Many commercial roofs have air handling units (AHUs) located on them, which can complicate firefighter roof operations due to their operation and weight. Understanding AHUs and planning accordingly may be critical for safe and effective operations.

What is an AHU?

Every building requires mechanical (HVAC) systems to be habitable. Depending on a building’s occupancy, mechanical systems serve different functions. They keep the occupants of buildings comfortable and can provide environmental protection from a process or transfer of heat.

AHUs are part of the mechanical system of a building. Their function is to heat, cool, filter, and humidify the air and then distribute that air throughout the building. They come in many different shapes and configurations, from large custom units to small “packaged” type units. Being able to recognize them and understand their operation can be important for firefighter operations.

Pre-Incident Planning

Firefighters can use satellite imagery and building plans to identify AHU locations. At the very least, this technique can be used to determine rooftops that are worth investigating to gather intel on the equipment on the roof. Coordinating with code enforcement for obtaining plans and construction updates can also be beneficial. In addition to AHU locations, firefighters should know roof access points, utility shut-offs, and structural high weight load areas. If you notice new rooftop equipment being installed, try to confirm that the work is being done with the proper engineering and permitting.

Initial Size-Up

Upon arrival at a scene requiring roof operations where AHUs are present, the incident commander and roof team must conduct a comprehensive size-up to include the following:

• Determine fire location relative to AHUs

• Assess roof integrity, especially near heavy units

• Watch for smoke movement through penetrations or ductwork

• Check for energized equipment and potential utility leaks (gas, refrigerant, water)

Key Hazards

Roof operations near AHUs present a distinct set of hazards that require attention. Firefighters should consider the risks below during operations:

• Collapse risk from fire-weakened supports or ponding water

• Trip/fall hazards from cluttered, uneven surfaces

• Electrical shock from energized components

• Chemical exposure from damaged units

• Mechanical hazards from moving parts or residual heat

• Poor visibility due to smoke, weather, or night conditions

Tactical Objectives

Firefighter objectives during roof operations involving AHUs typically include:

• Ventilate effectively in coordination with interior crews

• Preventing fire spread through ductwork

• Shut down utilities if safe

• Protect adjacent AHUs and rooftop exposures

Operational Guidelines

During rooftop operations involving AHUs, crews should gain access to the roof using designated, predetermined access points, when available, and always don full PPE. Locations of AHUs, roof penetrations, disconnects, and visible hazards should be identified. If fire is threatening the AHU, secure the electrical disconnect and any fuel or refrigerant shut-offs if safe to do so. If the fire involves the AHU itself, be prepared for electrical arcing and pressurized leaks. Periodically reassess roof conditions for sagging, spalling, or moving near heavy units. Perform vertical ventilation at locations away from HVAC equipment, because this could spread fire to areas of the building that aren’t involved in the fire. Roof crews should maintain radio contact with interior and command teams to communicate ventilation openings and utility shut-offs with all crews. Be mindful of exit routes in case conditions deteriorate quickly. Assign someone to monitor fire conditions and/or structural stability deformities if possible.

Special Considerations

Special considerations for rooftop operations include several aspects. Lightweight roofs fail more quickly when exposed to fire. The presence of multiple pieces of equipment (AHU’s, fans, condensers, solar panels, etc) can hinder movement. Weather can increase slip hazards, and darkness can hide visible risks.

Training

Fire departments should incorporate AHU-specific roof operations into their ongoing training programs. Practice identifying AHUs and their associated hazards on actual rooftops, review procedures for isolating utilities, plan ventilation tactics, and discuss rescue operations for a firefighter who has become injured on the roof.

Conclusion

Roof operations involving AHUs can be challenging. Success depends on planning, scene assessment, execution, and training. By understanding the unique risks posed by AHUs and employing the right preparation and tactics, firefighters can operate safely and effectively in these complex environments.