A packaged rooftop unit utilizes a two-stage gas burner for heating. The system short-cycles frequently on the high fire stage during cold weather, but operates without issue on low fire. All safety controls (limit switch, rollout switch) have been checked and are functioning. Which of the following is the most likely underlying issue?

Question: A packaged rooftop unit utilizes a two-stage gas burner for heating. The system short-cycles frequently on the high fire stage during cold weather, but operates without issue on low fire. All safety controls (limit switch, rollout switch) have been checked and are functioning. Which of the following is the most likely underlying issue?

Answer options:

• The heat anticipator setting on the thermostat is too aggressive for the high fire stage. ✅ The high fire input rate is oversized for the current heating load or the airflow is insufficient for the high fire stage.
• The pressure switch for the high fire stage is intermittently failing, causing nuisance trips.
• The gas valve's second stage solenoid is sticking, leading to erratic pressure delivery.

Correct answer: The high fire input rate is oversized for the current heating load or the airflow is insufficient for the high fire stage.

Explanation: Short-cycling on high fire, especially when low fire operates fine, often indicates that the high fire is providing too much heat too quickly for the current load, or that the system's airflow cannot adequately dissipate the heat generated by the high fire, tripping the high limit switch.