Your questions answered: System design back to basics for plumbing, mechanical engineers

In this Q&A, learn more about plumbing codes, piping and water systems

By Consulting-Specifying Engineer April 5, 2024
Webcast presenters explained how to calculate pressure zones in high-rise buildings, and used a water riser diagram as part of the example. Courtesy: Consulting-Specifying Engineer

Plumbing, piping and pumps are essential parts of a building’s design. From fire pumps to hydraulic systems, all commercial buildings have them, or some portion of them based on the building type and occupancy needs.

A model plumbing code available for design engineers is the International Plumbing Code (IPC). . It provides the minimum design criteria for code compliance for the design of plumbing systems. Furthermore, it is the basis for these systems to perform at a level to prevent water contamination and promote longevity.

During the webcast on Plumbing: System design back to basics for plumbing, mechanical engineers on Feb. 29, 2024, the presenters didn’t have time to answer all the questions. Review additional responses here.

Expert presenters:

  • Steven Smith​​​​, CPD, GPD, Associate Principal, Lead Plumbing Designer, Page, Houston
  • Thomas Mikulasovich, PE, CPD, LEED AP, Associate Partner, Syska Hennessy Group, Charlotte, N.C.

Question: Can vent pipes run at any slope just to keep condensate moving or is there a code minimum?

Steven Smith: Per IPC, the vent just must slope no minimum slope required.

Question: Can we pitch vent pipes at less than 1/8 inch per foot or even flat, with no pitch to avoid structural conflicts? Or does it depend on the state where we intend to install the gravity piping system?

Steven Smith: The vent pipe needs to slope.

Question: Can the presenter dig deeper into the the International Energy Conservation Code (IECC) impacts of the reduction in distances between the circulating hot water system and public lavatories?

Steven Smith: You can loop the hot water main down the wall or you can use flow diverter valves to circulate down the wall. Both add cost and complexity to the design.

Question: Why would overflows not be specified, when possible? Is the extra cost prohibitive?

Steven Smith: There is no way to clean the overflow area.

Question: I would assume there is a maximum flowrate for eyewash fixtures to minimize injury, is that correct?

Steven Smith: The fixtures are designed to flow the code 20 gallons per minute (gpm) amount and are tested to achieve this amount.

Question: Table 906.1 only applies to stack vents and vent stacks, not individual vents. Correct?

Tom Mikulasovich: That is correct. Sections 906.2 and 906.4 cover individual, branch, circuit and multiple branch vents.

Question: Why do we vent?

Steven Smith: Without vents, the sanitary system would not flow properly.

Question: Is it appropriate for air admittance valves to serve as a substitute for typical soil vent pipe terminations (with cowl)?

Steven Smith: Some jurisdictions do not allow air admittance valves. If they are allowed code dictates how they are to be used.

Webcast presenters explained how to calculate pressure zones in high-rise buildings, and used a water riser diagram as part of the example. Courtesy: Consulting-Specifying Engineer

Webcast presenters explained how to calculate pressure zones in high-rise buildings, and used a water riser diagram as part of the example. Courtesy: Consulting-Specifying Engineer

Question: The mentioned diameters in those tables are inner or nominal?

Steven Smith: Inside diameter.

Question: In your design experience, do you find that using larger drainage piping at a shallower pitch (1/8 inch per foot) or smaller drainage piping at ¼ inch per foot is the better design application for fitting drainage piping into tight spaces?

Steven Smith: Depends, but usually the slope is the biggest issue so bigger pipe with 1/8-inch slope usually works better.

Question: What is the most energy-efficient way to provide tepid water (hot water heater) to an emergency shower or multiple emergency showers in a building.

Steven Smith: Serve the emergency showers from the circulated domestic hot water main.

Question: Are there any benefits to using polyvinyl chloride (PVC) or cast iron for sanitary piping?

Steven Smith: Depends on the application, both have benefits just depends where they are located. Cast iron is quieter but more expensive. PVC does not meet the 25/50 flame smoke rating. The list goes on and on.

Question: What are the two devices for measuring flow in a compound meter?

