HVAC D-2 License Practice Test 2026 – Your Comprehensive All-in-One Guide to Exam Success!

The burner in a residential gas boiler is governed by a temperature-based safety control. The high limit acts as a protective switch in the burner circuit; if the water temperature reaches the preset maximum, it opens the circuit and stops the burner to prevent overheating. In normal operation, the thermostat calls for heat and the boiler’s aquastat manage the burner to bring water to the desired temperature, but the high limit provides the ultimate shutoff if things push beyond safe limits. That safety role is why this option best fits the question.