Expansion tank
A small tank that absorbs pressure as your water heater heats water.
Water expands as it heats. In a closed plumbing system, that extra volume has nowhere to go, which spikes pressure and stresses your water heater and pipes. An expansion tank — a small tank near the water heater — absorbs that pressure. If yours waterlogs or fails, you may notice the TPR valve dripping. Many areas now require one on new installations.