Tom Mikulasovich: A positive displacement chamber measures low flow, while a turbine chamber records high flow.

Question: I am a novice in the design realm, but would like to expand my knowledge base. Do you have any recommendations on how I can develop my mechanical building design skills?

Steven Smith: There are many colleges and junior colleges that offer design classes. I would look in your local area and see what is available.

Question: What is the benefit of have two check valves for DCV backflow prevention. Is it redundancy only, or another reason?

Steven Smith: Code dictates what type of backflow preventer is used in what application.

Question: Please describe how the tank work in the picture you showed with booster pump.

Tom Mikulasovich: The tank is a hydropneumatic tank. It is pressurized and helps regulate system pressure. Having the tank results in the pumps not having to come on every time the system pressure drops from the use of a fixture because the tank will help regulate the system pressure. When the tank pressure is used up then the pump will come on.

Question: What are access requirements for WHAs?

Steven Smith: Usually, access is not required but needs to be verified with the local jurisdiction.

Question: Should we not factor in diversity anymore when sizing the water main?

Steven Smith: The charts have diversity already considered in sizing mains.

Question: When you say max press = 80 psi: Who is responsible for that? Does the water supply utility have a max requirement?

Steven Smith: The engineer is responsible that is why you would install a pressure reducing valve.

Question: Please clarify where backflow preventers be installed within a building’s plumbing system cross-connections.

Tom Mikulasovich: Anywhere the possibility of contamination of the domestic water supply can occur. For example, at HVAC makeup water connections, connections to ice machines, coffee makers, boilers and at hose bibbs.

Question: Is it advisable to put the water hammer arrestors when I have only when I have furniture with automatic closing valves or also when I do not have them?

Steven Smith: They are required at any fast-acting valve.

Question: When you add up fixture count, what kind of coincident flow percentage is recommended? In other words, how many faucets do you think flow at the same time?

Steven Smith: The tables in the code account for the coincident flow.

Question: What is 0.434 used for? Pressure at the base of a stack?

Tom Mikulasovich: It is 0.434 psi/foot. Each foot of water is equal to 0.433 psi. It is used for calculating static pressure, etc.

Question: Do you need a pressure tank for a variable speed booster pump?

Steven Smith: A pressure tank is usually provided.

Question: Does New York require plumbing to pick up meters (and put them inside) due to the temperature?

Tom Mikulasovich: New York City required water meters to be part of the plumbing scope of work because there is no place they can be located outside the buildings, in addition to the weather concerns.

Question: When the incoming water does not come in on the lowest floor or basement, how does that impact your calculations?

Steven Smith: The calculation is the same you just start from the level where the water is located.

Question: Can you please verify flushometer valve pressure?

Steven Smith: Usually 35 psi.

Question: What was the name of the American Society of Plumbing Engineers (ASPE) manual recommended for plumbing design?

Steven Smith: Illustrated plumbing codes design handbook available through ASPE.

Question: Is the ASPE book by Julius Balanco available in digital format?

Tom Mikulasovich: Refer to the ASPE website for available ebooks.

Question: Who is responsible for selection plumbing fixture, architect or plumbing engineer?

Steven Smith: The plumbing fixtures are shown on the plumbing documents but can be the architect, owner or engineer that picks the fixtures.

Question: In your example, instead of having two zones for water distribution, can we have one zone with booster pump at the supply and add reduction pressure valve on the lower floors?

Steven Smith: Yes, that is common for apartment buildings.

Question: Do we see any difficulties in using heat pump domestic water heaters? It is now a requirement in the Seattle Energy Codes.

Steven Smith: If they can meet the HW demand, you are good.

Question: How do you handle wastewater (sanitary sewer) in ultratall buildings?

Steven Smith: Code dictates the requirements.

Question: As plumbing engineers, do you specify gas regulators?

Steven Smith: Depends on who is doing the gas design, but usually yes.

Question: How much flow should be designed for the relief line on an RPZ backflow preventer?

Steven Smith: It depends on the pipe size but you should account for the full flow rate.

Question: What is the maximum cold water supply pressure water heater with expansion tank in commercial high-rise building?

