Roger W. Griffith, P.E.

Flexible Air Ducts versus Flexible Air Connectors – What Is the Difference?

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Flexible Air Duct

The mechanical codes cover both flexible air ducts and flexible air connectors. They are similar in appearance, but they are not the same.

A flexible air duct conveys air in an HVAC duct system. A flexible air duct typically connects a main trunk to an air inlet or outlet. The mechanical code does not limit their length. However, design requirements typically limit the length of flexible air ducts to six feet. This limitation avoids high-pressure drops and kinking in long sections of flexible air ducts.

A flexible air connector is designed to connect different duct system components, e.g., connecting equipment to ductwork. Flexible air connectors are meant to absorb vibrations or slight misalignments and are limited in length to 14 feet.

The “flexible air ducts” listing label is rectangular and states “Listed Air Duct.” The listing label on “flexible air connectors” is circular in shape and states, “Listed Air Connector.” Both are listed to UL 181 Class 0 or Class 1 requirements.

International Mechanical Code (IMC) Requirements, 2024 edition

Definitions

FLEXIBLE AIR CONNECTOR. A conduit for transferring air between an air duct or plenum and an air terminal unit or between an air duct or plenum and an air inlet or air outlet. Such conduit is limited in its use, length, and location

Chapter 6: Duct Systems

Section 603: Duct construction and installation

603.6: Flexible air ducts and flexible air connectors

603.6.1: Flexible air ducts.

Flexible air ducts, both metallic and nonmetallic, shall be tested in accordance with UL181. Such ducts shall be listed and labeled as Class 0 or Class 1 flexible air ducts and shall be installed in accordance with Section 304.1.

603.6.1.1: Duct length

Flexible air ducts shall not be limited in length.

603.6.2: Flexible air connectors.

Flexible air connectors, both metallic and nonmetallic, shall be tested in accordance with UL181. Such connectors shall be listed and labeled as Class 0 or Class 1 flexible air connectors and shall be installed in accordance with Section 304.1.

603.6.2.1: Connector length.

Flexible air connectors shall be limited in length to 14 feet.

603.6.2.2: Connector penetration limitations.

Flexible air connectors shall not pass through any wall, floor, or ceiling.

603.6.3: Air temperature.

The design temperature of air to be conveyed in flexible air ducts and flexible air connectors shall be less than 2500F.

603.6.4: Flexible air duct and air connector clearance.

Flexible air ducts and air connectors shall be installed with a minimum clearance to an appliance as specified in the appliance manufacturer’s installation instructions.

Section 607: Duct and transfer openings.

607.7: Flexible ducts and air connectors.

Flexible ducts and air connectors shall not pass through any fire-resistance-rated assembly.

Chapter 3: General Regulations

Section 304: Installation

304.1: General

Equipment and appliances shall be installed as required by the terms of their approval, in accordance with the conditions of the listing, the manufacturer’s installation instructions, and this code. Manufacturer’s installation instructions shall be available on the job site at the time of inspection.

Does the Plumbing Code Prohibit Trap Seal Devices?

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Trap seal
Trap Seal Device

Drain traps must be prevented from losing their water seals. Loss of the water seal allows sewer gas to enter the building.

The International Plumbing Code (IPC) Section 1002.3, 2018 edition, discusses the types of prohibited traps. Specifically, the first type of trap that is prohibited is “Traps that depend on moving parts to maintain the seal.” Some interpret this language to prohibit the use of trap seals.

The Code Commentary states that “a trap is intended to be a simple U-shaped piping arrangement that offers minimal resistance to flow. Prohibited traps do not have a simple U-shaped design or configuration.” A P-trap with a trap seal still has the simple U-shaped design or configuration. Regarding this first prohibition, the Code Commentary states, “Item 1 refers to mechanical traps that use moving parts such as floats or flappers.”

Note that this code language is in reference to traps, not trap seals.

