Lights Flicker When Heat Pump Turns On: Causes, Diagnosis, and Fixes

The flicker of lights when a heat pump starts is a common issue that can indicate benign startup behavior or underlying electrical problems. This article explains the likely causes, safe diagnostic steps, and effective fixes to help homeowners identify whether the issue requires a simple adjustment or a licensed electrician. Knowing when flicker is normal and when it isn’t protects equipment and prevents safety hazards.

Symptom	Possible Cause	Typical Fix
Brief dim when compressor starts	Inrush current / voltage sag	Acceptable or add soft-start device
Frequent/long dimming	Loose neutral or undersized wiring	Tighten connections, upgrade wiring
Lights & appliances affected	Service panel overload or poor grounding	Panel upgrade, load balancing

Why Lights Flicker When A Heat Pump Starts

When the heat pump’s compressor and outdoor fan motor energize, they draw a high inrush current for a fraction of a second. That sudden demand can cause a brief voltage drop at the meter and throughout the home wiring, appearing as a light flicker.

Flicker can be normal if it is short, infrequent, and limited to lights near the outdoor unit circuit. Persistent, large, or widespread flicker often signals an electrical problem that needs correction.

Common Causes Of Flicker

High Inrush Current From The Compressor

Compressors and motors require significantly more current to start than to run. The starting surge can be 3–7 times the running current, producing a momentary voltage drop that causes lamps to dim briefly.

Loose Or Corroded Neutral Connections

A loose neutral at the service equipment or meter can create large voltage swings when loads change, causing flicker and potentially dangerous overvoltage on some circuits. This is a serious condition requiring prompt attention.

Undersized Or Aging Wiring

Homes with old or undersized branch circuits and service conductors are more susceptible to voltage drop during heavy loads. Long cable runs and inadequate wire gauge increase the likelihood of visible flicker when large appliances start.

Shared Circuits Or Poor Load Distribution

If the heat pump shares a subpanel or runs on the same phase as sensitive lighting circuits, the startup sag will be more noticeable. Improper panel load balancing can worsen the effect.

Faulty Capacitor Or Motor Problems

A weak run/start capacitor or failing motor causes excessive starting current and repeated attempts to start, which exaggerates flicker and can indicate imminent compressor failure.

Service-Side Issues And Utility Problems

Sometimes voltage sag originates at the utility transformer or the service drop. Signs include flicker that affects multiple homes in the neighborhood or persistent low voltage regardless of local load changes.

How To Diagnose The Problem Safely

Visual And Behavioral Checks

Note when and how the flicker happens: does it occur only when the heat pump starts? Is it brief or sustained? Are other appliances affected? Documenting patterns helps narrow causes.

Inspect Basic Connections

Homeowners can visually inspect for obvious loose connections at the outdoor unit disconnect and the indoor air handler junction box, but should not touch live terminals. Always shut power off at the breaker before accessing wiring.

Measure Voltage And Inrush Current

A qualified technician can use a multimeter to measure steady-state voltage and a clamp meter or power analyzer to capture inrush current during startup. Voltage dips over 5–10% at the service during startup merit further investigation.

Check For Loose Neutral At The Meter Or Panel

A loose neutral often causes large voltage swings across circuits and is dangerous. Only a licensed electrician or the utility should tighten meter base or service neutral connections.

Observe Neighboring Homes

If neighbors experience similar flicker when the heat pump starts, the problem may be with the utility transformer or distribution system, and the utility should be notified.

Practical Fixes And Upgrades

Install A Soft-Start Or VFD

A soft-start device or variable frequency drive (VFD) reduces inrush current by ramping the compressor motor up slowly. This can significantly reduce flicker and decrease mechanical stress on the compressor.

Replace A Weak Capacitor Or Repair The Compressor

Replacing a failing start/run capacitor or addressing compressor motor issues can lower starting current and stop repeated hard starts that amplify flicker and risk failure.

Tighten Connections And Repair Corroded Wiring

Tightening loose service, panel, and unit connections and replacing corroded conductors reduces resistance and stabilizes voltage during load changes. This is a common and effective fix.

Balance Loads And Upgrade Panel Or Service

Redistributing major loads across phases and circuits, or upgrading the service from 100A to 200A, can prevent overload-related voltage drops. Panel upgrades may be necessary for older homes with modern HVAC and electrical demands.

Install Dedicated Circuit Or Change Wiring Routes

Providing the heat pump with its own properly sized circuit and minimizing shared neutrals reduces interaction with lighting circuits. Using shorter, heavier-gauge conductors decreases voltage drop.

When To Call A Professional Or The Utility

If flicker is accompanied by tripping breakers, burning smells, sparks, or lights that brighten and dim widely across circuits, contact a licensed electrician immediately. These signs can indicate hazardous conditions such as a loose neutral or overloaded service.

When multiple homes are affected or the meter shows persistent low voltage, contact the electric utility for evaluation and possible transformer or service repairs.

Cost Estimates And Timeframes

Minor fixes like tightening connections or replacing a capacitor typically cost $100–$400 and may be completed in a few hours. Installing a soft-start device or VFD often ranges $400–$1,200 including labor.

Panel upgrades and service capacity increases are more expensive, commonly $1,500–$4,000 or more depending on permits and complexity. A professional diagnosis usually costs $75–$200 unless applied toward repairs.

Preventive Maintenance To Avoid Flicker Issues

• Schedule annual HVAC tune-ups that include capacitor checks and motor inspections to reduce excessive starting current.
• Have the electrical panel inspected every 5–10 years to identify loose connections and aging components.
• Ensure the heat pump disconnect and outdoor unit terminals are free from corrosion and tight.
• Consider proactive load analysis and balancing if adding major appliances or EV chargers.

Safety Tips And Best Practices

Never attempt service on the meter or service neutral; these are utility or licensed electrician responsibilities. Turn off breakers before working on branch circuits and use insulated tools and PPE when appropriate.

Document symptoms with timestamps and photos or videos if possible. This information speeds diagnosis and helps technicians reproduce the issue during inspection.

Frequently Asked Questions

Is A Brief Flicker Normal When A Heat Pump Starts?

Yes. A brief, localized flicker lasting a fraction of a second when the compressor engages is normal due to inrush current. Frequent, prolonged, or widespread flicker is not normal and should be investigated.

Can A Soft-Start Void Equipment Warranties?

Most manufacturers permit soft-start devices, but homeowners should verify with the HVAC warranty terms and have the device installed by a licensed technician to avoid warranty complications.

Will LED Lights Flicker More Than Incandescents?

LEDs and CFLs are more sensitive to voltage fluctuations and may show more noticeable flicker than incandescent bulbs. Upgrading fixtures or ensuring stable voltage reduces LED flicker.

Key Takeaways And Next Steps

Short, rare dimming during heat pump startup is often normal; persistent, strong, or widespread flicker indicates an electrical issue. The safest route is to obtain a professional electrical inspection to identify whether the cause is localized (capacitor, wiring, connections) or service-side (utility equipment or undersized service).

Homeowners should gather observations, schedule an HVAC and electrical inspection, and consider soft-start options or wiring upgrades when recommended by licensed professionals to restore stable, safe power and protect equipment.