This looks like a reasonable list of expansion tank issues to check for:
Avoid Common Problems – Expansion Tank Maintenance
Unfortunately, thermal expansion tanks don’t last forever. Most tanks eventually become waterlogged, making them unable to perform their function. Professional plumbers recommend replacing them every five to ten years. Here are a few ways to look after your expansion tank:
- Watch out for condensation. This can be caused by a waterlogged expansion tank or could just be the result of a very humid climate. Either way, it could pose a danger if your water heater is situated near any electrical devices.
- Do the “tap test.” Simply tap or knock on your expansion tank and listen to the sound it makes. If it makes a hollow, ringing sound you know there’s still air inside it. If it makes more of a dull sound, it means your tank is full of water and needs to be replaced.
- Feel the tank. Feel the top and the bottom of your expansion tank. The top of the tank, which should be filled with compressed air, should be cool to the touch. The bottom of the tank, where the water goes, should be warm. If the top of the tank feels the same as the bottom, you know you’ve got a problem.
- Pay attention to your water heater’s pressure release valve. Remember the pressure release valve? It starts releasing water if there’s too much pressure for the expansion tank to handle. If you notice the release valve is constantly dripping, it could mean your expansion tank isn’t working anymore.
- Upgrade your tank before it fails. You don’t have to wait until you notice a problem to invest in a new expansion tank. A good rule of thumb is to replace your tank whenever you replace or service your water heater.
- Replace a faulty tank as soon as possible. If your expansion tank isn’t doing its job, your home’s at risk for water damage or, even worse, a water heater explosion. As soon as you notice that something’s not right, get a hold of a plumber and start taking steps to get a new expansion tank installed.
If your expansion tank is the same age as your PRV, I certainly wouldn’t rule out that it needs replacing, but no sense starting there.