What is the primary purpose of an expansion tank in a closed system?

• A. Absorb thermal expansion and maintain pressure
• B. Filter water
• C. Increase pump speed
• D. Generate steam

Answer: (A)

The main idea is to provide a buffer for the extra water volume that results when the fluid heats in a closed loop. In a closed system, water expands as it heats, and with nowhere to go, pressure would rise and stress components. An expansion tank gives that extra volume a place to go by using a compressed air cushion (or a bladder inside the tank). As the water expands, it pushes into the tank, compressing the air or bladder and keeping system pressure within safe, designed limits. This protects the boiler, pipes, and fittings and helps avoid nuisance pressure relief and unnecessary pump cycling. It doesn’t filter water, increase pump speed, or generate steam, which is why those options don’t fit.