Diagnostic guide
Why Do My Lights Flicker When the Heat Pump Turns On?
Last reviewed · Reviewed by the Pro Electrician technical team
Short answer
Lights dim when a heat pump starts because the compressor motor requires a massive instantaneous surge of starting current (inrush current) that momentarily pulls down voltage across your household electrical panel. Common causes include undersized service panels, weak utility transformers, loose neutral connections, or failing run capacitors.
Understanding Compressor Inrush Current
When your heat pump compressor is stationary, it requires immense electrical torque to overcome mechanical head pressure and start turning. For a few cycles (less than 200 milliseconds), the motor draws its Locked Rotor Amperage (LRA), which can reach 60 to 80 amps on a standard residential unit. This sudden demand causes a brief voltage sag across your main electrical bus, causing LED drivers and incandescent bulbs on other branch circuits to dip in illumination.
- Compressor motors draw up to 5 times running current for a fraction of a second upon startup
- Voltage drop exceeding 5% indicates an electrical bottleneck in service wiring or panel lugs
- Failing motor capacitors force compressors to draw high current for longer durations
When Flickering Signals Dangerous Electrical Hazards
While a split-second dip is common, prolonged or severe dimming signals serious electrical issues. A loose main neutral connection in your electrical panel or at the Puget Sound Energy utility weatherhead creates severe phase imbalance—causing lights on one phase to dim while lights on the opposite phase flare dangerously bright. This condition can destroy electronics and requires emergency utility isolation.
Solutions: Soft Starters and Panel Heavy-Ups
To eliminate compressor light flicker, electricians install an electronic HVAC soft starter (such as a Micro-Air EasyStart) directly inside the heat pump condensing unit. Soft starters ramp up voltage gradually over several cycles, slashing startup inrush current by 60% to 70%. If the flickering is caused by an undersized 100-amp panel, upgrading to a 200-amp service with heavy copper feeders eliminates the electrical bottleneck entirely.
