Things You Know

By Timothy Allinson, P.E.

Murray Company, Long Beach, Calif.

Once upon a time, at a personal growth seminar called "The Forum," I learned that information can be classified three ways: 1) things you know you know; 2) things you know you don't know; and 3) those pesky things that you don't know that you don't know. The third category is dangerous territory. If I am able to discover all the things I don't know that I don't know about this industry before I retire, I suppose I will have achieved plumbing nirvana.

For example, I know that I know a great deal about UPC, and I know that I don't know a great deal about IPC. The other day, I was reviewing plans with a Los Angeles plan checker. It is an arduous process, but once in a while you discover something that you didn't know that you didn't know, which makes the plan check process quite valuable.

UPC Table 6-7

One thing I discovered that I didn't know that I didn't know is depicted in UPC Table 6-7, "Flushometer Fixture Units for Water Sizing Using Table 6-5." I have seen this table many times, but dismissed it repeatedly since I don't size my systems using the simplified Table 6-5. However, there is more to Table 6-7 than meets the eye.

Since Table 6-5 is based on Hunter's Curve No. 2 (systems with flush-tank water closets), you cannot arbitrarily throw a few flush-valve water closets into the mix. This came up on a recent project-a high-rise condo with flush-tank water closets and two flush-valve toilets off the lobby. In this case, I learned that since the system sizing for this building assumes flush-tank fixtures, the two flush valves on the ground floor had to be assigned 40 WSFU each as dictated by Table 6-5. As a matter of course, I had supplied these fixtures with 1.5-in. piping. However, with 80 fixture units added to the sizing process, the 1.5-in. pipe grew in size to 2 in. While it might be overkill, this is what the code dictates in a roundabout fashion.

The Bottom of Hunter's Curve

One source of local debate is generated by the fact that Hunter's Curve stops short of the low-end values of fixture units vs. flow (Figure 1). The way the curve is drawn it is impossible to convert the demand of tank system (curve 2) with less than 7 WSFU to gpm, or the demand of flush valve system (curve 1) with less than 10 WSFU to gpm. The curves need to be extrapolated to zero (Figure 2). The base of curve two is fairly close to a 1 to 1 ratio: 1 WSFU equals about 1 gpm; 2 equals 2, and so on. As you work up the curve, the inherent diversity weights the values toward the fixture units scale, and this is clear when the demand exceeds 7 fixture units.

I recently applied this logic to a project as it pertained to 1/2-in. pipes serving single lightweight fixtures such as lavatories and sinks. While the design worked in theory, what I didn't know that I didn't know was that the local plan check department had its own interpretation of the fuzzy section of Hunter's Curve-and their interpretation was a great deal more conservative than mine. They had produced their own chart that was extremely restrictive for the use of 1/2-in. pipe. No matter what I argued, if it didn't fit with their chart, they wouldn't accept it.

Yoke Vents

The UPC section 907.0, like most other codes, has a requirement for yoke vents connecting the waste and vent stacks in high-rise systems. What I didn't know that I didn't know is that IAPMO, as well as many local agencies, has an interpretation that the yoke vent must rise vertically and must turn horizontal until it is 42-in. above the floor. As this is not written in the code, I have no idea how this interpretation came to be. In most cases it is not a big deal, but, when a yoke vent is to be located where a stack offset occurs, major problems can result.

Static Gain & Friction Factors

Perhaps I have been naïve, but prior to joining my present firm, I never distinguished between the sizing of down-fed water risers and the rest of the system piping. After joining Murray Co., I learned from Dick Drenske (founding member of ASPE) that the office standard is to create (at least) two sizing charts-one for horizontal and up-fed piping based on the calculated friction factor, and another for the sizing of down-fed risers based strictly on velocity. I am embarrassed to admit that I didn't know that I never realized this distinction.

Water gains 0.433 psi/ft. in a down-fed system, which is equal to 43.3 psi/100 ft. This is a very large friction factor-one that exceeds the maximum friction factor for piping sized based on velocity. For example, a 3/4-in. pipe at 8 ft./s experiences a loss of about 15 psi/100 ft. This is approximately one-third the available friction factor based on static gain. As the pipe size increases, the velocity-related friction factor decreases, so there is never a condition where the static gain rule does not hold true.

Gas Regulator Relief Vents

Having spent most of my career in New York City, where the gas utility delivers only low-pressure gas, I had my first experience with gas regulators when I moved to California. Their selection is not terribly complicated, and they are usually outdoors so relief venting is not an issue. Recently, I had my first exposure to a gas regulator inside a building with a relief vent that had to be piped 12 stories up to the roof. "How do you size that pipe?" I wondered. Another thing that I didn't know that I didn't know.

Unlike the gas piping itself, gas-relief vents are not governed by code. Even manufacturers balk at sizing them. After conducting some research, I came up with two rules of thumb-one twice as conservative as the other. The less-conservative rule-the one I decided to use-is described here.

The first 20-ft. of relief vent piping can be sized to match the port size (i.e., a 1-in. pipe for a 1-in. relief port). If the relief is longer than that, it should be increased one pipe size for each additional 20-ft. In my case, the port was 3/4 in. and the pipe was 120-ft. long. Since 120 is five orders of 20 greater than the first allowance of 20, I made the relief vent five pipe sizes larger than the port size-or 2.5 in.

Combining Hot & Cold Fixture Units

Lastly, I recently had a fairly unique design condition that threw me into the territory of something I didn't know that I didn't know. This particular building had a condition where a cold-water supply line supplied the hot-water heaters feeding the entire building, but the cold water to only half the building. How do you combine all the hot-water fixture units with the cold-water fixture units for half the building? I had never had this situation before.

If you combine totals directly you wind up with excess fixture units-possibly more than the entire building. The situation was further complicated because I had applied the 75% rule allowed by the UPC (for hot- and cold-water fixtures, the fixture unit load for each service can be reduced to 75% of the value stipulated in the fixture unit chart).

This is confusing, so let's assign some numbers to make it clear. The pipe in question had a connected load of 2,133 cold-water fixture units (CWFU) serving the upper building zone, and 4,036 hot water fixture units (HWFU) to the water heaters that supply the entire building. The sum of these two values is 6,169 WSFU. However, the entire building only has 6,141 WSFU, and it does not make sense that this pipe should have more fixture units than the pipe serving the entire building.

To resolve this issue, I used the 75% rule and a washing machine. The washer had a total of 4 WSFU. When hot and cold are separated, there are 3 CWFU and 3 HWFU. If you combine these directly they total 6 WSFU, which is greater than the actual value of 4. But, if instead you add one-third of the hwfu to the cwfu you get the correct value of 4. I applied this logic to the pipe example above, and added one-third of 4,036 hwfu to the 2,133 cwfu to get 3,478 wsfu for the combined load. This seemed a much more accurate expression of the actual demand on the pipe than the inflated value using direct addition. It made sense to me anyway.

Unfortunately the plan checker didn't buy into my logic. He took the more-conservative route, and insisted that the hot-water demand should be added directly to the cold-water demand. If the total exceeds the total cold-water demand for the building, then he allowed the use of the lesser value. This results in a pipe that is larger than it needs to be, but it is hard to use logic with building officials to solve issues that are not addressed in the code.

I hope you will find these examples useful. Do you have examples of things you didn't know that you didn't know? Perhaps you could write to us and share them.

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.