Water Hammer Arrestors Versus Air Chambers

You may want to reconsider how your specifications are written for future projects to help protect the health of the nation or other countries you are designing projects in.

Water hammer, the banging sound you sometimes hear when you quickly shut off a faucet, is controlled by either air chambers or water hammer arrestors (also called shock absorbers).

One hospital found high legionellae counts in water hammer arrestors.  After removing the water hammer arrestors, the percentage of water outlets testing positive for Legionella dropped significantly, and no new cases of legionellosis were identified (Memish, Z. A., C. Oxley, J.  Contant, and G. E. Garber. "Shock Absorbers as a Source of Legionella pneumophila." Presented at the 4th International Symposium on Legionella, 1992.  In: Barbaree, J. M., R. F. Breiman, and A. P. DuFour, eds. Legionella: Current Status and Emerging Perspectives.  Washington , D.C. : American Society for Microbiology, 1993; 261-262).

Air chambers, however, which will normally be required if water hammer arrestors are not used, may be even more conducive to Legionella growth than are arrestors, because air chambers become waterlogged and harbor stagnant water. Water hammer arrestors hold less water than air chambers because they have membranes that separate the water from the air. In some types of water hammer arrestors the membranes are diaphragms-a dome shaped sheet of rubber or stainless steel that's fixed at the sides of the tube but flexible in the middle so that it moves up and down with variations of water pressure, somewhat like a trampoline. Other arrestors have piston-type membranes made of either plastic or brass.

Studies have not been undertaken to compare water hammer arrestors with air chambers with respect to legionellae growth, or one type of water hammer arrestor with another type, so there is no data on which to base decisions. However, it would seem that two Legionella-prevention factors should be considered in comparing one type of arrestor with another: (1) the amount of water it contains, and (2) whether the components are made of rubber, stainless steel, or brass. The less water the arrestor holds, the better. In most cases piston type arrestors will hold less water than will the diaphragm types, but there may be exceptions with various models. Components made of stainless steel or brass will be less conducive to Legionella growth than will components made of rubber or plastic.

Therefore, for future plumbing renovations, consider water hammer arrestors with piston-type membranes until better technology becomes available to control water hammer. If using a diaphragm type arrestor, prefer a model with a stainless steel diaphragm instead of one with a rubber diaphragm.

(Excerpted from Management Plan for Legionella and Other Waterborne Pathogens (http://hcinfo.com/managementplan.htm), by Matthew R. Freije)