Key advantages include reduced corrosion and failure, and quicker dousing of a fire.

Including residential, more than 90% of all sprinkler systems installed today are wet-pipe systems. But not all building interiors are climate-controlled. Rising energy costs have led many to consider leaving portions of their buildings unheated. When this occurs, the only fire protection system options available are dry-pipe, preaction, heat tracing, or anti-freeze.

The traditional anti-freeze system has been defined as follows: “a wet-pipe system that contains an anti-freeze solution and is connected to a larger wet-pipe system that it uses for a water supply. It supplies automatic sprinklers to protect small unheated areas.” Where the unheated area is large, it is generally understood that the most economically viable alternative is to augment the wet-pipe system with an auxiliary dry-pipe system to protect the area subject to freezing temperatures.

The design of an anti-freeze system depends on whether or not it includes a backflow prevention device. If it doesn’t, the arrangement of the supply piping and the valves would be as shown in the Figure 1 cross-section.  In this detail, designated points #3 and #4 refer to a 3/4” gate valve and fill cup. Point #6 refers to a supervised indicating valve, #8 to a 1/2” test valve, #5 denotes a 1” auxiliary drain valve, and #10 refers to a check valve with a 1/32” hole drilled in the clapper.

Figure 1.

Often, an installer will simply shave off a corner of the clapper instead, to allow for fluid expansion.  NFPA 13 allows that the check valve be omitted entirely if all anti-freeze piping falls below the elevation of the indicating valve (all antifreeze solutions are heavier than water).  The two test valves (noted as #8) must be installed at least 48 inches apart.  It should be noted that the gravity-fed fill cup may be placed almost anywhere downstream of the check valve and is normally installed at a high point of the piping.

Figure 2 is the NFPA 13-required supply arrangement for anti-freeze systems containing a backflow preventer.  In most jurisdictions, this device must be a reduced-pressure type.  This component will prohibit water from flowing back into the water supply, but over-pressurization can occur when pressure builds in the system as a result of thermal expansion caused by temperature change.

When the fluid expands, the excess is discharged into the expansion chamber.  Within the vessel, the gas-charged bladder compresses as the fluid enters the shell.  All system components are thereby protected because the chamber allows the trapped antifreeze to expand.  Expansion tanks vary in size depending on the volume of solution contained in the anti-freeze system.

Figure 2.


All About Anti-Freeze

The two types of anti-freeze are propylene glycol (PG) and glycerin. Section of NFPA 13 states that “an antifreeze solution shall be prepared with a freezing point below the expected minimum temperature for the locality.” As an example, the freezing point for the Chicago area is -22°F, which means that a glycerin solution used would contain 60% glycerin and 40% water. If propylene glycol is used, the correct mixture would be 50/50.

The solution must be thoroughly mixed with water before filling any system. An over-concentration of antifreeze would be costly and may reverse the protection desired, because beyond a certain point of over-concentration  (around 75%) the anti-freeze will thicken and cease to lower the freezing point of the solution.  Installers should know (this is common sense) not to mix any solutions in containers or drums that may be contaminated with other substances.

It is imperative that the system be tagged with the manufacturer of the anti-freeze, including all details and specifications of the fluid. When the solution is field-tested (annually - as required by NFPA 25), testers can determine the applicable concentration through the use of (hopefully) reliable laboratory-grade hydrometers or refractometers of sufficient quality. For hydrometers to produce accurate readings, the fluid temperature must be between 60°F and 68°F, or else a temperature correction shall be applied to the reading.

After five years or so following installation, the solution may break down due to small leaks, and concentrate will have to be added. After a stagnation period that comprises 8-10 years without servicing, the system will require complete solution replacement. If not properly maintained, the far ends of the piping will no longer have the suitable mixture. When any section of the system contains inadequate anti-freeze, freezing and breakage of sprinklers, pipe and fittings will result. Not only will this comprise a filthy and expensive mess, a frozen system cannot suppress a fire.

If the volume of an anti-freeze system exceeds 40 gallons, section requires that an additional drain/test connection be added at the remote system proximity - to provide another point for solution concentration level testing (it is expected that the upcoming 2010 edition of NFPA 13 will alter this section to include all anti-freeze systems regardless of size).

When systems are larger than 150 gallons, an additional test valve must be added for each 100-gallon increment. Survey reports have shown that rubber-gasketed adjustable drop nipples installed in anti-freeze loops have a tendency to leak.

In spite of the fact that anti-freeze systems have been comprised of steel or copper piping for years, they are generally not used with CPVC piping. When they are, only glycerin can be used (per NFPA 13 and 25 requirements) as PG solutions will chemically attack CPVC pipe joining compounds. With glycerin solutions, contamination may arise from the use of recycled or off-grade glycerin.  Also, only thread paste sealant compatible with the particular CPVC piping material is allowed.

Sealants and gaskets used in CPVC piped systems must also be checked for compatibility, and most manufacturers provide chemical compatibility Web sites such as to resolve any questions posed by the consulting engineer. Petroleum or solvent-based lubricants or sealants must never be implemented. Mixing steel and CPVC in the same system is generally a bad idea. For systems of steel or CPVC, field survey reports have found that rubber-gasketed adjustable drop nipples installed in anti-freeze loops are prone to leakage.


Cost Concerns

Engineers make things fit into the budget.  Prior to the 2002 edition of NFPA 13, the standard included the following advisory: “because of the cost of refilling the system or replenishing to compensate for small leaks, it is advisable to use small dry valves where more than 40 gal are to be supplied.” That section has disappeared from the code, meaning that it’s now permissible to supply 50 sprinklers or more with an anti-freeze system. But since solution costs exceed $20 per gallon, why would you do that?

Over time, a dry-pipe system will require less maintenance, fewer service calls, and is the safer system with regard to potable water. But if the system in an unheated area exceeds 30 sprinklers, it may still be advisable to choose an anti-freeze system for any of the following reasons:

  • to avoid the purchase of an excessive number of dry pendent sprinklers;

  • to avoid the installation of a plethora of drum-drip assemblies - if the new piping arrangement will contain numerous low points;

  • if the noise from an air compressor (i.e., behind a theatrical stage) is not acceptable to the owner;

  • there is virtually no floor space available for a dry valve;

  • plenum space is so tight that the potential dry pipe cannot be properly pitched;

  • or any additional electrical wiring has not been planned for or desired.

One other advantage of the anti-freeze system is that the interior of the piping will not scale as it would on a dry system. The reduction in corrosion, then, will extend the life of the piping system. Also, the anti-freeze system has a life safety advantage considering that, with dry systems, there is a longer lag time (up to 60 seconds) for water to hit a spreading fire after a sprinkler fuses.

In addition, there is an increased probability for system failure with the dry system set-up. With anti-freeze systems, all that has to work is the sprinkler itself. Barring any accidental valve closures, the system will work efficiently to quickly extinguish any fire