A Brush with Grayness

By Timothy Allinson, P.E.

Murray Company, Long Beach, Calif.

One of the more memorable evenings of my single life in NYC was a date I had with a lovely gal whom I took to a club in lower Manhattan called The Tunnel. The Tunnel was very unique: It was constructed at the end of a long-deserted NYC subway tunnel (hence its name) that served as the focal point of the subterranean dance floor. The tunnel itself had been renovated to serve as a visual conduit for a LASER light display projected from a point far down the tunnel toward the dance floor. It was very dramatic, very effective.

At one point during the evening, my date and I became separated, as was often the case during bathroom breaks at NYC clubs. As I walked away from the mouth of the tunnel in an effort to find my date, I passed a woman (and her entourage) whom I quickly recognized as Deborah Harry (aka  Blondie, of 1980s rock star fame). As we passed, she gave me a look that made my knees weak, and I thought (all too presumptuously), "I must go talk to her!" As I turned to pursue Ms. Harry for conversation, I ran smack into my date, so, being a gentleman, I focused my attention on her, and all attempts at further communication with the infamous blonde were thwarted.

Since then I have seen Ms. Harry in recent interviews, and her blonde hair of days gone by seems much grayer, as does a percentage of my own hair. I continually tell my wife that the gray hairs are just very blonde strands, but I think the excuse is getting old -- nearly as old as the follicles themselves.

This diatribe about grayness reminds me of a story about my old boss, Robert Benazzi, of JB&B fame, the engineering firm that designed the original WTC, now engineering the new WTC. Twenty-odd years ago, Benazzi and I went to a meeting with the NYC Department of Buildings to argue a technical issue. I opened the dialogue, but was quickly shushed by the building official. He looked at me and said, "I don't want to hear you speak. I want to hear him speak (referring to Benazzi)." He quickly explained, as he pointed to his temple, "I respect what he has up here ... I don't mean his brains ... I mean his gray hair ... his experience."

At the time I was insulted (even though we won our argument with the official), for being the object of prejudice because of my youthful appearance. Since then, I have realized that, no matter how smart you are, there is no substitute for gray-haired experience. These days, my chest hair is mostly gray, as are a few strands on my head, so I consider myself an unofficial member of the gray-haired club. Perhaps I have arrived, perhaps not yet. But regardless, there is still no substitute for experience, and, even in my relative youth, I will be the first to admit that fact.

In early December I had the opportunity, through the Orange County chapter of ASPE, to hear the new ASPE president, Julius Ballanco, speak at an OC chapter technical seminar. It was very gracious of Julius to speak, since his schedule was packed with bicoastal commitments. But he was there, carrying a wool overcoat in 80-degree heat, which served as convincing evidence of the time and temperature extremes of his travels.

Julius, as most of you know, is a man of gray-haired distinction ahead of his time. Prematurely gray, I think his hair serves as proof of his cerebral acumen. He gave a four-hour speech on a two-pronged subject: venting systems and hot water recirculation. I will attempt here to give a condensed summary of both topics, with my subjective commentary in italics.

Venting systems

The primary purpose of the sanitary venting system is to protect the trap seal at the fixtures by relieving pressure fluctuations. Positive pressure surges would act to blow the trap seal out of the fixture, whereas negative pressure will act to siphon the trap seal dry. Loss of the trap seal will allow sanitary gases to enter a building.

Water flowing down a stack drags air with it. This air must originate from somewhere, hence the need for the vent system. As air is dragged down it produces negative pressure upstream and positive pressure downstream. The pressure fluctuations are widely held as mostly negative, but the positive pressure at the base of a stack can be much more dramatic than the negative pressure experienced upstream.

Pressure fluctuations in the stack can vary from -7" w.c. to +7" w.c. Branch vents act to prevent the fixtures from seeing these pressure fluctuations. Horizontal flow creates only minor pressure fluctuation. Julius noted that the standard bathroom group produces only 0.9 psi in pressure fluctuations in the horizontal drain. This can be alleviated with a vent as small as a drinking straw.

