CSLB C-20 HVAC ContractorHeating SystemsEasy
A homeowner reports that their gas furnace cycles on and off rapidly, never completing a full heating cycle. The technician observes that the main burner lights, but then extinguishes after a short period, and the blower never fully engages. What is the most likely cause of this short cycling?
- AA faulty thermostat calling for heat intermittently.
- BLow gas pressure to the furnace manifold.
- CA dirty air filter restricting airflow and tripping the high-limit switch.
- DA clogged condensate drain line causing a pressure switch error.
Show answer & explanationAnswer & explanation
Correct answer: C. A dirty air filter restricting airflow and tripping the high-limit switch.
A dirty air filter significantly restricts airflow, causing the furnace's heat exchanger to overheat. This triggers the high-limit switch, which shuts down the burner to prevent damage, leading to short cycling.
Why the other options are wrong
- A. While a faulty thermostat can cause short cycling, the description of the burner lighting and then shutting off due to overheating points more directly to an airflow issue.
- B. Low gas pressure would likely prevent the burner from igniting properly or cause a weak flame, not necessarily lead to rapid cycling due to overheating after ignition.
- D. A clogged condensate drain line typically affects condensing furnaces and would trigger a pressure switch error, preventing ignition or causing a lockout, rather than short cycling after ignition due to overheating.
High-Limit Switch Function
The high-limit switch is a safety device in a furnace that monitors the temperature within the heat exchanger compartment and shuts off the burner if the temperature exceeds a safe limit.
- Prevents overheating of the heat exchanger.
- Typically resets automatically once temperature drops.
- Commonly trips due to restricted airflow (e.g., dirty filter, closed vents).
Memory trick: Hot air needs a clear path, or the furnace will halt.