In autumn, my thoughts wander toward my favorite cold weather activities: football tailgating, hayrides at the pumpkin patch, and reminding our clients to drain the condensate from their drum drips before they freeze and break their sprinkler piping.

Wouldn’t life be simpler without freeze-protected sprinkler systems? Unfortunately, water is not the perfect suppression fluid for every possible situation. Frozen sprinkler piping usually results in pipe breaks, and the resulting water damage requires costly cleanup and repairs. 

Even so, freeze protection can be equally frustrating and costly for the entire construction team and the building owner through increased design and installation costs, as well as ongoing maintenance for the life of the building. 

For these reasons, designers should carefully consider when protection from freezing is required.

How can a sprinkler designer be certain that freeze protection is needed? Fortunately, design and installation requirements are consistent throughout the National Fire Protection Association’s (NFPA) 13, 13D and 13R. For this column, we will focus on when the NFPA sprinkler standards require protection from freezing, including examples of when it’s not required.

The good news is that the NFPA’s code-correlating committees have aligned the requirements in all three primary sprinkler installation standards. The section requiring freeze protection for sprinklers is titled “Systems in Areas Subject to Freezing.” Although the specific section number differs, the standards state, “Where any portion of a system is subject to freezing and the temperature cannot be maintained at or above 40 F (4 C), the pipe shall be protected against freezing.”

With some installations, it is obvious if the temperature will be maintained at the 40 F (4 C) minimum. Other times, it isn’t so simple. Let’s review some different installations for the need to provide freeze protection.

Refrigerated Spaces

Our first example is a relatively simple one. Coolers and freezers in retail stores and warehouses are common spaces requiring freeze protection for sprinkler systems. Most retail coolers are maintained at 40 F (4 C) or less, and some freezers are held at -10 F (-23 C) to prevent your ice cream from melting completely during the drive home from the store. 

Even coolers maintained above 40 F will have localized areas with colder, even freezing, temperatures, mandating the freeze protection requirement.

It is important to clarify that piping routed outside of a refrigerated space using dry pendent or dry sidewall sprinklers does not require protection from freezing. In a future column, we will discuss more methods to provide freeze protection in these various installations. Remember that any piping, valves or components inside a freezer must be protected.

Concealed Spaces and Attics

In many buildings, the area being protected may be heated, but the sprinkler piping is routed in concealed spaces above ceilings and behind walls. Unfortunately, those concealed spaces may experience temperatures below 40 F
(4 C), triggering the requirement for freeze protection. Some examples are attics, entry vestibule ceilings and cupolas with combustible construction. 

Designers should not rely on a generic box note on the submittal plan requiring the owner to always provide a minimum 40 F (4 C) environment. Instead, designers should coordinate with the design team exactly how the freeze protection will be addressed for each situation. 

One typical protection approach is to add pipe insulation in the concealed space with the sprinkler piping between the heated area and the pipe insulation. Another common solution is to add a unit heater and local thermostat to maintain temperatures above the 40 F (4 C) threshold. 

It is not recommended to rely upon open-return air grating in drop ceilings to warm concealed spaces above ceilings for several reasons. First, unless a heat transfer analysis is performed, the temperature in the concealed space may not be maintained to the minimum temperature. 

Additionally, adding large openings to drop ceilings could delay the activation by allowing heat above the ceiling instead of collecting heat around the drop-ceiling sprinklers.

Outdoor Spaces

For our next example, consider a sprinkler system installed outside below a canopy or porte-cochère. Most of the United States experiences minimum temperatures below 40 F (4 C), while some locations are more difficult to determine. 

Again, all three installation standards have the same methodology for determining when freeze protection is mandated: “The weather temperature used to determine if an unconditioned portion of a system is subject to freezing and required to be protected … shall be the average annual extreme minimum temperature obtained from an approved source.” [Emphasis added.]

All we need is the average annual extreme minimum temperature for a given location — but what is “an approved source”?

Typically, any source allowed by the authority having jurisdiction (AHJ) is acceptable, and the annex for all three standards suggests the following sources:

• National Oceanic and Atmospheric Administration — National Climatic Data Center;

• National Weather Service;

• Plant Hardiness Zone Maps (see planthardiness.ars.usda.gov);

• American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE);

• Other approved sources.

This author’s favorite data sources are the National Weather Service NOWData searchable database and ASHRAE’s Weather Station tables (requires paid account or paper copy of “ASHRAE Handbook — Fundamentals”). Next, we’ll discuss how to easily obtain the average annual extreme temperature for any location in the United States.

National Weather Service 

To use NOWData, navigate your favorite web browser to www.weather.gov/wrh/climate (Figure 1), then select the appropriate region of the country. 

For example, we will determine the average annual extreme minimum temperature for Fort Myers, Fla., a relatively temperate climate. Figure 2 shows suggested inputs to obtain the average annual extreme temperatures for the most recent 25 years. This is typically acceptable, but confirm with the AHJ that the selected date range is acceptable. NFPA standards do not specify a minimum range of dates.

The minimum temperatures for each month are presented in tabular form and summarized at the bottom of the table. Note the highlighted value in Figure 3 is the average annual extreme minimum temperature within the selected time period.

Based on NOWData historical information, the average annual extreme minimum temperature for Fort Myers over the past 25 years is 36 F (2 C). Based on this weather data source, freeze protection is required.

According to ASHRAE 

Now we’ll review another weather data source, the “ASHRAE Handbook — Fundamentals.” Access to current data is behind a paywall; however, many free sources of the data, such as local libraries, are readily available. 

This source can be overwhelming to understand. The needed information is displayed in the middle of the page: the Extreme Annual Temperature Mean Minimum DB (dry bulb) temperature. Be sure not to choose the WB (wet bulb) value. Refer to the highlighted value in Figure 4.

That highlighted value, 34.8 F (2 C), is ASHRAE’s value for the average annual extreme minimum temperature for Fort Myers. Therefore, freeze protection is required by NFPA 13, 13D and 13R for outdoor sprinkler system installations. 

Incidentally, the time period reflected in the ASHRAE Handbook also encompasses a 25-year span whenever historical data is available. Note that the time period may lag the NOWData by a couple of years. For example, the Fort Myers data is from 1994 to 2019 (shown at the top of the figure).