Call a plumber. Check that you re-opened all the valves you closed.
A hot water heating systems water pressure should be 12-25 psi , the air in the expansion tank should be set around 10 psi but can only be accurately tested with no water pressure against it. Usually u can tell if the tank is bad when the relief valve on the boiler starts releasing water
About 40-70 psi.
House pressure, 75 PSI maximum.
If you have a leak on the hot water somewhere in the house and the water input into the tank is not running, the tank will lose it's water. And if it is an electric tank, the elements will burn out. Gas hot water tank should be no problem. Pressure valve does not have to be open.
Water expands when it is heated, this expansion increases the pressure. The bladder tank absorbs the extra pressure, protecting the hot water heater.
This depends on whether the hot water comes from a combination boiler or from a hot water tank with a header tank feeding it. For a combination boiler the fault would probably be at the boiler, possibly from scale in the heat exchanger. For the system that uses the header tank and a hot water cylinder it could be that the float valve in the header tank is stuck an there for it is not refilling the tank or debris has entered the tank and is blocking the outlet.
You should not ever hear your tank filling unless you are using hot water somewhere. Your tank is under pressure always so it would never be filling if you were not using hot water.
No. Nothing to do with it. Much more likely there is an air leak between pump and pressure tank, or pressure tank needs a new aircharge.
Install on cold water line.
Its a thermal expansion tank to absorb the pressure when the cold water heats in a hot water tank.
go to www.kwikot.com and there you can view and download a file on a hot water pressure tank
You probably haven't re-opened the cold water inlet valve.