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ASSE 1016 Shower Valves

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Showers pose a risk for hot water scalding.  In addition to the potential for hot water scalding, thermal shock from rapid changes in shower water temperature can cause slips and falls.

To protect the user, plumbing codes require water delivery from showers to not exceed 120 degrees F. Water heater thermostats are no longer considered a safe means for controlling water temperatures when the code requires a maximum water temperature at showers and tub/showers. The plumbing codes now require the installation of the American Society of Sanitary Engineers (ASSE)  1016 shower valves to prevent scalding and thermal shock.

ASSE 1016 contains requirements for three different types of shower valves.

  • Type P: Pressure compensating. This is the most common type of shower valve.
  • Type T: Temperature compensating
  • Type P/T: Pressure and temperature compensating

Design engineers have to decide what type of ASSE 1016 shower valves to specify. First, for individual showers and tub/showers, a Type P pressure compensating valve is required to prevent thermal shock. Unless thermal shock protection is provided by another device, such as an ASSE 1066 pressure-compensating device, a Type P or Type P/T valve is required.

Second, the question becomes, is the additional thermostatic protection provided by a Type P/T shower valve needed? This often depends on the hot water system and the amount of temperature variation that can be expected at the shower fixture. Some hot water systems have large temperature variations, for example, residential storage-type water heaters with no recirculation. Due to the phenomenon of  “thermal stacking,” hot water temperatures in these systems can easily vary by as much as 10 – 15 degrees F. Another consideration is whether the facility has a higher risk for scalding, such as hospitals, nursing homes, and assisted living centers. In these instances, a Type P/T shower valve is a good idea.

By contrast, some commercial hot water systems with master mixing valves, continuous recirculation, and/or precise controls may have nearly constant hot water temperatures at the shower fixtures. For these systems, a Type P shower valve may be sufficient to limit temperatures at the shower to a maximum of 120 degrees F.

Here is a conservative approach: if system temperatures are not stable or unknown, install a thermostatic device (ASSE 1017 master mixing valve in conjunction with ASSE 1016 Type P shower valves, ASSE Type P/T shower valves, etc.) at some point between the water heating source and the delivery point. Although this is not a code requirement, maintaining a maximum temperature of 120 degrees F is. A thermostatic device will greatly reduce temperature swings in the hot water system and also protect the end-user in case of a failure of the water heater thermostat.  Design engineers should evaluate the hot water system and make an informed decision about scald protection. Specifying what “everyone else does” or “what meets the code” may not provide the required level of safety needed to prevent scalding.

Given the allowable temperature variation in the ASSE 1016 standard, at what temperature should shower valves be set to ensure a maximum water temperature of 120 degrees F? Outlet temperatures for ASSE 1016 valves can vary by +/- 3.6 degrees when tested at 45 psi. At higher operating pressures, the temperature swing could be greater than the +/-3.6 degrees F. Therefore, I recommend the high-limit stops on shower valves be set to provide a discharge temperature of 110 degrees F. If a shower valve is set to 110 degrees F, the actual discharge temperature may vary from 105 to 115 degrees F. This temperature range provides a comfortable shower without the risks of hot water scalding.

Options: integral check stops:  Always specify shower valves with integral check stops to prevent the hot water from crossing over into the cold water piping.

Plumbing specifications should require high-limit stops to be set at the time of installation. The design engineer should also check the hot water temperatures during site inspections prior to project completion.



Sleeper Sofas and Gas Appliances


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Sleeper sofa


Gas appliances could pose a threat to people when they are sleeping, unaware of any impending danger. The threats include oxygen depletion and elevated levels of nitrous oxide or carbon monoxide. For this reason, the fuel gas codes restrict the use of gas appliances in sleeping rooms.


 


The 2018 International Fuel Gas Code (IFGC), Section 303 – Appliance Location contains the following criteria.


