Which device helps control boiler pressure by accommodating water expansion?

• A. Flow check
• B. Automatic air vent
• C. Pressure reducing valve
• D. Expansion tank

Answer: (D)

Water expands when heated, and in a closed boiler system that expansion makes the pressure climb. An expansion tank is designed to absorb that extra volume by providing an air-filled space that water can occupy as it expands. It has a diaphragm or bladder that keeps water on one side and air on the other; as the water heats and expands, it pushes into the tank, compressing the air cushion and preventing a dangerous rise in pressure. This keeps the system operating safely and helps avoid frequent boiler cycling or relief-valve trips. The expansion tank is usually connected near the boiler and is pre-charged to a set air pressure; if its charge is lost or it becomes waterlogged, it won’t absorb expansion effectively. Other devices manage different roles—flow checks prevent backflow, automatic air vents remove stray air, and pressure-reducing valves lower incoming pressure—but they don’t accommodate water expansion the way an expansion tank does.