Is it possible for an expansion tank to deflate?

Yes, an expansion tank can deflate over time, especially if it develops a leak or if the diaphragm inside it fails. This can lead to a loss of pressure in the system and reduce its effectiveness in absorbing excess pressure from heating systems. Regular maintenance checks can help identify and resolve issues with the expansion tank before they lead to significant problems. If deflation occurs, the tank may need to be repaired or replaced.