Commercial Construction Considerations: HVAC Systems

The proper use of the HVAC features can assist firefighters but they can also be your worst enemy.

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.

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