A water heater expansion tank is a small pressurized vessel that absorbs excess water volume created when water heats and expands, protecting plumbing systems and water heaters from overpressure damage. These safety devices contain a rubber diaphragm separating water from pressurized air, allowing flexible volume accommodation as heating cycles occur.
Technical Specifications
Expansion tanks range from 2 to 12 gallons in capacity, with residential systems typically using 5 to 10-gallon models. Precharge pressure measures 40 to 60 PSI, matching the system’s static pressure. Maximum working pressure ratings reach 100 to 150 PSI. Tanks measure 12 to 18 inches in height with connections via 3/4-inch NPT ports. A 40-gallon water heater heats approximately 1 to 2 gallons of water per temperature rise of 30 degrees Fahrenheit.
Importance and Compliance
Expansion tanks prevent pressure relief valve discharge and internal tank damage caused by thermal expansion. Most building codes require expansion tanks on closed-loop systems where backflow preventers block water return to municipal supplies. Installation costs range from $150 to $300. Without expansion tanks, pressures can exceed 100 PSI within minutes of heating, leading to leaks, failed diaphragms, and equipment failure within 5 to 10 years.