A vaporstat control is sensitive to _____?

• A. temperature
• B. pressure
• C. humidity
• D. flow

Answer: (B)

A vaporstat is a pressure‑sensitive control used on steam boilers to regulate firing by monitoring the steam pressure in the system. It uses a diaphragm or bellows that responds to the vapor pressure; when the pressure reaches the set point, it opens the electrical circuit to the burner (cutting off fuel), and when the pressure falls below the drop‑out point, it closes the circuit to re-ignite. This keeps steam pressure within a narrow range, which is essential for consistent heating in steam systems. It's not primarily measuring temperature, humidity, or flow—the control action is driven by pressure in the vapor space, which directly governs how much heat gets delivered to the radiators.