7”), and if 

they are uniformly insulated. The design must avoid 

nonuniform freezing, which can lead to large stress 

concentrations with subsequent permanent deformation and 

eventual burst under freeze. 

None of the 126 metal fittings in the freezer has broken. 

Water beneath the metal fittings could be expected to freeze 

faster than water beneath the polymer pipe wall, which has 

~1/1000th of the conductivity of the metal. Having frozen 

first, the metal fittings would avoid directly seeing the 

affects of direct piston action. Both brands of metallic 

fittings seem suitable at this stage, as none of these fittings 

has broken. 

5
5. CONCLUSIONS AND FUTURE WORK hoop Direction around the circumference of the pipe 

ice Ice 

The market for PSWH can be extended northward if a water Water 

freeze-tolerant piping system can be identified to serve as a 

fail-safe back to a reasonably reliable primary freeze 

protection method. Stress-strain data indicate that PEX 

piping systems may be freeze-tolerant, as strains from 

freezing are in the quasi-linear region. However, there is 

ambiguity in defining the region, and strain from freeze puts 

the material uncomfortably close to the nonlinear region 

where permanent deformation is evident. Tests of available 

PEX piping systems were performed inside a freezer. Kitec 

and its connectors are not freeze tolerant. Tests showed that 

short pipe sections of order 3-5” in length with metal 

connectors on both ends tended to burst, and must be 

avoided. Tests also showed that longer sections (> 7”) of the 

two chemically-crosslinked brands are freeze-tolerant. For 

an irradiation cross-linked material, three longer sections 

broke, and that pipe should not be considered freeze 

tolerant. The polysulphone connectors prevented early 

formation of blockages, but several broke and should not be 

used in a freeze-tolerant piping system. None of the two 

brands of metallic connectors have broken. There is good 

indication that the two available PEX piping systems may 

be freeze-tolerant if installed in long sections with uniform 

insulation and with metallic connectors. 

7. ACKNOWLEDGMENTS

The authors acknowledge funding from the U.S. Department 

of Energy’s Solar Energy Technology Program, Solar 

Heating and Lighting (SH&L) subprogram We gratefully 

recognize encouragement from Tex Wilkins and Glen 

Strahs, SH&L program managers, to pursue PSWH market 

extension concepts. Ron Judkoff, director of the NREL 

Buildings and Thermal Sciences Center, has been a 

masterful enabler of good science. 

8. REFERENCES

(1) Solar Energy Technology Program: Multi-year 

Technical Plan 2007-2011. The document is available at 

http://www1.eere.energy.gov/solar/about.html. 

(2) Salasovich, J., Burch, J., and Barker, G., “Pipe Freeze 

Probability for Passive-solar Water Heating Systems in 

the United States”, Proc. ASES 2004, ASES, Boulder, 

CO. 

(3) Burch, J, and Heater, M., “Pipe Freeze Prevention for 

Passive Solar Water Heaters Using a Room-air-driven 

Natural Convection Loop”, to be published Proc. 

ASES2006 Denver, CO, July 2006. ASES, Boulder, CO. 

Future work depends on available funding. Pipe freezing is 

a long-standing, serious problem with high damages 

possible with burst piping. A novel solution as proposed 

here requires more thorough investigation and gradual 

accumulation of field experience before widespread 

adoption. For initial installations, it would be wise to 

include additional safeguards (such as a thin-film shroud 

around piping with outlet to drain) until favorable field 

experience is accumulated. Pipe properties may change with 

exposure to chemicals and heat over time that could 

decrease the linear region significantly and could affect 

freeze tolerance. In addition, the length dependence of 

freeze-induced bursting needs to be investigated with finer 

resolution than used here. 

(4) Heater, M., “Passive Freeze Protection for Passive Solar 

Thermal DHW Systems”, 12/04, M.S. Thesis, ACEE 

Department, University of Colorado, Boulder, CO. 

(5) Darbaheshti, M., Burch, J., Gawlik, K., and Johnson, T., 

“Freeze Protection for Potable Water Piping Using 

Natural Convection Loops,” Proc. ASES 1999, ASES, 

Boulder, CO. 

 (6) United States Patent, US005806511A, Method and 

Apparatus to Provide Freeze Protection for Solar Water 

Heating Systems, 1998. Assigned to D. Hart, Watershed 

Energy Systems. 

(7) Burch, J., and Salasovich, J., “Water Consumption from 

Freeze Protection Valves for Solar Water Heating 

Systems”, Proc. ASES2005, ASES, Boulder CO. 

6. NOMENCLATURE (8) Farrington, R. B. “Polybutylene Pipe Freeze/Thaw 

Reliability Testing. ” Proc. ASES1987, ASES, Boulder, 

Symbols CO. 

L Pipe length (9) ASTM Standard D 638-9B “Standard Test Method for 

Tensile Properties of Plastics.”, ASTM Committee D-20. 

March 1999. 

r Pipe radius 

V Volume of pipe as cylinder 

(10) Gordon, J. 1996. “An Investigation into Freezing and 

Bursting Water Pipes in Residential Construction.” 

Research Report 96-1, University of Illinois at UrbanaChampaign. 

ε Strain, ∆l/lo

σ Stress 

ρ density 

Subscripts

axial Direction parallel to the pipe axis 

6
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4. TITLE AND SUBTITLE

Northward Market Extension for Passive Solar Water Heaters by 

Using Pipe Freeze Protection with Freeze-Tolerant Piping: Preprint 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-550-39664 

5e. TASK NUMBER

SH06.2003 

6. AUTHOR(S)

J. Burch: NREL 

M. Heater, M. Brandemuhl, and M. Krarti: University of Colorado 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

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13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Combining a freeze protection method with fail-safe, freeze-tolerant cross-linked polyethylene (PEX) piping may 

provide a means to significantly extend the market northward for passive solar water heaters. Stress-strain data on 

PEX piping materials indicate that the 3.5% hoop strain from uniform freezing is likely tolerable for many cycles, but 

data are not definitive because it is not clear where permanent deformation starts occurring. Four PEX piping 

systems were freeze-thaw-cycled ~450 times. One brand showed no freeze tolerance, whereas two other brands 

appear freeze-tolerant, with no bursts in lengths greater than 7” or less than 2”. Two geometries were identified that 

promote nonuniform freezing and that should be avoided: 1) nonuniform insulation increasing from ends toward the 

middle of the pipe in longer sections; and 2) pipes of ~4” in length with metal connectors on both ends. All metallic 

fittings have survived, but several polymer fittings broke and should be avoided when freeze-tolerance is desired. 

Further testing and analysis is needed to better understand the length dependence of freeze bursting and effects of 

piping aging, and field testing should proceed cautiously. 

15. SUBJECT TERMS

solar domestic water heating; solar water heating; cold-climate solar domestic water heating; cold-climate solar water 

heating; freeze-protection for solar water heating 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

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c. THIS PAGE 

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17. LIMITATION 

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UL 

18. NUMBER 

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Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18