Why My Furnace Short Cycles — Diagnosis and Fix
Published: July 23, 2026 — BC Wide Home Services Ltd, doing business as BC Wide Heating & Air Conditioning — Greater Vancouver, BC
Most Common Cause — Dirty Filter
A dirty air filter restricts airflow, causing the heat exchanger to overheat quickly. The limit switch trips, shutting off the burners to prevent damage. Once the heat exchanger cools, the switch resets and the furnace starts again — only to overheat and shut off again. This creates a short cycle. Replace the filter first before calling for service.
Other Common Causes
Oversized furnace: heats the home too quickly, reaches thermostat setpoint in minutes, shuts off, then starts again when the temperature drops. Thermostat location: too close to a heat source (near a vent, in direct sunlight, near the kitchen). Limit switch failing: switch that trips at too low a temperature. Failing blower motor: cannot move enough air, causing overheating.
Professional Diagnosis Cost
Short cycling diagnostic: $150-300 service call. If a dirty filter resolves the issue, no service call needed. If the cause is more complex, the diagnostic fee is waived if you proceed with the recommended repair. Common short cycling fixes: filter replacement (free/DIY), thermostat relocation ($200-500), blower motor repair ($500-1,200), or limit switch replacement ($250-500).