Don’t let an ad campaign fool you. An automatic air vent installed in accordance with NFPA 13 requirements on a gridded system will only remove air from the cross mains, and the highest branch line on which it is installed. Most of the remaining branch lines will still contain the original volume of air (at atmospheric pressure), which is then reduced in volume by the system working water pressure to about 10% of the original volume (assuming a 2½-in. diameter pipe).

Without an air vent, all the air volume (at atmospheric pressure) will be trapped in the system, compressed by the system working water pressure, and contained in all but the lowest few branch lines. The faster the system is filled, the more lower branch lines that are filled nearly water solid. A photo from a recent advertisement for an automatic air release device appears to show somewhere near 40% trapped air volume above the air/water interface and may be a valid representation of a branch line on a system without an air vent.

We would be interested in a comparison of a pipe with an automatic air vent vs. a manual air vent over time highlighting a comparison of the corresponding air/water interface. Regardless of manual or automatic, we expect most branch lines will still have an air/water interface and will still be subject to a similar level of corrosion.

The best chance of removing as much air as possible from a sprinkler system would be to provide a gang air vent line tied into all the branch lines terminating above the system high point in a vent (manual or automatic). Even in this scenario, removing all air is dependent on how level the branch lines are installed. This scenario is specifically referenced in NFPA 13 Annex material as not being necessary. This
Annex material also makes clear that “it is neither the intent nor practical to exhaust all trapped air from a single location.”

Our conclusion is that an air vent as required by NFPA 13 does reduce but does not eliminate the air trapped in a sprinkler system. One might question the real value of providing an air vent in minimal compliance with NFPA 13.  HGI prefers to specify manual air vents over automatic for our clients to economically minimize the risk of corrosion.