Every closed water heater and closed boiler loop has a small tank that most homeowners never look at. It exists because water expands when it heats, and in a closed system with a check valve or pressure reducer on the supply, that expansion has to go somewhere. The expansion tank is where it goes. When the tank works, the pressure gauge rises a little during a heating cycle and settles back. When the tank fails, the relief valve ends up doing the tank's job, and that is a worse arrangement.
ACHR News ran a feature titled Inside NIBCO's Bet on American Manufacturing, which takes a firsthand look at NIBCO's Virginia copper plant and the people, processes, and investments behind "products HVAC and plumbing contractors rely on." That story is about manufacturing, not your basement, but it points at the same chain. Someone made each part in your system, someone installed it to a spec, and it will age on its own schedule. The expansion tank is one of those parts, and it has a wear pattern you can check with a tire gauge.
Identify the tank you have
Look at the tank body near the water connection. Most residential units are a steel or composite shell with a fitting on the bottom and an air valve on top, usually under a plastic cap. The air valve works like the one on a tire. Inside, a bladder or diaphragm separates the air side from the water side. Older non-diaphragm tanks have no bladder, so the air gets absorbed into the water over time. Those tanks need a full drain and recharge rather than a quick top-up at the valve. Write down which type you see before you touch anything.
Measure the static pressure first
The air pressure in the tank should match the static pressure of the cold water system, measured with the system cold and no water drawing. Attach a gauge to the hose bib closest to the heater, open it fully, and read the number. Then close it. That reading is your target for the tank precharge. Do not set the tank from the factory label alone. The factory setting is a starting point, not a measurement of your house, and your incoming pressure may be different from the default.
Check the air side with the water off
Shut off the cold supply to the heater. Open a hot tap downstream to relieve pressure, then close it. Drain the water side of the tank by opening its drain fitting until water stops. An accurate precharge reading requires an empty water side, because water in the tank holds air pressure in a way that hides the real number.
Now put a tire gauge on the air valve. If the reading sits well below your static target, add air until it matches. If the valve will not hold air, or the reading falls to zero within a day, suspect the valve core or a failed bladder. A bladder that has ruptured often lets water into the air side, so you may also see water when you press the valve. Either way, the tank is no longer doing its job.
Watch the gauge through a heating cycle
Once the precharge matches your target, restore the cold supply and bring the system back to normal. Watch the pressure gauge on the boiler, or the gauge on the heater line, through a full heating cycle. A healthy tank lets pressure rise a modest amount and settle back when the water cools. A gauge that climbs toward the relief setting and holds there, or a relief valve that drips on every cycle, points to an undersized tank, a waterlogged tank, or a relief valve that is failing on its own. Log the static reading and the peak reading with the date, and keep the log with your warranty paper.
Know when to stop and call
Stop and call a licensed plumber if the relief valve discharges steadily instead of briefly, if you see rust or weeping at the tank seams, or if you cannot reach your target precharge with a gauge you trust. Those are pressure-vessel and relief-valve decisions, and they belong to a qualified professional. Your job is to bring a clean set of numbers so the visit starts with data rather than guesses.
The expansion tank is a small part with a simple job. It takes the pressure swing that heating creates and gives it somewhere to go. A tire gauge, a hose bib, and five minutes with the drain fitting will tell you whether it still does that job. Check it once a year, write down what you find, and the next time the relief valve starts to drip, you will know whether you are looking at a tank problem or a valve problem.