said by Bob4:The expansion tank should have an air valve on it like a tire does. Turn off the water and open a couple of faucets. Then use a pressure gauge to check the pressure. It should be set a few psi lower than the water pressure of the house. Best to use a bicycle pump if you need to add air to it as it is very easy to add too much when using a compressor. said by sempergoofy:
How does one determine if the expansion tank has failed? Tap it and listen? Or is there another a procedure?
Tapping it, if you have a good ear. Another way is to turn off the main water shut off, then open a faucet for a sink. If the water flows under pressure for a while (30 seconds?), the expansion tank is OK. If the water flows under pressure for just a few seconds, the expansion tank is no good.
Perhaps someone else has another way to test it.