Commercial Construction Considerations: HVAC Systems

Heating, ventilation, and air conditioning (HVAC) systems in commercial buildings can be of great assistance to firefighters, but they can also be our worst nightmare. In order to truly understand how HVAC systems will affect firefighting efforts, one must have a basic understanding of these systems.

Comfort heating and cooling is accomplished using either air or water as the heating or cooling medium. In many commercial buildings, these two mediums are used together. However, for the purposes of this article we will look at them independently. The impact on firefighting is very different.

There are many different ways of heating and cooling buildings and depending on your location there will be differences due to climate. However, these basic concepts need to be understood.

Air Handlers
Due to ventilation requirements, all buildings use some type of air handler to distribute air throughout a building. It is nothing more than a big box that has an inlet, an outlet, a fan or two, and a means to heat or cool the air.

Generally, there are two types of units -- those that are installed in mechanical rooms and those installed outside. When located outside, they are often on the roof, although you can find them on the ground. The air handlers are connected to duct work that supplies and removes air from the space or spaces it serves. You will also find exhaust fans and make up air units serving spaces, exhaust fans simply remove air from a space, and make up air units provide tempered make up air from outdoors.

The size of a unit is typically referred to in cubic feet per minute (CFM) and the amount of air the unit can move is measured in this way. So, a 30,000 CFM can move 30,000 cubic feet of air per minute. In units above 2,000 CFM, the unit will be provided with duct smoke detectors. Duct smoke detectors are special detectors that are located with the duct and generally span the entire width of the duct. These are tied into the fire alarm system and will activate the alarm just like any other detector. Units over 2,000 CFM are required to be shut down when the fire alarm is activated. It is important to remember that these are current code requirements and there are many buildings that do not have these features.

Units Can Introduce or Recirculate Products of Combustion
The problems that the air handler could give firefighters are pretty straightforward. When we are trying to control ventilation of the fire to provide for a safe and orderly suppression, these units will continue to add fresh oxygen or recirculated products of combustion to the fire room on the supply side. If the system serves multiple spaces the products of combustion will quickly fill the entire space served by the unit. On the return side the air handling unit will move the air in and in some cases super heated air, which can create further fire entering the ductwork or the plenum space.

A return air plenum is one of the biggest concerns for firefighters. The plenum is the space above the ceiling and below the floor or roof. When we introduce super heated products of combustion into this space through the return air portion of the HVAC system, we may not even know it but need to be aware of the risks. As I have said in each one of my articles this is why it is very important for you to know the buildings in your response area. You need to know which buildings are equipped with fan shutdowns and which do not. You need to know which buildings have a return air plenum and which have return air duct work.

The code previsions requiring fire dampers where any duct work, either supply or return, go through a fire rated partition are of great help to firefighters. They keep the unit from providing oxygen to the fire prevent the return from getting out of the fire area. When conducting site visits ask the facilities management staff if they have an inventory of the fire dampers in the facility and how often they check them for proper operation.

Locations and Associated Hazards
Now, let's look at the difference between the air handling units that can be found inside the building and those you can find on the roof.

The units inside a building will be found anywhere that they can be hidden. In many buildings you will find them in dedicated equipment rooms and the units will be mounted on the floor. In an equal number of buildings, you will find the units hanging above a corridor or above the space they serve.

The fact that these units are hanging creates a huge danger for firefighters. Typically, these units are hung with unprotected threaded rods, which will fail quickly when exposed to fire conditions. Usually these units use hot water as a heating medium therefore a gas burner is generally not an issue.

Rooftop units pose two specific and different dangers to firefighters. The units are mounted on curbs on the roof, and in most cases the building structure around these units was designed to withstand their weight. Sometimes these units are installed on existing buildings with little regard for the increased load on the roof. Either way the danger of these units falling through the roof is very real. Many of today's buildings use lightweight construction so the roof structure itself can fail in a very short period of time, so if the roof structure fails a unit weighting several thousand pounds will be coming with it.

If no regard has been given to the structure of the building when the unit was installed, there are even greater chances that any substantial increase in heat may lead to failure of the roof structure and the unit falling on firefighters working in the building. Some units are powered natural gas or propane, and these hazards should not be treated any differently then we do every other time.

Exhaust Fans in HVAC Systems
Exhaust fans can be very helpful in our ventilation practices. When used properly they can remove the hot gases of combustion from the spaces we are working in. If we are to use the buildings exhaust fans to assist us in this process we need to know where they are located and how they operate. It's also vital to know what areas they serve and how big they are. If you do not know these facts, I would suggest performing ventilation in traditional methods.

HVAC systems are integral parts of a building, but they can help us and hurt us. The next article will discuss boilers, chillers, and condensers and how they will affect us during firefighting operations. Please take the time to learn about these systems in the commercial buildings you serve, you may find that having this information will make a big difference in outcome of a fire.

MATTHEW STIENE, a Contributing Editor, is a senior facilities manager for Mecklenburg County, NC, and a firefighter with Robinson Volunteer Fire and Rescue, in Charlotte. He is a licensed professional engineer in North Carolina, New York and Pennsylvania, and is a certified facility management professional. To read Matthew's complete biography and his archived articles, click here. You can reach Matthew by e-mail at