Which device maintains boiler pressure around 10 to 15 psi in the system?

• A. Thermostat
• B. Circulator
• C. Pressure relief valve
• D. Automatic fill valve

Answer: (D)

In a hot-water boiler system, keeping the pressure within a safe, steady range is essential for reliable circulation. The device that does this by adding make-up water when pressure drops is the automatic fill valve. It senses the boiler’s pressure and opens to admit water whenever the pressure falls toward the lower setpoint, typically keeping the system in a 10 to 15 psi range as it heats and cools. When pressure rises due to heating, the valve closes to maintain the normal operating pressure and prevent excess buildup.

The other devices have different roles: a thermostat controls when the boiler fires based on temperature, not pressure; a circulator moves water through the loops; a pressure relief valve acts as a safety to vent water or steam if pressure becomes too high, not to regulate normal operating pressure.