Julius went into a lot of detail about the various types of vents (individual, common, wet, combination, island, bathroom group, stack, circuit, relief, Sovent and the Philly single stack). Every code has restrictions on these various venting methods, so be sure to read the code that applies to your project or territory.

Julius discussed Air Admittance Valves (AAVs) at length. I have not personally had any experience with these devices, since I've never worked in a jurisdiction where they are legal, and I have always held a great deal of skepticism about them. I learned about the limitations of AAVs that were previously a mystery to me. These limitations include being effective only in installations up to five stories in height. Anything greater than that requires a vent from the furthest fixture to a stack. Stack AAVs are limited to seven stories. I guess my skepticism was both warranted and relieved by learning of these limitations. Keep in mind that most of my experience has been in high-rise design, where these devices would not be applicable.

Hot water recirculating systems

First, I am confused by the term "recirculation." I know of hot water return and hot water circulation, but hot water "recirculation" seems redundant. It is a circulatory system, not a "recirculatory" system. Even Spell Check dislikes the word. Maybe I should try Spell Recheck.

Anyway, hot water circulation is one of two methods of keeping the hot water hot in long pipe runs, the alternative being electric heat trace cable. ASHRAE and the International Energy Code limit hot water dead-ends to 100 feet, while most other codes are silent on the length limit. Even 100 feet is far longer than good design practice would stipulate. 100 feet of 3/4" copper contains more than 2.5 gallons of water that would be wasted as the user waits well over a minute for the arrival of hot water at the kitchen sink.

Julius recommends keeping circulation velocities down to one or two ft/sec, since hot water is aggressive toward copper and velocity acts to increase corrosion. These low velocities have the added benefit of reducing the required head of the circulation pump, but this is at the expense of larger copper tubing.

Julius reviewed fairly detailed calculations for determining circulation flow rate based on heat loss. I will not go into detail here since I covered the subject in detail in my September 2003 article. Suffice it to say that the circulation must be sufficient to compensate for heat losses between the heater and the furthest fixture with a tolerable temperature loss of five degrees, sometimes stretched to 10 degrees.

Julius described the circulation pump head requirement as "almost zero feet." I take exception to this statement. The head requirement is the friction loss experienced in the longest circulated loop. In a large building this can be significant. As I mentioned in my September 2003 article, I have been guilty of designing circulation systems too much by the book as Julius presented in his talk. In reality, the contractor is stuck having to balance the system. If the flow and head of the pump is designed too precisely, the system will never achieve balance. A good rule of thumb is to allow at least 0.5 to one gpm per loop or riser, depending on the size of the building. Larger systems will require more flow through each loop to compensate for heat losses. This rule of thumb serves also as a much faster way of designing the system.

Julius touched briefly on my favorite subject, summarized in my February 2005 article, that of combining the circulation of systems operating at different pressures. Julius stated that such a design can work if PRVs and pumps are selected properly, but I disagree. When circulation systems of differing pressures are combined as one, the systems will never stay in balance, no matter how carefully the pump heads and PRVs are selected. That's my story and I'm sticking to it. Never circulate through a PRV.

Julius also spoke briefly about a new hot water circulation system that uses 3/8" diameter PEX tubing that runs inside the main hot water supply pipe, thus reducing heat losses. It seems to be a fascinating system that will be the subject of a future article once more information becomes available about this new product.

Lastly, Julius gave an interesting example of a hospital with a unique hot water circulation system. In this particular hospital the system was configured for an off-hours sanitization cycle. During the middle of the night, the system would send a signal turning off all hot water fixtures electronically so that no hot water could be used. It would then circulate 140 F water through all the loops to kill any Legionella that might be growing in the piping. This is certainly a very unique and proactive approach to controlling Legionella, but I would expect it to be rather costly from a control standpoint.

Julius' seminar was very interesting, even though I did take exception to a few of the things he said. I guess no two engineers will ever see 100% eye-to-eye. We all have our own unique experiences that form our opinions and theories as to what we consider optimal design.

Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design-build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed Accredited Professional. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.