Jon Nisja, Data and Fire Protection Specialist

Ceilings play a significant role in effective fire sprinkler operation. This series of blogs will explore some of the ceiling-related definitions in NFPA 13, Standard for the Installation of Sprinkler Systems, and apply some fundamentals of heat spread to better understand the role ceilings play in fire sprinkler activation and discharge. 

Basic Fire Physics – Heat Transfer

To understand the interaction between ceilings and sprinklers, a review of some basic fire physics is in order, primarily the issue of heat transfer. In a fire, heat is given off via three primary methods: Conduction, radiation, and convection.

Conduction is the transfer of heat from one material to another through contact. The materials can be solids, liquids or gases. A classic example is placing a metal pan onto the burner of a stove. The heat from the gas-fired or electric burner is transferred to the pan and then to the food or liquid in the pan.

Radiation is the transfer of heat through energy waves. Examples of radiant heat transfer would include the warmth from sunlight on an otherwise cool day or the heating of food or liquids in a microwave.

Figure 1 – Three methods of heat transfer

Convection is the transfer of heat through air or fluids. An example would be a convection oven where hot air is circulated around food. Fire protection engineering and research have shown that the majority of the heat released in a typical fire is through convection (typically estimated as 2/3 to 3/4 of the total heat release).

Figure 1 shows the three types of heat release using a pan and a gas stove burner. The flame is heating the pan and its contents by conduction. The gas flame and hot pan are giving off radiation that could potentially ignite nearby combustible materials. The contents of the pan are releasing convective heat to the surrounding space (primarily upwards).

The warm convective air tends to rise and spread outwards in a V-shaped pattern. Since convective heat is the majority of the heat released in a fire and since convective heat rises, the placement of fire sprinklers near the ceiling becomes critical.

A ceiling stops the vertical convective heat spread and allows it to spread horizontally and activate ceiling-mounted sprinklers. Figure 2 shows the vertical fire and heat spread until it reaches the ceiling and then spreads across the ceiling. From a sprinkler activation standpoint, the best ceiling is one that is smooth, horizontal, flat, and monolithic in shape (covering the room from wall to wall).

Ceilings that have irregular surfaces, change elevation, are sloped, have numerous openings, or have some sort of structural or decorative features that obstruct heat from reaching fire sprinklers tend to slow down sprinkler activation times.

Figure 2 – Vertical heat spread and impact on fire sprinklers

Future blogs on this topic will examine various ceiling configurations, apply the definitions from NFPA 13, and identify preferred fire sprinkler placement and locations.

Summary

Ceilings play a crucial role in sprinkler performance. Most of the heat released in a fire is convective heat. Convective heat (or convection) primarily travels upwards and tends to accumulate near the ceiling. It then spreads horizontally at the ceiling and activates nearby fire sprinklers. Fire sprinklers installed at or near the ceiling tend to activate the fastest. Upcoming, future blogs will explore ceiling configurations and definitions in more detail.

Jon Nisja is a Data and Fire Protection Specialist with the National Fire Sprinkler Association (NFSA). Prior to working for NFSA, Jon served as a fire marshal since 1985 in two communities and as a state fire safety supervisor for the Minnesota State Fire Marshal Division. From 2005-2022, he supervised the State Fire Marshal’s fire protection, training, and data teams. He is the past president of both the Fire Marshals Association of Minnesota and the International Fire Marshals Association. Jon can be contacted at nisja@nfsa.org.