What Is Short Cycling?
Short cycling occurs when a heat pump starts up, runs briefly (less than 5–10 minutes), then stops — repeating this pattern frequently throughout the day. This is problematic because:
- Each startup causes additional wear on the compressor
- Efficiency drops significantly during short run periods
- The system never reaches optimal operating temperature
Causes of Heat Pump Short Cycling
1. Oversized System
An oversized heat pump reaches the target temperature too quickly, triggering a shutdown before the system can run efficiently. This is the most common cause. A properly designed system should run for 20–30+ minutes per cycle in typical conditions.
2. Incorrect Thermostat Placement
A thermostat positioned near a heat source (radiator, sunny window) will read a higher temperature than the room average, causing premature shutdowns.
3. Low Refrigerant
Low refrigerant charge can trigger high-pressure or low-pressure safety cut-outs repeatedly, causing short cycling.
4. Dirty Air Filter
Restricted airflow causes the system to overheat, triggering thermal protection cut-outs and resulting in short cycling.
5. Faulty Control Settings
Incorrect hysteresis settings (the temperature differential between on and off) can cause the system to cycle too frequently. Your installer or engineer can adjust these settings.
Solutions
Have a qualified engineer:
- Check refrigerant charge
- Clean the air filter
- Review thermostat placement
- Assess whether the system is oversized
- Adjust control settings
Weather compensation control can also help, as it modulates compressor speed rather than cycling.
Sources: MCS certification and UK field trial.
