Expansion tanks are used in many hydronic systems in order to absorb pressures that build up as a result of increasing temperatures. These devices safely compensate for increased systems pressures so that pressure relief valves do not have to open every time a high-pressure event occurs. In indirect glycol systems the loop must be manually re-pressurized after pressure relief valves open.
Expansion tanks used specifically on the solar side of an indirect glycol system absorb increasing system pressure by allowing some of the fluid to enter the tank. Tanks used in this scenario are exposed to different conditions than typical hydronic expansion tanks and must meet requirement over and above typical tanks. Expansion tanks used in this manner must be able to tolerate propylene glycol solutions and operate at elevated temperatures (> 200°F) & pressures (> 100 PSI). When specifying an expansion tank for use on the solar side of an indirect glycol system always verify that it can meet these requirements.
Expansion tanks are comprised of a steel tank shell that contains a butyl rubber bladder. A cushion of pressurized air exists between the steel tank shell and the bladder which can be changed, typically, via a Schrader valve. Once connected to the system fluid is able to enter the tank whenever the pressure in the solar loop exceeds that of the expansion tank’s charge. Because of this the expansion tank must be charged with enough pressure that fluid is not able to enter the tank unless there is a high-pressure event in the solar loop. When this occurs fluid is pushed out of the loop and into the tank to help regulate pressure. When the solar loop’s temperature and pressure return to operating levels the tank’s pressure compresses the bladder and pushes the fluid inside back into the system. This ensures stable pressures and no loss of fluid.