How does a pre-charged expansion tank work?

A pre-charged expansion tank is designed to manage the expansion of water in a closed heating system. The correct choice highlights that the bladder within the tank is pressurized to match the system's fill pressures. When water in the system heats up, it expands, creating excess pressure. The pre-charged bladder allows for this expansion by compressing the air in the tank, which helps to maintain a balanced system pressure.

This mechanism effectively absorbs the increased water volume when it heats up, preventing the pressure from rising too high, which can lead to system failures or inefficiencies. The pressurized bladder acts as a cushion that allows for controlled expansion and contraction of the water within the closed-loop system, ensuring that pressures remain within safe limits.