A thermal expansion tank is a small tank on the water supply at a tank water heater that gives heated water room to expand. You need one if your home has a closed system, meaning a check valve, backflow preventer or pressure reducing valve stops that extra water from pushing back into the city main.
Water grows in volume as it heats. In an open system the extra backs out toward the main; in a closed system it has nowhere to go, so pressure climbs each time the heater runs. Inside the tank, a rubber bladder separates the water from an air cushion that compresses to absorb it. Without one, the usual signs are a dripping relief valve on the heater and toilets that refill on their own.
It is also a code item. The 2021 International Residential Code, which Plano adopted for homes, requires a device for controlling pressure where a backflow preventer or check valve serves a storage water heater, or where expansion pushes pressure past a pressure reducing valve’s setting. Other DFW cities adopt and amend their own editions, as the Texas plumbing code guide explains, and the question usually comes up at a water heater replacement.
There are limits. The check valve rule is written for storage heaters, so a tankless heater may not need one; confirm with your city. A tank does not lower high street pressure either, which is the pressure reducing valve’s job. And tanks wear out: once the bladder fails, the tank fills with water and stops working. A healthy one sounds hollow when tapped high up, and its air valve releases air, not water. The devices that close a system sit on the main supply line or inside some newer meters, and a licensed plumber can confirm which you have.