Recirculating Hot Water Can Corrode Pipes

At a recently opened hotel near Lake Tahoe, a guest reported a small leak in the ceiling. Investigation revealed the source: a pinhole leak in a copper pipe that was part of the 15-story building'srecirculating hot-water system. A similar leak was reported soon after … then another … and another.

By the time our firm was consulted, nearly every room showed water damage on walls, ceilings or both. Ultimately, the recirculating system had to be repiped at a six-figure cost—and more, if you include lost revenues.

Recirculating hot water systems, designed to provide nearly instantaneous hot water at the tap, have become a common amenity, especially in hotels and large high-end multifamily residential complexes, where a high level of convenience is expected. The walls in such buildings often contain cable, telecom, gas, electric, HVAC, security and other service infrastructure. A water-pipe failure within these walls is virtually guaranteed to produce a cascade of costly consequences.

Typically, a recirculating pump is used to pump hot water from a water heater through the copper hot-water pipeline, at the same time returning water that has cooled from the hot-water lines back to the water heater to be reheated. With hot water circulating constantly in well-insulated pipes, users—even those far from the water heater—never have to wait for the water to get hot. The system can also assist in determining if water and energy conservation measures are being met.

Unfortunately, these systems can be vulnerable to flow-accelerated corrosion, sometimes called erosion corrosion, which in turn can quickly mushroom into a costly and disruptive problem.

When corrosion occurs, several factors may be implicated, including aggressive water chemistry and improperly soldered joints. In most cases, though, the primary, or even sole, culprit is excessive flow velocity. In an effort to ensure instantaneous delivery of hot water, an oversized pump has been installed, with the result that the pipes are literally eroded away by their own contents.

When water flows through a copper pipe, dissolved oxygen reacts with the copper to form a protective oxide coating. But the flowing hot water is constantly trying to dissolve or erode the protective oxide film, and the higher the velocity, the greater the abrasive effect. The surface is most affected in areas where turbulence develops, such as below bends, elbows and tees, which is where the tell-tale pinholes are commonly found. Continued corrosion causes thinning of the tube wall, which may start as a series of deep U-shaped pits and a general roughening of the surface, which in turn causes additional turbulence and accelerates the corrosive process.

Flow-accelerated corrosion is a well-known and serious hazard in the electric power industry, where it damages boiler feed water systems, and National Regulatory Commission regulations include specific inspection procedures for monitoring and maintenance of erosion-corrosion/flow-accelerated-corrosion (EC/FAC) in nuclear plants.

In principle, a few straightforward precautions should be enough to prevent copper pipe corrosion in recirculating hot-water systems. The most important is to reduce or regulate flow velocity, mainly by using a pump of appropriate—not excessive—capacity and tubing of adequate diameter. Most knowledgeable designers recommend a minimum ¾-in. pipe size for the hot-water return system piping. Smaller ½-in.-dia. pipe may be adequate for some small systems, but should never be used in larger, commercial applications.

Other precautions also should be observed, such as reducing the number of joints if possible (to minimize turbulent flow), avoiding abrupt changes in direction wherever possible, and assuring quality work (e.g., taking care that burrs are removed from tube ends before joining, joints are soldered according to ASTM standards, etc.). But simply controlling flow velocity would eliminate the problem in the great majority of cases.

So if the solution appears simple, why does the problem persist? And persist it does. Over the years, we have encountered over a dozen situations like the Lake Tahoe hotel. For example, at a 400-unit North Bay Area condominium complex, corrosion damage caused massive dislocation and other inconveniences as well as extensive, costly repairs and reconstruction.

One reason is that many recirculating hot-water systems are installed without engineering guidance. The plumbing contractor, who may be perfectly competent, installs the system, paying due regard to local plumbing code requirements and the Uniform Plumbing Code. But the codes, unfortunately, are silent as to the specific requirements for a circulating hot-water system.

Would review or supervision by a specifying engineer solve the problem? The answer is, “Yes, but…” The “but” refers to the fact that relatively few engineers understand that flow velocity in these systems should, in my opinion, always be kept below 3 feet per second (fps), even though the American Water Works Assn. and the Copper Development Assn. both published the standard as long ago as the mid-1980s. [Note: CDA recommendations are that the flow velocity not exceed 4 to 5 fps, and suggests that 4 fps would be a more appropriate figure here.] To mention just a few examples,

• A recent National Assn. of Corrosion Engineers publication stated that “flow maximums should not exceed 3 to 4 fps for water >60°C”

• A “special report” from Hotel Online warns that “water velocities exceeding 4 fps” could be a “contributing factors” to corrosion

• A 2006 advisory from the Copper Development Assn. assures readers that “corrosion can be mitigated if systems are designed so that velocities do not exceed 4 to 5 fps (1.2 to 1.5 mps) in circulating systems”

• The Canadian Copper & Brass Development Assn. recommended “designing all hot water recirculating systems to keep velocities below 5 fps for temperatures up to 60°C (140°F).”

Thus, even review by an engineer may not guarantee secure protection against flow-accelerated corrosion.

To further complicate the picture, potential corrosion problems often go uncorrected because they may not manifest themselves for many years. The issue rarely shows up clearly because many of the overbuilt systems are routinely operated at less than their full design capacity for a variety of reasons, such as low initial occupancies or low demand for hot water. Often, water velocities may be throttled down below pump capacity for reasons that have nothing to do with corrosion. This can mask the underlying design vulnerability, since the system may never be run at a dangerously high velocity. On the other hand, the mismatch remains, and the potential for trouble exists should someone later decide to run the system at full capacity.

To repeat: these systems should be so designed that hot water temperatures generally do not exceed 140°F (60°C), depending on local codes requirements, and flow velocity is no greater than 3 fps. Copper is a soft and malleable material, and vulnerable to exposure to fast-moving hot water. Oversized circulation pumps or undersized distribution lines are a recipe for disaster. The penalty for ignoring that can be painful and costly for building owners and occupants alike.