A water treatment system can add a check valve, backflow device, or pressure regulator to the supply. Once water cannot flow back into the street or well line, heated water expands in the water heater and has to go somewhere. An expansion tank gives it room. Most of these tanks have a rubber diaphragm and an air valve on the end, similar to a tire valve. If the air charge is gone or the tank is waterlogged, the expansion tank stops doing its job, and pressure can climb.
The part a homeowner can check
Find the expansion tank on the cold supply line near the water heater. Shut off the water supply and open a hot tap to relieve pressure. With no water flowing, tap the air valve with a tire gauge and compare the reading with the air charge listed on the tank label or in the installer's paperwork. If the tank has no air, the gauge reads very low, or water comes out of the valve, the tank needs service. Do not overinflate it to fix a low reading without knowing the target.
The parts to hire out
Replacing the expansion tank, changing the piping, adjusting a pressure regulator, replacing a relief valve, or sizing a new tank all belong to a licensed plumber. These tasks involve soldering, pressure settings, and discharge routing that can cause damage or injury if done wrong. A relief valve must discharge to a safe, visible location, and a plumber can confirm that.
Questions to ask before the work starts
Ask what pressure the system is set to and why. Ask whether the installer checked the expansion tank when the check valve or backflow device went in. Ask whether the expansion control is part of the quote or an extra line item. If the quote does not mention it, the answer is still worth getting in writing before you sign.