Where polyethylene pipe challenges metal for slab radiant heating

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Canadian Builder, 13, 4, pp. 55-56, 1963-04

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Where polyethylene pipe challenges metal for slab radiant heating

Platts, R. E.

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WHERE

POTYETHYIENE

PIPE

CHATTENGES

Ti|IETAT

FOR

STAB

RADIANT

HEATING

A N A L Y Z [ D

by

R. E. Platts

Reprinted From

Canadian Builder, Vol. XIII, No. 4

April 1963, p. 55

Technical Paper No. 151

of the

Division of Building Research

1e*fls

-JUN l) $63

NRC 7372

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THT

N2Lt2

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no. L5L

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NATIONAL RESEARCH COUNCIL

CANADA

DIVISION OF BUILDING RESEARCH

OTTAWA

Price 10 cents

April 1963

Where polyethylene pipe challenges meti

By R. E. PLATTS

Housing Section, Division of Building Research' N.R.C.

Although hot water heating systems are now being installed in all types of buildings, their use would undoubtedly increase if a way could be found to re-duce the material and fitting costs that result from the great amount of piping required. At present most installations still use metal pipe, but it is generally agreed that the flexible coils of poly-ethylene tubing can be placed in the concrete slabs at lower cost and in less time.

Authorities have been slow to allow this use of polyethylene, basing their caution on lack of experience with the comparatively new material, and also on its thermoplastic nature: the ability of polyethylene to sustain high stresses for long periods is sharply reduced as tem-peratures rise. As now reported, an ex-tensive review of laboratory testing and field experience supports this use of low density polyethylene pipe if proper con-trol of quality and working conditions can be assured.

Laboratory Testing

Low density polyethylene pipe has been on the market for about 14 years, but it is only 10 or 1l years old in terms of known sustained use and con-trolled testing. As with any new mate-rial, predictions of its working life are merely predictions, but the type and amount of test work are particularly impressive.

Although concrete surrounds and con-fines most of the pipe in slab radiant heating, it is best to design the pipe for possible unsupported areas as is later discussed. Hoop tension resulting from water pressure must be considered in relation to temperature and time. Ex-tensive testing of these effects has been done for cold water piping at 70 F

temperatures and 80 to 100 psi water

pressures and higher. Standard polyethyl-ene pipe is designed for these conditions with working stresses of 300 to 550 psi (hoop tension in the pipe wall), and tests indicate that working life for sus-tained use should be at least 20 years or more. Lower temperatures greatly in-crease the working life at these pressures (this particularly supports the use of polyethylene for underground service and artificial ice piping) but higher temperatures drastically cut the working life: at 105"F and 550 psi hoop stresses, failures can occur at 1000 hours.

When used for radiant heating, how-ever, unusually high temperatures for polyethylene (100"F to 150"F) are com-bined with unusually low stresses (about 55 psi hoop tension). Little test work has been intentionally aimed at such a com-bination but useful American and Ger-man work has been reviewed and interpreted for these conditions.

American tests indicate that at 140"F and 55 psi hoop tension, low density polyethylene should suffer little creep expansion, probably less than 5Vo in 20 years or more-not a critical amount(r). Creep expansion is the usual mode of failure for a low density material at normal temperature and stress, but un-der these particular conditions the ma-terial will eventually probably experi-ence brittle failure (pinholes or slits). The known American work cannot be used to estimate time-to-failure at these low stresses.

German work, however, has involved extensive testing to 10,000 and 30,000 hours (3.4 years) at a wide range of temperatures. Cfhis allows a particular method of extra-polation to be based on the failure stress/time curves, since their shape can be found within 10,000 hours at higher temperatures.) Incidental to its intended purpose, much of this work can be interpreted for radiant heating conditions. Extrapolations indicate work-ing lives of these orders for low density

polyethylene at sustained hoop stresses of 60 psi:(z)

CONCLUSIONS:

65Both

_{laboratory and field}

ex-perience show that low density

polyethylene pipe should be futly

suitable for concrete slab radiant

heating systems

if proper control

is exercised.tt

So states Mr. R. E. Platts of

the Division of Building

Re-search, NRC in h,is conclusions

in this special article written for

Canadian Builder. He adds:

'?ipe must meet recognized

standards. Maximum temperature

should be positively limited to

130oF and preferabty 120oF by

means of aquastat cut-off valves

on the feed line fion

the

'blender' to the slab. Pressures

should not exceed 15 psi for

sus-tained use a,t these temperatures."

Temperolure

Time-lo-Foilure, 6O psi hoop sfress S o o c ( l 7 6 o F l 2 , 0 0 0 h r

6 0 " c { l 4 O o F l 5 0 , 0 0 0 r o r 00,000 hr

5o.c (l 22otl

... .... o,.turi'.;trJ'1,

The predictability of extrapolations down to such low stresses is not known, of course, but these estimates are prob-ably conservative. Quality control has been upgraded since this work was car-ried out. CSA Specification Bl37(3) in

for slab radiant heating

particular demands quality control test-ing at 100"F and over 600 psi hoop tension for 1000 hours without failure; most of the older pipe would fail this test. In addition, the above are sustained-load times; radiant systems will run at their maximum temperatures only occa-sionally in the winter, and will be off in the summer, so that the working life estimates given may be doubled. Finally, of course, it must be noted that most of the embedded pipe is confined by concrete, and so the chances of local failures will be greatly decreased. This suggests that 120"F and 55 psi hoop stress (about l5 psi water pressure) should allow indefinite working life for polyethylene radiant heating systems. Field experience supports these recom-mendations.

