What are the differences between open and closed expansion tanks, and which one is suitable for domestic hot water systems?

Open expansion tanks are vented to the atmosphere, allowing air to enter and exit the tank as the water level changes. Closed expansion tanks, on the other hand, are sealed systems that use a bladder or diaphragm to separate the water and air. For domestic hot water systems, closed expansion tanks are generally recommended, as they prevent air from entering the system and reduce the risk of corrosion and contamination. Closed tanks also provide a more accurate and reliable means of controlling system pressure.