What is the primary purpose of a high limit control in a gas-fired hydronic system?

• A. Prevents overheating
• B. Maintains constant pressure
• C. Controls pump speed
• D. Regulates gas supply

Answer: (A)

The main function of a high limit control in a gas-fired hydronic system is to act as a safety shutoff that prevents the water from overheating. It monitors the boiler water temperature and, if the temperature rises above a preset limit, it shuts off the burner to stop heat generation. This protects the boiler, piping, and fittings from damage and helps prevent unsafe conditions like scalding. In normal operation, the aquastat manages the setpoint, and the high limit only intervenes as a backup safety device (some models auto-reset after cooling, others require manual reset). It isn’t used to maintain constant pressure, control pump speed, or regulate the gas supply during normal operation.