READER FAVOURITE FROM HPAC’S ARCHIVE: Tubing Options
Look for polymer tubing to soon dominate the market worldwide.
April 1, 2013 by John Siegenthaler
Most of the early water-based heating systems were designed around steam boilers. At the time, piping options were limited to mostly threaded black iron and steel. As the market developed to include both steam and liquid water based systems, rigid iron and steel piping remained the staple. They were essentially the only materials available for piping such systems. Figure 1 shows a typical system design from 1949, which was based on the used of rigid iron or steel piping.
Over the last 50 years, the high cost of skilled labour, energy and transportation, as well as tight construction schedules, have greatly reduced the amount of black iron and threaded schedule 40 steel piping used in smaller hydronic systems. This is not a reflection on the quality of the material, it is just a reality of the market.
The first use of copper tubing is believed to be to convey water within the great pyramids of Egypt over 5000 years ago. Mass production of copper tubing dates back to the early 1900s. Its use for hydronic heating blossomed in the post World War II housing boom. Both rigid and annealed (soft temper) copper quickly became the tubing of choice for residential and light commercial hydronic systems in North America.
The type of copper tubing used in hydronic heating systems conforms to the ASTM B88 standard. It is available in three wall thicknesses referred to as types K,L, and M. All three thicknesses have the same outside diameter, which is exactly ” added to their nominal pipe size. This makes all three types compatible with the same fittings and valves. The relatively low pressures used in hydronic systems allow the thinnest type, (type M), to be used in most situations. However, some codes require type L for any potable water piping.
Until the last decade, copper tubing was largely joined by soft soldering using 50/50 tin/lead solder. In most jurisdictions, this joining technique can still be used for piping carrying non-potable water.
However, most codes now mandate non-lead solders for all copper piping carrying potable water. Within the U.S., all valves and other components used in potable water systems must be classified as no-lead by January 2014.
Copper tubing remains a staple for smaller hydronic systems, but a couple of things have changed over the last two decades. First, copper tubing is now used mostly within the mechanical room and certain portions of the distribution system, rather than being the only type of tubing in the system. This is attributable to increased used of polymer tubing, such as PEX, for a wide variety of radiant panel systems. Second, new “flameless” joining techniques, such as press fitting, shown in Figure 2, have captured the attention of many installers. These techniques have reduced or in some cases eliminated the use of soft soldering as a joining technique.
Copper tubing is known for its relatively high corrosion resistance and excellent thermal conductivity. It remains the standard material for situations where high rates of heat transfer are required, such as from the element in a fin-tube baseboard. Copper also remains a highly valued commodity worldwide and its current price reflects market demand. This demand and pricing has also driven development of a wide variety of polymer-based tubing products for use in both hydronic systems and potable water distribution.
In the early 1980s, crosslinked polyethylene tubing (PEX) made its way from Europe to North America. At first, it was generally viewed as a product limited to use in radiant floor heating systems. Today, PEX tubing is largely recognized as a universal piping material, with a wide variety of applications in both hydronics and potable water distribution.
Most of the PEX tubing used in North American hydronic systems conforms to the ASTM F876/877 standard. It is commonly available in nominal sizes of ” to two-inch, and with pressure temperature ratings of 80 psi at 200F, and 100 psi at 180F. The type of PEX used in most closed loop hydronic systems is known as “barrier PEX.” It has a thin layer of a compound called EVOH (ethylene vinyl alcohol), which greatly reduces the rate at which oxygen molecules can diffuse through the pipe wall. This reduces the potential for oxygen based corrosion of ferrous metal components, such as cast-iron circulators or steel expansion tanks.
Another popular hydronic piping material, often referred to as “composite” pipe, bonds an inner and outer layer of PEX to an aluminum core. The aluminum is protected against contact with the fluid within the pipe, as well as the material in which the piping is embedded or otherwise surrounded. The aluminum core provides a structural property that significantly reduces the linear expansion of PEX-AL-PEX relative to PEX. It also helps the pipe remain in the shape it is bent to and serves as an excellent oxygen diffusion barrier.
