ASHRAE Building Commissioning Professional (BCxP) Certification Practice Test

In a closed heating or cooling system, water expands when heated, and there needs to be space for that extra volume. An expansion tank provides exactly that: a place for the expanding fluid to go so pressure stays within safe limits. The tank is kept partially filled with water and contains air (or a bladder/diaphragm that creates an air-water separation). As the water warms and expands, it pushes into the tank, compressing the air or moving the diaphragm, which absorbs the increased volume and keeps system pressure from spiking. This arrangement protects components, reduces pump cycling, and prevents relief valves from opening due to overpressure. While there is some dampening effect from the air cushion, the primary purpose is to accommodate volume expansion of the fluid, not to store potable water. A fully filled tank would have no expansion space, so that option wouldn’t work.