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I love getting feedback from readers. Not only is it interesting to learn what others have to say, it is evidence that I am not completely wasting my time writing these articles.
Back in June, I wrote an article about the rational equation, namely: Q = ciA.
This equation calculates the flow from a rainfall event. In short, “Q” is the calculated flow in CFS (cubic feet per second), “c” is the unitless coefficient of imperviousness, which is generally 1 for rooftops (hard surfaces) “i" is the rainfall intensity in inches per hour, and “A” is the area receiving rainfall in acres. I poked fun at this equation, calling it the irrational equation, because it made no sense to me that inches per hour times acres should produce CFS. That was short sighted of me, and it resulted in more reader feedback than I have ever received about an article.
Many readers pointed out to me that if you do a simple conversion of units this equation is, in fact, rational. Here is the conversion.
For all intents and purposes, 1.008 ft3/s equals 1. So, the units do, in fact, make sense and I have egg on my face. Thanks to all those who emailed me with this rationalization.
Another reader emailed me a question about ASSE 1070 mixing valves and public lavatories. The UPC reads:
413.1 Limitation for Hot Water Temperature for Public Lavatories. Hot water delivered from public-use lavatories shall be limited to a maximum temperature of 120F by a device that conforms to ASSE 1070, Standards for Water Temperature Limiting Devices, or CSA B125.3, Standard for Plumbing Fittings. The water heater thermostat shall not be considered a control for meeting this provision.
The IPC has similar verbiage. This code section endeavors to prevent scalding at public lavatories in the event a water heater’s thermostat goes haywire and allows the water heater to produce scalding hot water. The UPC has the same requirement for bathtubs and whirlpools (I’m surprised they used a brand name there) in section 414.5 for tubs that have deck mounted bath fillers [without pressure or temperature balanced control valves]. This latter parenthetic statement is my own verbiage, not that of the code, to express what I believe to be the intent of the code.
Getting back to the 1070 valve requirement at the public lavs, some people interpret this to mean that every public lav has to have a 1070 valve. That is not the case. One 1070 valve can serve a battery of lavs as long as it is sized accordingly.
The issue of hot water delivery is a complex one. In today’s world of water conservation, public lavs are typically specified with flow restrictors that have flow limitations as low as 0.4 gpm or less. Further, the run time on the faucet cycle is often set to 7 seconds, so each faucet use produces less than 0.05 gallons of water. That’s just 6 ounces. And, since the water to the faucet is generally tempered, only a portion of that 6 ounces is hot water – perhaps 4 ounces or so. How is hot water ever expected to reach the lavatory?
The only practical way to deliver hot water under these circumstances is to circulate the hot water in the plumbing chase, which can make circulation systems very complicated. The more circulation loops that are required, the harder the system becomes to balance and the more expensive it is to install. Heat tracing might be the more practical means to achieve hot water delivery, or a hybrid system could be the best compromise, where the main is circulated and the branches to the fixtures are heat traced. However, with plastic piping materials becoming more popular, heat tracing would not be an option for plastic piping.
In high-rise buildings with vertical water distribution, you have the option of a circulated hot water riser right behind the fixture(s), and this is a good solution to fast hot water delivery. But, for large low-rise buildings with fixtures scattered all over the floor, hot water delivery remains a challenge. If a private toilet room with just one water closet and lavatory is located in a remote area, a local electric water heater might be more practical than dragging the hot water loop all the way to that one fixture. The only down side is that instantaneous electric water heaters use a lot of power – generally 3 kW or more. Utilizing too many of these will then create problems for the electrician and an increase in electrical use.
Lastly, another reader emailed me about hot water for emergency showers. Providing hot water to emergency showers can be approached many different ways. First, it depends on how cold the cold water supply is. Here in So Cal our cold water is 60°F or higher, so we can meet the ANSI requirements for tepid, which is from 60°F to 100°F, without heating the water at all. But, some specs call for the water at emergency showers to be 85°F, so that would require hot water.
Second, if you have multiple emergency showers, do you assume they are all operating at the same time, or just one at a time? In general, I assume one at a time, but it is conceivable that there could be an accident involving more than one person. If you have a large central hot water system, this is not an issue. But, if it is a facility with limited hot water demand – perhaps just a pair of toilet rooms and a pantry – then the emergency showers will dictate the size of the water heater.
First, I would confirm with the user that the system will be sized for just one emergency shower operation (per hour). Second, confirm the water temperature. We had a recent project where the specs called out for 85°F, but it turned out the user wanted 60°F because warm water would accelerate the reaction of the type of chemical accident they anticipated.
The shower is required to produce 20 gpm for 15 minutes, or 300 gallons. Since the water is tempered, roughly half of that 300 gallons will be hot water, depending on your cold water temperature. So, the water heater will have to produce at least 150 gallons of hot water (per hour) at 120° F or higher. To that you have to add your domestic load, because in theory you do not want the domestic usage to compromise your emergency supply.
Some manufacturers make self-contained systems that have dedicated hot water supplies. This can be costly, so it should be addressed as another question for the user. The downside of these products is that since the emergency showers are rarely used except for testing, the water heater produces a lot of standby losses of wasted energy. In addition, the water might become a little stagnant since testing is infrequent and there is no regular flow through the water heater.
Thanks again to all the readers who provided the feedback to inspire this article!
Addendum: After finishing this piece I heard a comedy bit by Demetri Martin on XM radio, Comedy Central Channel 95. Demetri has a very funny piece about electronic plumbing fixtures that you can find by Googling "Demetri Martin People Watching" - it's the latter half of the 5-minute piece. Anyway, in the bit I heard while driving home, Demetri was joking about faucet handles. "You turn on the C and cold water comes out. You turn on the H and more cold water comes out. So, I understand that the C stands for Cold. But what does the H stand for? I must be Ha Ha Sucker!" I found the statement ironically appropriate.
Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He 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. Allinson is a past-president of aspe, both the New York and Orange County Chapters. He can be reached at firstname.lastname@example.org.