Field Experience

It is still general practice with hot water radiant heating systems to instal metal pipe at a depth of about 4 in.; this requires a water temperature of 120'F to 150'F to obtain a floor sur-face temperature o f80oF as the radiant "panel". In such installations, however. there is a lag in heating and cooling as outdoor conditions change, and so re-cent practice, to allow faster response, is to run the pipe close to the surface of the concrete and at temperatures from 90'F to 110'F.

Since polyethylene has much lower heat conductivity than metals, these temperatures would have to be boosted to 110 and 120'F to allow the same heating results. As already mentioned, these temperatures should be suitable for polyethylene since the pressures in radiant systems are very low-about 10 to 15 psi. They are maintained at this level mainly to prevent air entry at pump glands.

The usual temperature control on these systems, however, is by a "blender" which mixes boiler water with recirculat-ing by-pass water, and these "blenders"

can allow full boiler water at 180 to 200'F into the radiant coils for some minutes at the beginning of a cycle. These temperatures can quickly weaken or cause failure of unconfined polyethyl-ene pipe even at very low pressures.

Some installers maintain that the crete slab should fully surround and con-fine the polyethylene pipe-thus making the pipe merely an effective liner for the concrete. With this in mind a few installers use only "blender" controls with polyethylene systems, allowing pos-sible over-run temperatures as noted. At least one installer runs his systems at 150'F and can point to trouble-free installations that were six years old when reviewed itr 1957(4).

Failures in the embedded piping have occurred in enough instances however to show that it is risky to consider the plastic pipe as a liner only, with the concrete fully confining it. Evidently small pockets occur, probably because of "snagging" of large aggregate in the concrete against reinforcing mesh or against the plastic pipe itself. These voids allow the adjacent pipe surface to be loaded in hoop tension, and even though the pressure is low, the plastic will slowly fail if temperatures rise too high. For example, out of five large installations completed near Toronto ten years ago, two failed because of in-adequate temperature control, while the other three have performed satisfac-torily. The supplier involved recom-mends llOoF as the maximum water temperature.

On the average, suppliers and install-ers recommend 120'F as the top,limit for polyethylene pipe used for slab radiant heating but some state a top limit of 130'F. Nearly all require aquastat ctrt-off valves between the blen-der and the plastic heating coils. Many emphasize this point strongly, and some even recommend two aquastats in series in case one fails(4).

Quality control of the polyethylene itself is of prime importance. Pipe ex-truders have sometimes yielded to the temptation to throw reclaimed scrap into the "pot", and quality has suffered dras-tically. Users must demand full assur-ance that pipe meets recognized speci-fications such as CSA B 137 or CS 197-60(s, s).

In reviewing reported installations up to I I years old in several countries, no failure has been noted where polyethyl-ene quality and water temperature have been properly controlled. Further, both laboratory and field experience indicate that the "scale" that plagues metal piping does not form in plastic pipe.

Temperature-expansion stresses in the confined pipe have been questioned and should be briefly mentioned. Although the coefficient of expansion is high (about 0.00018 in./in./"C) the effective modulus of elasticity is very low partic-ularly at higher temperatures (about 70OO lb/rn.,/in. at 50"C). Thus the re-sulting compressive stresses in the pipe wall are only in the order of 37 psi as the pipe is heated from 20'C to 50.C (68"F to 122'F\.

References

1. Carey, R. H. Creep ild stressrupture behavior of polyethylene resins. Industrial and Engineering Chemistry, Vol. 50, July 1958.

2. Niklas. H. and K. Eifflaender. Results of long-time tests on tubes of polythene and polyvinyl chloride. Plastics (England), April 1 9 5 9 .

3. Polyethylene pipe for cold watef servies. Canadian Standards Association. 235 Montreal Road, Ottawa, Canada. CSA 8-137.

4. Plastic pipe - is it practical for radiant heating? Domestic Engineering (U.S.), Feb-ruary 1957.

5. Flexible polyethylene plastic pipe. U. S. Dept. of Commerce. Commercial Standard CS 197-60. (For sale by Superintendent of Docu-ments, U.S. Gov't Printing Office, Washington 25, D.C.) - _{T h i s p o p e r i s o c o n l r i b u l i o n f r o m l h e} D i v i s i o n o f B u i l d i n g R e s e o r c h , N o t i o n o l R e s e o r c h C o u n c i l , o n d i s p u b l i s h e d w i t h l h e o p p r o v o l o f t h e D i r e c t o r o f f h e D i v i -s i o n .

A list of all publications of the Division of Building

Research is available and may be obtained from the

Publications Section, Division of Building Research,

National Research Council, Ottawa, Canada.