I have determined that my existing expansion tank has failed (The system is Wiel Mclane and is about 7 years old.) and will need to be replaced. There is no shutoff above the tank so I found the valve that feeds water into the boiler and is in the same line as the backflow valve and the reducer valve.
I have purchased a new expansion tank and also a valve to install above the tank so if this happens again I will be better off.
I understand how to remove the tank and add the new one but my question is - do I need to relieve pressure in the boiler before unscrewing the old tank? And also once the new tank is installed do I need to bleed air and add water that was lost in the replacement process?