Why Do My Lights Flicker or Dim When a Heat Pump Starts?

When a heat pump starts, its electrical demand changes quickly. That can cause a brief voltage dip, which some lights, particularly LEDs, show as momentary flickering or dimming.

A very slight dip may not indicate a serious problem. However, obvious, repeated, worsening, or whole-home flickering is worth investigating because it may reveal a weakness somewhere in the electrical foundation.

Why the lights react

The heat pump does not need to share a circuit with the lights for them to be affected. Every circuit eventually connects through the switchboard and the home’s main electrical supply.

When the outdoor compressor and fan begin operating, the heat pump’s electrical demand increases. If there is enough resistance within the circuit, switchboard, mains connection, or local supply, the voltage available to other parts of the home can fall briefly.

LED lighting can make even a small voltage change visible. Its electronic driver may respond immediately, while another type of light might barely change.

Many modern heat pumps use inverter technology to vary compressor output more gradually. Even so, the load still changes as the system begins heating, increases output, or enters another operating cycle.

The pattern provides useful clues

The way the lights behave can help indicate where the issue may sit.

If only one light flickers, the cause may be the lamp, fitting, dimmer, LED driver, or local lighting circuit.

If lights across several rooms dim at the same time, the issue is more likely to involve a shared part of the electrical foundation, such as the switchboard, mains connection, or incoming supply.

A single brief dip when the heat pump starts is different from lights that continue pulsing while it runs. It is also important to notice whether the problem is new, becoming more noticeable, or worse when other appliances are operating.

Lights that alternately brighten and dim, rather than simply dipping once, deserve particular attention.

What could be causing it?

The heat pump circuit

The heat pump circuit may be too long, too small for the load, incorrectly connected, or affected by a deteriorating termination. The isolator, circuit protection, and equipment itself may also need checking.

This does not necessarily mean the heat pump was installed incorrectly. Conditions can change as connections age, equipment develops a fault, or more electrical demand is added elsewhere in the home.

The switchboard and internal connections

A loose or high-resistance connection can exaggerate voltage changes. The issue could involve the heat pump circuit, a neutral connection, the main switch, an older switchboard component, or another shared connection.

Older homes may also have distribution arrangements that were designed around much lower electrical demand. The heat pump may simply be making an existing constraint visible.

The mains or local electricity supply

The cause may sit outside the heat pump circuit altogether. Northpower notes that loose or corroded connections, an overloaded network, or an undersized conductor supplying a house can contribute to voltage fluctuation and dimming lights. Northpower’s fluctuating power guidance

An electrician can take measurements within the home and determine whether the electricity network needs to be involved.

The lighting itself

Some LED lamps, drivers, and dimmers are more sensitive to voltage changes than others. If only one fitting reacts, replacing or testing the lighting equipment may resolve the issue.

However, sensitive lighting can also expose a genuine voltage problem. Replacing the lamp should not be used to disguise flickering occurring throughout the home.

When should you arrange an electrical check?

WorkSafe identifies flickering lights as one possible warning sign that a home’s wiring may need attention or upgrading. It does not prove the entire installation is unsafe, but it should not be dismissed without considering the wider pattern. WorkSafe electrical wiring guidance

Arrange an assessment if the flickering:

• Affects several rooms or most of the home

• Is pronounced, repeated, or becoming worse

• Continues after the heat pump has started

• Is accompanied by buzzing, crackling, heat, or an unusual smell

• Causes appliances to reset or operate incorrectly

• Includes some lights brightening while others dim

The Electrical Workers Registration Board advises isolating the power and contacting a licensed electrical worker when abnormal flickering occurs alongside strange smells, arcing or crackling noises, or appliances not operating correctly. EWRB safety guidance

What should a proper assessment establish?

A useful assessment should do more than recommend a larger switchboard or blame the heat pump.

It may include observing the heat pump’s operation, measuring voltage while the load changes, checking circuit design and conductor size, inspecting accessible connections, reviewing switchboard condition, and assessing the incoming mains supply.

For Dunedin homes, this whole-system view is particularly valuable because an older electrical foundation may now be supporting heat pumps, induction cooking, EV charging, and other loads that were never anticipated when the home was wired.

The takeaway

A slight momentary dip does not automatically mean you need a switchboard upgrade. It is a clue that should be considered alongside the severity, duration, location, and history of the flickering.

If the cause is unclear, a Power Integrity Review™ can help establish whether the issue sits with the lighting, heat pump circuit, switchboard, mains connection, or wider supply before deciding what work is actually needed.