What are the differences between open and closed expansion tanks, and when would I use each?

Open expansion tanks are vented to the atmosphere and are typically used in open systems where the tank is not pressurized. Closed expansion tanks, on the other hand, are pressurized and used in closed systems where the tank is subjected to system pressure. Closed tanks are more common in modern hydronic systems due to their ability to maintain a positive pressure and prevent air from entering the system. Open tanks are often used in older systems or in applications where the system pressure is relatively low. The choice between open and closed tanks depends on the specific system requirements and design.