Ignite Your Future: Ace the 2026 NFPA 54 National Fuel Gas Code Practice Exam!

A limit control is the device designed to prevent boilers from reaching unsafe conditions by interrupting heat input when a preset high limit is reached. In steam boilers it blocks further pressure rise, and in hot water boilers it handles both high pressure and high temperature by shutting off the burner or valve to stop heating. This proactive interlock is what keeps the system from exceeding safe limits, which is why it’s required. A pressure relief valve only vents excess pressure after the limit is exceeded, not to prevent it, and a thermostat or safety switch don’t serve the same protective high-limit interlock function for boiler pressure/temperature.