CSLB C-20 HVAC ContractorHeating SystemsMedium
A gas furnace's blower fails to start, causing the heat exchanger to overheat. Which safety control is designed to shut off the burners in this situation?
- AFlame rollout switch
- BPressure switch
- CHigh-limit switch
- DDraft inducer relay
Show answer & explanationAnswer & explanation
Correct answer: C. High-limit switch
The high-limit switch monitors plenum or heat exchanger temperature and opens to shut off the gas valve if temperatures exceed a safe threshold, such as when blower failure prevents adequate airflow across the heat exchanger.
Why the other options are wrong
- A. A flame rollout switch detects flames escaping the burner compartment, not general overheating.
- B. A pressure switch verifies draft inducer operation before ignition, unrelated to overheating.
- D. A draft inducer relay controls inducer motor power, not temperature safety.
High-Limit Switch
A bimetal safety switch that senses supply plenum temperature and shuts off the gas valve if the furnace overheats due to restricted airflow.
- Located in the supply air plenum near the heat exchanger
- Auto-resets once temperature drops to safe range
- Prevents heat exchanger damage from blower failure or dirty filters
Memory trick: Too hot? High-limit shuts it down.