Understanding NFPA's Standard 90A for air conditioning and ventilation systems

Mastering the National Fire Protection Assn. (NFPA) 90A, “Standard for the Installation of Air Conditioning and Ventilating Systems,” can be daunting. Here are some tips: Divide the work. Architects and structural engineers on the project team will define the massing, compartmentalization, interior construction details, means of egress, function of the ceiling and raised floor plenu...


Mastering the National Fire Protection Assn. (NFPA) 90A, “Standard for the Installation of Air Conditioning and Ventilating Systems,” can be daunting. Here are some tips:

Divide the work. Architects and structural engineers on the project team will define the massing, compartmentalization, interior construction details, means of egress, function of the ceiling and raised floor plenums, and fire and smoke separations for inclusion in the project's code-compliance drawings.

Critical to successful building systems integration, all project changes must be documented in the code-compliance drawings so team members can continually reassess impacts on their systems. Even a subtle change in design can compromise the HVAC system and its life safety features, not to mention initiating a change order that affects contract values and schedules.

Define the HVAC system. Defining ingredients that make up the HVAC system is crucial to 90A compliance. Screen requirements for outdoor air intakes, flame-spread and smoke-developed product index ratings; access panel requirements in ductwork; flexible duct connector length restrictions; and protection for exposed fan inlets are all details that need to be documented in project specifications.

Taking it a step further, 90A requires drawing of fan arrangements for proper inspection and maintenance, identifying specific locations of air outlets, inlets and light fixtures used as return air paths, and indicating fan rooms used as an extension of ceiling plenums or used as a return air path.

Seemingly insignificant details can affect the HVAC system design. For example, the need to provide access panels in return and exhaust system ductwork affects the integrity of the ductwork itself, i.e., one needs to estimate how much more duct leakage will occur and provide fan capacity accordingly.

Integrate all the systems. Superimposing HVAC infrastructure on architectural and structural systems is where the fun begins. Protection of duct, pipe and conduit penetrations in fire-rated walls and partitions, floors, roof-ceiling assemblies, shafts and smoke barriers is a focus of 90A. The standard includes a handy diagram outlining a couple dozen of the most common penetrations in these building components.

Next, make sure that fan rooms comply with all requirements. This is particularly important for rooms that house fan systems serving multiple floors with ducts routed into vertical shafts. Standard 90A dictates which ducts can share shaft space with each other. For example, kitchen exhaust duct risers cannot be installed in the same shaft as supply and return air duct risers. Note: When this situation is met in an existing building, there are techniques that can be implemented to achieve separation without having to reconstruct shafts and duct risers, including special duct wraps for kitchen exhaust duct risers.

Requirements for fire and smoke dampers and combination fire/smoke dampers in smoke barriers are also outlined in 90A as well. Fire dampers need to be installed in the plane of the wall, partition or floor and must be rated for dynamic conditions. Smoke dampers can be installed up to 2 ft. away from the smoke barrier (the concept is that the duct itself provides some protection from the passage of smoke). Access panels must be provided in the ductwork of all fire and smoke dampers and combination fire/smoke dampers, and they must be installed immediately adjacent to the device.

Additionally, there may be cases where project architects and structural engineers choose to extend a fire-rated shaft below the elevation of a floor slab in order to protect a duct riser. When the shaft is extended down into the ceiling plenum of the floor below, a fire damper can be installed in its vertical plane, instead of having to install it in the floor slab, making installation and maintenance easier.

The 90A standard also lists specific requirements for the provision of smoke dampers in an air-handling unit (AHU), as opposed to smoke dampers out in the duct distribution portion of the system as discussed above. Smoke dampers are required in HVAC systems with a capacity greater than 15,000 cfm in order to isolate the AHU from duct distribution. The smoke damper on the return side of the system should be installed upstream of its fan. The smoke damper on the supply side of the system should be installed downstream of its supply fan. Exceptions include systems located on the floor that they serve exclusively, as well as systems positioned on a roof that control only the floor immediately below.

Finally, 90A defines a need for duct-mounted smoke detectors downstream of all filters, but upstream of all branch takeoffs for supply fans with capacities greater than 2,000 cfm. The same goes for those of return fans with a capacity greater than 15,000 cfm. Additionally, duct-mounted smoke detectors also are required in each return duct inlet that serves more than one story for systems with a capacity of greater than 15,000 cfm. These smoke detectors also should be able to sense smoke on a particular floor of multi-story building. Duct-mounted smoke detectors, however, are not required in a return air system if the spaces served by the HVAC system are protected by ceiling-mounted smoke detectors. Duct-mounted smoke detectors are also not required downstream of exhaust fans.

Sequence of operation. Once all system components have been furnished and installed, engineers need to help clients and contractors understand how life safety systems work. The 90A standard requires smoke dampers installed at the AHU to close automatically when the HVAC system is not in operation. Activation of any duct-mounted smoke detector noted above shall automatically stop the fans associated with the HVAC system and close the smoke dampers, unless the system is intended to function as part of an engineered smoke control system.

Last but not least, acceptance testing is required in order for all parties to confirm that the systems have been installed and are operating according to design requirements. A form of commissioning, acceptance testing must occur even if the owner chooses not to have any other elective commissioning performed on other building systems.

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