Expansion tank is waterlogged. Turn off boiler. Tank should be horizontal tank in ceiling with 1/2 pipe going to it. Between boiler and tank is shutoff valve. Turn it off. There also should be a drain in bottom of tank. Dont hook a hose to it as it will just pull a vacuum and not drain. Hold a bucket under it and let it drain. Just keep doing it till its empty. Close drain. Open valve. Look at pressure gauge on boiler. If you have a 2 story house you want the pressure to be about 15 to 18 pounds pressure after the water partially fills the tank. Good luck.