Steven Smith: Water heaters are available for high pressure applications.

Question: Could you share your experience regarding the booster position in relation to the water meter?

Steven Smith: The pump can be before the meter of after depends on the design. Example you could have a main water meter then booster pump and individual water meters for each apartment unit.

Question: What is the maximum pipe length for hot water circulation system?

Steven Smith: There is no limit.

Question: Where is the best location of WHA in plumbing design and what is the code and sundered and, what is the best practice for location of WHA is?

Steven Smith: PDI-WH 201.

Question: What are your thoughts on press-on fittings? Megapress, Propress, etc. Compared to soldered, threaded, brazed fittings.

Steven Smith: Use them all the time have not had any issues if they are installed properly.

Question: What is your recommended method to reduce excessive noise radiating from wastewater or sanitary piping?

Steven Smith: You can insulate the pipe.

Question: Can anyone touch on the clean-out requirement topic?

Steven Smith: The requirements are in the plumbing code.

Question: For a commercial kitchen, what types of fixtures can use soft water and which fixtures must use domestic water?

Steven Smith: Usually, all the fixtures in a kitchen can use soft water.

Question: Would coated cast iron work better on grease waste for better flow?

Steven Smith: flow is not the issue; corrosion of the pipe is the concern.

Question: What is the maximum horizontal length of a vent pipe?

Steven Smith: Depends on the fixture count and the pipe size refer to the plumbing code vent size table.

Question: You mentioned floor mount urinals were one option. I thought they were not allowed anymore for sanitary and safety reasons? Would this be a local requirement?

Tom Mikulasovich: Local requirements would dictate if the fixture is not allowed by code.

Question: The Facility Guidelines Institute (FGI) update for 2022 reduced the maximum dead leg of a hot water recirculation line from source to fixture from 25 to 10 feet. Are projects seeing huge cost increases due to all the additional hot water recirculation lines that this requires? Do architects need to be more aware of this code change to adjust layouts to reduce further fixtures?

Tom Mikulasovich: I would assume there is some cost increase due to the increase in piping material. There should be coordination with the architects because the wall thicknesses may need to be increased due to the additional piping now within the wall.

Question: When you mentioned meter can go backward, does it mean it counts backward too?

Tom Mikulasovich: Yes, from the project I was working on the meter reading went backward on the dial. The manufacturer informed me that the device was not malfunctioning but if the flow goes backward the dial will go backward.

Question: Where is the logical place for water hammer?

Tom Mikulasovich: At the end of the branch line between the last two fixtures.

Question: Since the water requirements for the water restroom fixtures have decreased, will the water supply fixture unit calculation numbers get modified?

Tom Mikulasovich:  There is talk in the industry of revising the water supply fixture units (WSFU) based on today’s fixture flow rates. International Association of Plumbing and Mechanical Officials (IAPMO) has released a calculator for residential and is currently working on a commercial calculator as well.

Question: What about high-density polyethylene for hot greasy waste?

Steven Smith: I have no experience using this for grease waste.

Question: Do you separate DOM risers based on zones (street pressure zone, booster zone 01, booster zone 02, etc.)? This may be displayed on risers shown.

Steven Smith: Yes, that is how we show the different zones.

Question: How does domestic hot water (DHW) booster calculation or design differ from domestic chilled water (DCW) booster pump example provided? Do you prefer DCW booster pump only, or separate DHW booster?

Steven Smith: I serve the water heaters with the boosted domestic cold water to ensure the pressure of both systems are the same.

Question: Are there any cost-efficient materials for elevated temperature for sanitary discharge as an alternative to C.I.?

Steven Smith: Not that I am aware of. I use quenching valves to lower the discharge temperature to below 140. In kitchens the high temperature can be an issue just depends on how they are operating the kitchen and what equipment they have selected.

Question: Are there any limitations for usage of stainless-steel piping in lieu of copper for domestic water piping?

Steven Smith: Cost can be an issue.

Question: Should you account for hose bibbs or makeup water supply above the roof level when sizing the booster pump?

Steven Smith: Yes, you use the highest fixture or equipment pressure requirement.