Lights Dim When Heat Pump Turns On: Causes and Solutions

Lights Dim When Heat Pump Turns On is a common household issue that signals electrical stress when a heat pump starts. This article explains why the dimming occurs, how to diagnose the problem, and practical solutions ranging from simple fixes to professional upgrades. Readers will learn safety steps, cost ranges, and when to call a licensed electrician or HVAC technician.

Symptom	Likely Cause	Typical Fix
Lights dim briefly on startup	Inrush/startup current from compressor	Add soft starter/replace lights with less sensitive LEDs
Lights dim house-wide	Service voltage drop or undersized service	Upgrade service, improve neutral connection
Dim only on same circuit	Shared circuit or overloaded breaker	Move loads to another circuit or add dedicated circuit

Why Lights Dim When Heat Pump Turns On

The heat pump compressor motor requires a high startup current called inrush current that can be several times the running current. This sudden draw causes a temporary voltage drop in the home’s wiring, making lights appear dim for a second or two when the heat pump kicks on.

Voltage drop is more likely if wiring is undersized, the breaker is old, connections are loose, or the main service is near capacity. Other contributing factors include older incandescent or fluorescent fixtures and sensitive LED drivers that react visibly to brief voltage dips.

How Heat Pump Startup Current Affects Home Lighting

When a compressor starts, the motor’s impedance is low, producing a current spike. That spike travels through the electrical panel and service entrance and can momentarily lower the available voltage to lights and other loads.

Key variables include the heat pump’s horsepower, distance and gauge of wiring, panel condition, and whether other large loads are active simultaneously. Homes with a weak neutral connection or small main service (100A or less) experience more pronounced dimming.

Common Causes And Diagnostic Steps

1. High Inrush Current From Compressor

Compressor motors can draw 5–7 times their running current at startup. This transient can be enough to dim lights if the system isn’t properly managed or if the electrical network is marginal.

2. Shared Circuit Or Overloaded Panel

If lighting circuits share the same subpanel or run through the same branch circuit as the heat pump’s controls or outdoor unit, the local voltage drop is more noticeable. Overloaded panels near their limit also contribute to dimming.

3. Loose Or Corroded Connections

Loose service or panel connections increase resistance, causing greater voltage drops during heavy loads. A loose neutral is particularly hazardous and can cause large voltage swings and equipment damage.

4. Undersized Service Or Long Feed Runs

Older homes with 60–100 amp service may experience dimming when modern HVAC equipment starts. Long runs of small-gauge wiring also add resistance and magnify voltage drop during startup.

5. Sensitive Lighting Types

Certain LED drivers and older fluorescent ballasts show visible flicker during momentary voltage sag. Replacing or upgrading fixtures may reduce perceived dimming even if the electrical issue remains.

How To Diagnose The Problem

Start with simple observations: does dimming happen only when the outdoor unit starts, or during other motor starts? Note whether dimming is brief (a second) or prolonged. Identify if dimming affects the whole house or just lights on one circuit.

Useful diagnostic tools include a clamp meter to measure inrush current, a digital multimeter to check voltage at the panel during startup, and a licensed electrician’s power quality analyzer for comprehensive assessment.

Immediate Remedies To Reduce Dimming

Check And Tighten Connections

Have a licensed electrician inspect the main breaker, panel connections, and neutral for tightness and corrosion. Correcting loose connections often reduces voltage swings and improves safety.

Reduce Other Loads At Startup

Scheduling high-draw appliances (ovens, washers) to avoid simultaneous startup with the heat pump can help. Smart plugs or load management devices can stagger loads to prevent combined inrush events.

Use Less Sensitive Light Bulbs

Replacing old fluorescent or certain LEDs with bulbs that have robust drivers or incandescent alternatives will make dimming less perceptible, though this addresses symptoms rather than the root cause.

Install Whole-House or Circuit Surge/Voltage Stabilizers

Stand-alone voltage stabilizers or uninterruptible power supply (UPS) systems can smooth brief sags for sensitive electronics and lighting circuits. These solutions add expense but protect equipment and reduce visible flicker.

Longer-Term and Professional Solutions

Add A Soft Starter Or Variable Frequency Drive (VFD)

Soft starters or VFDs reduce compressor inrush by ramping motor voltage and current during startup. Benefits include reduced dimming, lower mechanical stress on the motor, and potential energy savings. Installation should be performed by qualified HVAC technicians.

Install A Dedicated Circuit For The Heat Pump

Ensuring the heat pump has a properly sized, dedicated circuit reduces interaction with lighting circuits. This is standard for HVAC installations but may be missing or undersized in older homes.

Upgrade Main Service Or Panel

If service capacity is insufficient or the panel is aged, upgrading to a higher amperage service (e.g., from 100A to 200A) and a modern panel can resolve persistent voltage drop issues and support additional loads safely.

Replace Or Rewire Long Or Undersized Runs

Reducing wire length or increasing conductor gauge lowers resistance and voltage drop. This is effective when the outdoor unit or subpanel is far from the main service or when branch circuits are undersized.

When To Call An Electrician Or HVAC Pro

Contact a licensed electrician immediately if there are signs of burning, arcing, flickering outside normal dimming, breakers tripping frequently, or if the neutral is suspected to be loose. These can be safety hazards and cause equipment damage.

Hire an HVAC technician when dimming coincides clearly with the heat pump startup and a soft starter or compressor diagnostics are needed. Coordinated work between electrician and HVAC pro is often the fastest path to a durable solution.

Cost Estimates And Timeline

Costs vary by scope: tightening connections or replacing bulbs can be under $200. Installing a soft starter typically ranges $500–$1,500 including labor. Panel or service upgrades can range $1,500–$4,000 or more depending on region and complexity.

Rewiring long runs or adding dedicated circuits typically costs $300–$1,200. Full service upgrades take a day or two; soft starter installs often take a few hours to a day depending on access and coordination.

Safety Considerations

Do not attempt major electrical repairs without a license. Working on service panels, mains, or HVAC electrical components can be lethal. Always turn off power and follow local code requirements. Permit inspections may be required for service upgrades.

Preventive Maintenance To Reduce Future Dimming

• Schedule annual HVAC maintenance to ensure starters, capacitors, and compressors are in good condition.
• Have the electrical panel inspected every few years for tightness, corrosion, and capacity concerns.
• Upgrade lighting to high-quality LED fixtures with robust drivers to minimize visible flicker.
• Consider whole-home load management or smart thermostats to avoid simultaneous heavy loads during startup periods.

Frequently Asked Questions

Is A Little Dimming Normal?

Yes, brief dimming of lights for a second or two when a heat pump starts is common and not necessarily dangerous. However, frequent or severe dimming warrants inspection.

Can A Soft Starter Harm The Heat Pump?

No. When correctly sized and installed, a soft starter reduces mechanical stress and prolongs compressor life. It should be installed by a qualified HVAC technician.

Will Replacing Bulbs Fix The Issue?

Replacing lights with better LED drivers can hide the symptom but does not fix electrical issues. Use this as a short-term measure while addressing root causes.

Key Takeaways

Lights dim when a heat pump turns on primarily because of compressor inrush current and voltage drop through wiring or service connections. Simple fixes include tightening connections and swapping bulbs; durable solutions often require a soft starter, dedicated circuit, or service upgrade. Professional diagnosis ensures safety and the most cost-effective remedy.

For persistent or severe dimming, prioritize calling a licensed electrician and HVAC professional to inspect panel health, connections, and the heat pump’s starting components to prevent equipment damage and ensure household safety.