Welding is an old and reliable technology. It basically involves melting the quick fitting together. Steel and polypropylene employ this method. Welding can be used for galvanized steel, but it is virtually impossible to repair the zinc coating on the interior of the pipes, so mechanical coupling is preferred.

Threading involves screwing the pipes together, usually with a female nipple between two male-threaded sections of pipe. Threading is common for steel and galvanized steel pipes. It is also common for some plastic pipe materials. It is used for SS but requires fresh dies and an anaerobic pipe compound to make leak-tight joints. Threaded joints withstand forces in all directions.

Flanging is expensive but virtually foolproof. Flanged joints can hold any desired pressure and can be dielectric to minimize corrosion (see Figure 2).

Figure 3: Mechanical couplings connect pipes, withstand forces in all directions, and can hold any desired pressure. Courtesy: Victaulic Mechanical couplings (see Figure 3) withstand forces in all directions and can also hold any desired pressure. Today we see a movement toward either shop-welded assemblies that are connected in the field by mechanical couplings or systems that are fully mechanically coupled, primarily in sizes above 2 in. Both rigid and flexible couplings are available. Some projects also include vertical risers that benefit from the linear flexibility of “flexible” couplings to avoid expansion joints or offsets that increase shaft sizes to prevent pipes from breaking due to shear forces at inflexible shaft walls. Flexible mechanical couplings also can replace flexible connections, depending on the geometry and vibration isolation of the pump or equipment.


Corrosion is very important to address in pipe systems. Generally, hydronic heating or cooling systems employ corrosion inhibitors and perhaps biocides. Nitrites and molybdates are the most common corrosion inhibitors. Some design firms specify only molybdates for chilled-water systems, but it allows either molybdates or nitrites for heating-water systems that raise the water temperature to above 140°F in winter. This is because in cool water, nitrites can be food for microorganisms; microbiological “bloom” can occur in chilled-water systems.

Separate inhibitors are added to protect “yellow metals,” such as copper. In glycol systems, most suppliers use a phosphate corrosion inhibitor because it also meets Food and Drug Administration rules for food-grade products, so they only need to make one product for food-grade and non-food-grade glycol.

However, at least one supplier uses nitrates, so each owner must keep records of what is in their building. Data regarding the efficacy of half nitrate and half phosphate treatment is not available; mixing glycols with different inhibitor chemistries is not recommended. Systems that contain glycol must maintain the glycol concentration at between 18% to 25%. Sources vary about the exact limit, but no manufacturer sells premixed glycol below 20% concentration; it is recommend to use nothing below 25%.

If this is not done, microorganisms may multiply rapidly because glycol is food. Glycol is an alcohol, and much like making wine, until the concentration becomes toxic the microorganisms will multiply. Never allow a domestic water makeup connection in a glycol system, or the concentration will slowly decrease until there is a major problem. A feed tank filled with premixed industrial (not automotive) glycol, a pressure switch, and a pump is recommended.

Steel is relatively immune to corrosion if it is in a high pH environment (e.g., steel rebar in concrete). The pH scale is logarithmic and commonly ranges from 0 to 14. It indicates how acidic or basic a solution is, with 0 being the most acidic and 14 being the most basic. A pH of 7 indicates neutrality. A pH range of 8 to 10.5 is commonly used for pipe systems that include steel. Steel is, however, subject to corrosion if the pH is low or individual chemicals attack the steel. Many corrosion-protection schemes rely on high pH, but this is a problem for systems that include boilers that have aluminum heat exchangers because aluminum is not compatible with high pH. The combination of steel pipe and aluminum heat exchangers requires a very narrow pH range in hydronic systems, typically 8 to 8.5.

Surface condensation is another issue. In the Midwest, it is common not to insulate PEX or other plastic pipe materials in some systems because condensation doesn’t form. But from an energy standpoint, PEX loses heat faster than copper pipe. This is because the larger outer diameter of PEX provides more surface area for heat transfer

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