 


303.3 Prohibited Locations. Appliances shall not be located in sleeping rooms, bathrooms, toilet rooms, storage closets or surgical rooms, or in a space that opens only into such rooms or spaces, except where the installation complies with one of the following:


 


There are six exceptions listed for paragraph 303.3. These six exceptions allow gas appliances in these prohibited locations when specific conditions are met.  [Read more…]
Careful! The Wrong Showerhead May Scald You


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Shower head


If not specified and installed correctly, showerheads can cause scalding!


Currently, standard shower valves and showerheads have a minimum flow rate of 2.5 GPM. There are water-saving shower valves and showerheads available with lower flow rates. WaterSense showerheads are rated at 2.0 GPM. There are also showerheads available that have 1.5, 1.0, and even 0.5 GPM flow rates. These various options for minimum flow rates in shower valves and showerheads create an opportunity for misapplication and dangerous results.


When the flow rate of a showerhead is less than the minimum flow rate of the corresponding shower valve, the potential for scalding increases for the following reasons: [Read more…]


Roger W. Griffith appointed to ICC Plumbing/Mechanical/Fuel Gas Code Action Committee


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ICC Code Committee meeting


Roger W. Griffith has been appointed to the Plumbing/Mechanical/Fuel Gas Code Action Committee (PMGCAC) of the International Code Council (ICC). The role of this code committee is to review and propose revisions to the International Plumbing, Mechanical, Fuel Gas, Private Sewage Disposal, and Swimming Pool and Spa Codes and the International Residential Code, chapters 12 through 33, which covers the mechanical, plumbing, and fuel gas systems in residential occupancies. I am honored to be appointed as a member of this committee for the upcoming 2018 – 2019 code development cycle.


These important codes define how mechanical, plumbing, and fuel gas systems must be designed and installed to protect the health, safety, and welfare of the public.


Tools and Resources for Mechanical Insulation


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National Insulation Association (NIA) - logo


When designing mechanical systems, engineers and designers often need help specifying the type and thickness of insulation for ducts and piping. Good resources are always valuable and welcome when time constraints leave little time for items like insulation and other accessories.


The National Insulation Association’s (NIA) new redesigned website is a helpful resource for all things insulation, from training, design helps, and specifications. Check out their new website and bookmark it for return trips. Enjoy!




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Thermal Expansion Tanks: Part 3


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Selection and Installation of Thermal Expansion Tanks 


thermal expansion tank support


See Part 2 for calculating the expansion tank volumes. Now, using the calculated acceptance volume and total volume for the tank, select an expansion tank from the manufacturer’s data that meets these requirements.


Be aware that in most states, unfired pressure vessels larger than a certain volume must have an ASME stamp. Check your state’s requirement for this limit. If the expansion tank you need is larger than this limitation, it must be constructed in accordance with ASME Section VIII requirements. ASME rated vessels are much higher in cost than non-ASME vessels. Installing multiple smaller tanks, which are below the volume limitation for ASME tanks, is also an option. Most tank manufacturers offer both ASME and non-ASME thermal expansion tanks.


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Thermal Expansion Tanks: Part 2 – Sizing


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How To Size Thermal Expansion Tanks For Hot Water Systems


thermal expansion tank


In Part 1 of this series, we looked at where thermal expansion tanks are needed. When it is determined that a tank should be included in the plumbing system, the next task is to determine the correct size for the tank. Referring to sizing tables in an expansion tank manufacturer’s catalog is the easiest method, although not the best method, for sizing thermal expansion tanks. Sizing tables provided by the manufacturers, give the tank model number and size based on the water heater’s volume and the system supply pressure. These tables, however, are based on three important assumptions that designers need to be aware of.


First, for most manufacturers, the tables are based on a maximum allowable line pressure of 150 psi. This is the maximum allowable working pressure of most water heaters and thermal expansion tanks. It is also the setting of the water heater relief valve. In other words, the thermal expansion tank, if selected by the tables, could accommodate the thermal expansion up to a system pressure of 150 psi, which is the relieving point for the water heater relief valve. Remember that the purpose of the thermal expansion tank is to avoid the relief valve from relieving. Sizing the thermal expansion tank for a maximum allowable pressure of 135 psi, which is 10% below the relief valve set point, is a better alternative.


