Challenges in DHW Distribution – Part 1

Over the years, I have addressed some of the many problems associated with large, multi-story apartment buildings DHW production and distribution systems. I’ve addressed the many methods of creating a direct cross connect between the hot and cold water piping, which causes a severe shortage of DHW to the units that are near this cross connection.

I’ve found that some cross-connection problems can be masked by other problems common to the DHW distribution system. Depending upon the architectural considerations as it pertains to the building layout compared to the location of the physical plant, finding the “ideal” distribution system is rather rare. In fact, in my 42 years of doing this, I can think of one system that was piped to an “ideal” consideration, that situation being a perfectly balanced recirculation return system. That happened to be on a high-rise hotel, which was piped in a parallel reverse return method. This type of system if done correctly, is a self-balancing system with equally circulated flows going through each hot water distribution main, as well as even balanced flows through the recirculation loops. Most systems are piped in a parallel direct return method, and in some cases a pyramidic  type of parallel. Regardless of the piping methodology that was employed in the installation, unless it was done in a parallel reverse return method, it is going to be difficult at best to achieve a good and balanced flow to meet all of the hot water recirculation needs of the building, regardless of the use of the building whether it’s a hotel, a senior retirement center or a multi-family apartment complex.

Water is like your ex-brother in law: it is wet, lazy and stupid. Given an opportunity, it WILL follow the path of least resistance.

The property maintenance people will complain to the service company responsible for maintaining the DHW system about the lack of instantaneous hot water availability within the building, but in most cases if the “source of supply” is hot, most of the distribution system is buried within the structure, and the contractor tells the management team that there is nothing that they can do about the situation. Generally speaking, the building plans used in the construction of the building and its physical plant are no longer available, so a plan of attack can’t be developed to attempt to address the problem. In some cases, the contractor will install a circulation return pump of significantly higher capacity in an attempt to force flow further into the system. In some cases, this effort will help to alleviate the hot water availability issue, but in most cases, it actually ends up causing more damage to the piping distribution mains and the recirculation return piping due to a condition known as hydraulic erosion corrosion.

The best visual example of HEC (hydraulic erosion corrosion) that can be used to explain this condition to the consumer is the Grand Canyon. The mighty Colorado river has cut its way through all of the strata, physically eroding its way into the Earth. You DON’T want the Grand Canyon to live in your piping distribution system. Generally speaking, HEC raises its ugly head in the smaller diameter supply and return piping in the form of pin holes popping up on the piping, typically a few inches after an elbow or a tee.

Part of this problem has to do with bad habits of the piping installers not reaming their pipes to full bore, but most of it has to do with excess velocities being maintained in the piping systems. This is either due to an imbalance in flow, or excess velocities caused by the application of an oversized pump. Remember, water is like your ex-brother in law: it is wet, lazy and stupid. Given an opportunity, it WILL follow the path of least resistance in an effort to get back to the pump for another free ride around the loop. The combination of the water tripping over the un-reamed pipe and being pushed through the pipe at an excessive velocity on a continuous basis causes the pin holing to break through the walls of the pipe, resulting in a leak and the associated water damage.

If you want to verify that it is HEC, mark the exterior of the pipe showing expected direction of flow, then, after the fitting and failed pipe have been cut out of the system  carefully cut the piping and fitting in half length-wise so that you can see the inside of the pipe. Remember when looking at the internal erosion damage that “Horses always walk up stream”. You will see U shaped signs of erosion, from horses walking upstream.

Tune in next month as we continue looking at this complex problem and newly available solutions.

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