PEX-AL-PEX is manufactured to the ASTM 1281 standard, and has pressure/temperature ratings of 125 psi at 180F, and 110 psi at 210F. It is commonly used in radiant panel applications, especially those involving aluminum plates for heat diffusion.
It is also readily adaptable to baseboard, panel radiators, and convectors. Figure 3 shows ½” PEX-AL-PEX tubing installed in combination with aluminum heat diffusion plates for a radiant ceiling application.
A wide variety of fittings are available for PEX and PEX-AL-PEX tubing to adapt to treaded fittings and valves, as well as to other types of tubing. The ½” PEX-AL-PEX to ¾” soldered elbow shown in Figure 4 is one example. It allows PEX-AL-PEX to pop up through the floor and connect to a ¾” copper tube within a typical fin-tube baseboard. It also has a tapping for an air vent.
A relative newcomer in North America, PE-RT stands for polyethylene raised temperature. Used for over 20 years in Europe, PE-RT is certified to ASTM F2623, and provides pressure/temperature ratings of 160 psi at 73F and 80 psi at 180F. PE-RT tubing is available with an oxygen diffusion barrier for closed hydronic systems. It is available in sizes ranging from ” to one-inch nominal inside diameter.
One key difference between PE-RT and PEX, is that PE-RT is a thermoplastic, whereas PEX is a thermoset polymer. A thermoplastic can be heated and melted, whereas a thermoset polymer cannot. This allows PE-RT to be joined to compatible polyethylene fittings using socket fusion. The outside surface of the tube, and inside surface of the fitting are simultaneously heated using a special tool, such as that shown in Figure 5.
At temperatures in the range of 450F to 465F, PE-RT assumes a semi-molten state. After a specified heating time, the tubing and fitting are simultaneously pulled off the heating tool and pushed together to form a very strong and permanent bond. PE-RT is also used in combination with an aluminum core to create a composite pipe referred to as PERT-
AL-PERT. It is currently available in Europe, but has not yet been marketed in North America. It can also be joined to PE-RT fittings using socket fusion as shown in Figure 6.
Another polymer pipe system that is relatively new to North America is PP-R (Polypropylene Raised Temperature). It is now available in sizes from ” up to 24″. Although the smaller sizes are available in coils, the majority of PP-R in North America is sold in four meter straight lengths. PP-R tubing has a fibreglass-reinforced core that, in combination with the polypropylene inner and outer layers, limits thermal expansion and allows sustained operating temperatures up to 160 F with a corresponding pressure of 70 psi and temporary operating temperatures of up to 195 F.
PP-R tubing is joined by butt fusion in larger sizes, or socket fusion for smaller sizes. Figure 7 shows an example of
a portable fixture tool now available for socket fusion of straight length PP-R tubing and fittings.
Another benefit of PP-R tubing is the ability to create tappings using a process called saddle fusion. A special tool is used to cut a hole in the side wall of the pipe. This tool removes the cut material from the pipe. Next, the surfaces of the pipe and those of a saddle adapter are heated to approximately 500F. The saddle adapter is then pushed into the hole. The semi-molten PP on the fitting and pipe wall immediately bond for a strong, permanent connection. An example of where this technique has been used to create an elongated manifold system for floor heating is shown in Figure 8.
THE FUTURE IS PLASTIC
The global trend for hydronic system design is based on significantly lower temperatures than those used in the early applications with iron and steel pipe. Think 100F rather than 200F. These lower temperatures enable modern heat sources like condensing boilers, heat pumps and solar collectors to achieve high efficiency. They also support more widespread use of polymer-based tubing versus metal tubing. Although copper, stainless steel, and even black iron will retain specific “niches” in future systems, polymer-based tubing options already dominate the European hydronics market, and are very likely to continue gaining market share in North America. <>John Siegenthaler, P.E., is a mechanical engineering graduate of Renssellaer Polytechnic Institute and a licensed professional engineer. He has over 34 years experience in designing modern hydronic heating systems. He is also an associate professor emeritus of engineering technology at Mohawk Valley Community College in Utica, NY. Credits: Fig 1 Bell & Gossett Fig 2 Viega Fig 4 Uponor Fig 7 McElroy Manufacturing Fig 8a Jack McAllister