Copyright Goodheart-Willcox Co., Inc. 244 Hydronic Heating: Systems and Applications 12.3 Radiant Heat Piping Systems Radiant hydronic piping systems use several different types of plastic piping to transport heated water through the “terminal device.” In this case, the terminal device is typically the floors, walls, or ceiling of a heated space. Regardless of whether the tubing is run through a floor, wall, or ceiling, the intent is still the same: using the heated water from the tubing to warm the thermal mass through which it is running. This thermal mass then conveys heat by means of radiation to warm the people and objects within the conditioned space. Considerations must be made when choosing the type of piping to be used and the piping con- figuration. This section covers different types of tubing and their applications as well as several different piping configurations typically used with radiant heating systems. 12.3.1 Radiant Piping Materials When selecting the type of piping material for a radiant heating system, several factors need to be considered. The selected piping should adhere to the following characteristics:■ ■■ It must have the ability to readily transfer heat. ■■ It must be flexible and expandable. ■ It should be easy to join together and connectable with other types of piping materials. ■ ■■ It must have longevity. ■■ It should be resistant to freezing temperatures. ■ It should be reasonably priced. The two types of tubing that will be discussed in this section are the most widely used in the radiant heating industry today: PEX and PE-RT tubing. 12.3.2 PEX Tubing PEX stands for crosslinked polyethylene. It is made from a high-density polyethylene polymer (HDPE), which is melted and continuously extruded into tubing. PEX is flex- ible, resistant to scaling and corrosion, and easier to install than metallic or rigid piping, Figure 12-12. It has been used in Europe since the early 1970s and was introduced to the United States around 1980. PEX pipe usage has increased ever since, and today it replaces copper piping for many applications, including domestic hot and cold water plumb- ing as well as radiant heating systems. Most PEX tubing is available in pipe sizes from 3/8″ up to 1″ in diameter. PEX tubing or piping is classified as type A, B, or C. The dif- ferent classifications are not different grades of PEX rather the letters are used to identify the manufacturing process. ■ ■ PEX type A is the highest quality. It is more flexible and has thermal memory—in other words, it can be heated and will return back to its original shape if it becomes kinked. ■ ■ PEX type B is less flexible than PEX type A. It is more susceptible to freezing under extreme conditions however, it is typically more competitively priced than type A or C. ■ ■ PEX type C is softer than type B, but it is more environmentally friendly to manufacture. It has little or no coil memory and is more prone to cracking. Goodheart-Willcox Publisher Figure 12-11. A solar collector storage tank is used to circulate water to the heating loops. Pawel G/Shutterstock.com Figure 12-12. PEX tubing is crosslinked polyethylene made from a high-density polyethylene polymer.