An expansion tank will help to regulate the pressure within your system. When the pressure gets too high, the inner diaphragm will fill allowing the excess water to have a place to escape. Without an expansion tank, your water heater can distort or even explode from the pressure of hot water inside with nowhere for the pressure to escape. Most plumbing codes also require an expansion tank, so without a proper method of containing the pressure in your system, it will not pass inspection.
Expansion tanks can be found at www.supplyhouse.com
