Why do my lights flicker when the furnace kicks on in my New Brunswick home?

Question

New Brunswick Electrical · Accepted Answer

Why Lights Flicker When Your Furnace Starts in New Brunswick

Lights flickering briefly when your furnace kicks on is one of the most common electrical complaints from New Brunswick homeowners, especially during our long heating season from October through April. A brief dim lasting less than a second is usually normal, but persistent or severe flickering can indicate an electrical problem that needs professional attention.

Why It Happens — The Normal Explanation

Your furnace blower motor draws a surge of electricity when it starts — typically 3-5 times its normal running amperage. A standard residential furnace blower pulls 8-12 amps while running, but during startup it can momentarily draw 25-40 amps. This sudden demand causes a brief voltage drop on the circuit and sometimes throughout your home's electrical system, which you see as a quick flicker or dim in your lights.

This is similar to what happens when a well pump starts in rural New Brunswick homes, or when a heat pump compressor cycles on — any large motor creates an inrush current at startup.

When Flickering Is Normal

A single brief dim lasting less than half a second each time the furnace starts is generally nothing to worry about. This is especially common in older New Brunswick homes with 100-amp electrical services, which were standard in builds from the 1950s through the 1980s across Moncton, Saint John, and Fredericton. The smaller service has less capacity to absorb the startup surge without a noticeable voltage dip.

When Flickering Signals a Problem

You should call a licensed electrician if you notice any of these patterns:

- Lights dim significantly (more than 30-40% brightness drop) — This suggests the voltage drop is excessive, possibly due to undersized wiring, a loose connection, or an overloaded panel.
- Flickering lasts more than 1-2 seconds — The motor may be struggling to start, which could indicate a failing blower motor, a weak start capacitor, or voltage issues from NB Power's supply.
- Lights flicker on circuits far from the furnace — If every light in the house dims noticeably, the issue is likely at the main panel or service entrance, not just the furnace circuit.
- Flickering is getting worse over time — Progressive deterioration often points to a loose connection that's getting worse, which is a fire hazard.
- You hear buzzing or humming from the panel when the furnace starts — This can indicate a loose breaker connection or a breaker that's beginning to fail.

Common Causes and Fixes

Undersized electrical service (100 amps or less): Many New Brunswick homes built before 1990 have 100-amp services that struggle with modern electrical loads — furnace, dryer, range, hot water tank, and electronics all competing for capacity. Upgrading to a 200-amp service costs $2,500-$4,500 in New Brunswick and resolves most flickering issues. This is especially relevant if you've added loads like a heat pump or EV charger.

Loose connections at the panel: Over time, breaker connections and bus bar contacts can loosen due to thermal cycling — New Brunswick's temperature swings from -30°C winter lows to +30°C summer highs accelerate this. A licensed electrician can inspect and re-torque all connections for $150-$250. This is one of the most common and dangerous causes of flickering.

Shared circuit: If the furnace shares a circuit with lighting (which violates current CEC requirements but is common in older homes), the flickering will be more pronounced. The fix is running a dedicated circuit for the furnace — typically $300-$500.

Failing furnace blower motor: A motor with worn bearings draws more startup current than normal. If flickering has gotten worse and your furnace is 15-20+ years old, have an HVAC technician check the motor's amperage draw.

NB Power voltage issues: In some rural New Brunswick areas, particularly during peak winter demand, utility voltage can sag below the standard 120V. If neighbours are experiencing similar issues, contact NB Power at 1-800-663-6272 to request a voltage check at your meter.

What to Do

1. Note the pattern — Does it happen every time? How long does the dim last? Which lights are affected?
2. Check your panel — Look for any signs of scorching, melted plastic, or a burning smell (if you find any, call an electrician immediately)
3. Have an inspection — A licensed TSANB-certified electrician can measure the actual voltage drop during furnace startup with a power quality meter. The CEC allows a maximum 5% voltage drop on branch circuits — anything beyond that needs correction.

Don't ignore worsening flicker — loose connections are a leading cause of electrical fires in Canadian homes, and New Brunswick's older housing stock is particularly susceptible.

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