Challenges in DHW Distribution — Part 2

This is a continuation of an article started in the December 2018 column addressing the many circulatory problems associated with older, multi-story buildings.

Another condition that has been discovered recently that will mimic HEC is acid-based flux corrosion. The appearance of flux corrosion differs from HEC (hydraulic erosion corrosion) in that there are less U-shaped signs than random holes in the piping system. The problem of flux corrosion has been eliminated through the introduction of water-based fluxes, which are washed out of the piping system, thereby eliminating that corrosion opportunity, but remember that this change in flux was within the last few years, and many older systems were performed with the petroleum-based fluxes, which don’t simply wash out of the system when being flushed.

Generally speaking, if the system is on a city water treatment system, the utility purveyors are required to maintain a certain degree of hardness (Langlier Index) which lays down a protective patina of calcium. In most cases, this protective patina will lessen the effects of the acid and avoid long term damage to the piping system. In most of today’s systems, flux acid corrosion is non-existent due to the various different non-soldered pipe joining methods (ProPress and PEX) employed in the distribution systems.

An alternative to the ball valve is the use of calibrated flow reading devices applied to each branch of the circulation return system.

In some situations, the circulation return mains may be accessible, but as it pertains to balancing, the only possibility of balancing the flows out is dependent upon 1/2” ball valves. The problem with using a ball valve to attempt to balance out a system is that the ball valve doesn’t even begin affecting the flow through the valve until it is 7/8th closed. So, you only have between 7/8ths closed and fully closed to try and balance the flow out on a large system. This is a very difficult and labor-intensive process, and all of your hard work can be completely canceled by a technician who didn’t realize that you had gone through and attempted to balance the flow out using only the ball valves, who goes and opens the valves completely, effectively eliminating any balancing act that had been performed.

An alternative to the ball valve is the use of calibrated flow reading devices applied to each branch of the circulation return system. This allows the installer to actually “see” how much flow is being moved through that given branch. While this is much better than guessing at the position of the ball valve, it is still rather labor intensive and requires a lot of work to get the system perfectly balanced out in a manner that eliminates the possibility of HEC occurring within the system.

I’ve found that limiting the flow to a velocity below the HEC threshold generally result in significant drops in the required flow rate overall in gallons per minute. In fact, in the “ideal” situation, where the heat source is below the distribution system, with hot water having a propensity to rise, and cool water having the potential to fall, the required additional input from a circulator can be minimal at best. Unfortunately, the “ideal” system is as rare as hen’s teeth. Hence, the systems start out with a small circulator, and then — due to complaints from hot water users on the far ends of the system — it is retrofitted with a significantly oversized circulation return pump that ends up causing HEC within the system.

In an ideal system, the flow through the hot water mains created by the circulation return pump, to the ends of the system and the beginning of the circulation return piping system would be controlled based on the presence of hot water at the far ends of the supply mains, and the beginning of the return mains. Up until the last few years, this was nothing more than a pipe dream. However, recently some innovative engineers have come out with numerous devices that do just that. Once hot water is achieved at the beginning of the circulation return main serving a given riser, the flow is automatically restricted by the presence of heat. It’s essentially a non-electric thermostatic flow control valve, similar to the ones we’ve been using in space heating applications for many years, except that this one is sensing the water temperature instead of the air temperature. I will refer to this device as a TFC, which stands for Thermal Flow Control.

Tune in next month as we continue to look at the solutions to this very complex pipe eating problem.

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