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Thermal Expansion Tanks: Part 1 – The Need


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thermal expansion tank with water heater


When are thermal expansion tanks needed and/or required in plumbing systems?


When heated, water expands. We know that. When water is heated in a water heater, the water expands, if possible. But what happens when water is heated in a water heater and there is no hot water usage?


This can occur after a period of showering/bathing during peak times followed by a period of no hot water usage. For example, evenings are a time when there is hot water usage for showering followed by a period of no usage while people are sleeping during the night.


During peak usage times, hot water stored in the water heater is distributed to the end-users, and cold water fills the water heater. The cold water in the tank is then heated, but when there is no subsequent hot water usage, the heated water has no room for expansion. If there is no place for the expanded water to go, the pressure in the system increases significantly. This leads to one of two possible scenarios:


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Duct Smoke Detectors – Locate In Supply or Return Ducts?


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duct smoke detector


Duct smoke detectors, when installed in HVAC systems, automatically stop the system fans upon the detection of smoke. For HVAC systems with airflows exceeding 2,000 cfm, duct smoke detectors are required. This includes where multiple air handling systems utilize common ducts with a combined design capacity greater than 2,000 cfm.


But when duct smoke detectors are required, where should they be located? Let’s look at the requirements in two of the prominent codes and standards.


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What’s Wrong With This Picture?


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Water Heater T&P relief valve discharge piping, installed incorrectly


If you stay in this business long enough, you see a bit of everything. Notice the water heater’s T&P valve in the above photograph. The T&P valve discharges vertically up. What’s wrong with that?


There are many specific code requirements (13 to be exact) for the discharge piping from water heater T&P valves (Ref: 2012 International Plumbing Code 504.6). This installation violates requirements 8 and 9, which are:


8.  Not be trapped.


9. Be installed so as to flow by gravity.


Obviously, with this incorrect installation, the discharge cannot flow by gravity. Also, water can fill and remain in the discharge pipe. Over time, stagnant water that remains in the discharge pipe can corrode the pipe and cause it to fail.


This is just a reminder. During field inspections, check the discharge pipe to verify code compliance.




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NFPA 13R Sprinkler Systems – Are You Double-Dipping?


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Sprinkler system riser


For residential occupancies, a 13R sprinkler system is permitted by the building codes for many applications, and a 13R system offers numerous economic advantages for the building owner. Most notably, with a 13R system, attics and concealed spaces do not have to be protected with sprinklers.


But, be careful! Although the building codes permit a 13R system for residential occupancies, a 13R system is not permitted when a 13 system is used by the Architect for tradeoffs to other code requirements.


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Floor Registers in Commercial Buildings


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HVAC floor register




Commercial buildings typically have ceiling diffusers or sidewall grilles, but occasionally there is a need to locate air supplies or returns on the floor of a facility. Floor registers are common in residential applications. Before locating them on the floor a commercial building, a careful review of the applicable codes is imperative.


Building codes typically do not prohibit floor registers in commercial buildings, however, the fire codes can restrict their usage. For example, NFPA 101: Life Safety Code, paragraph 9.2.1 states that “Air-conditioning, heating, ventilating ductwork, and related equipment shall be in accordance with NFPA 90A.” NFPA 90A: Standard for the Installation of Air-Conditioning and Ventilating Systems, paragraphs 4.3.7.3.1 and 4.3.8.3.1 state that “Air inlets/outlets shall be located at least 3 inches above the floor, unless provisions have been made to prevent dirt and dust accumulations from entering the system.”


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Roger W. Griffith, P.E.

P.O. Box 702

Jefferson City, TN 37760

(865) 471-8142


Email: rgriffith@griffithengineering.net



		






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