Why do my lights flicker when the furnace kicks on?

Why Lights Flicker When Your Furnace Starts

A brief, subtle flicker when your furnace starts is normal. A prolonged dimming or noticeable brightness drop signals a problem that needs attention.

Normal vs. Problematic Flickering

Normal: Lights dim for a fraction of a second (less than 1 second) when the furnace blower motor starts. Electric motors draw 3–7 times their running current during startup, causing a momentary voltage dip on the circuit. This is especially noticeable with a high-efficiency furnace that has a variable-speed blower drawing 800–1,200 watts at startup.

Problematic: Lights dim noticeably for 2+ seconds, dim repeatedly while the furnace runs, or flicker throughout the house. This points to one of several issues.

Common Causes

1. Undersized electrical panel
Many older New Brunswick homes — particularly those built before 1980 in Moncton, Saint John, and Fredericton — still have 60-amp or 100-amp panels. When a furnace motor starts alongside other loads (electric dryer, hot water tank, kitchen appliances), the panel can't deliver enough current without voltage drop. Upgrading to a 200-amp panel ($2,500–$4,500 installed) resolves this permanently.

2. Shared circuit
If the furnace shares a circuit with lighting, the startup surge directly affects those lights. The Canadian Electrical Code requires furnaces to have a dedicated circuit. If yours doesn't, an electrician can run one for $300–$600.

3. Loose connections
Loose wire connections at the panel, junction boxes, or the furnace disconnect create resistance. Under load, that resistance causes voltage drop and flickering. This is a fire hazard — loose connections generate heat and are a leading cause of electrical fires. Have a TSANB-licensed electrician inspect and tighten all connections.

4. Failing furnace blower motor
A motor with worn bearings draws more current than normal during startup, creating a larger voltage dip. If the flickering has gotten worse over time, have your HVAC technician check the motor's amp draw against its nameplate rating.

5. Utility-side voltage issues
If flickering happens across your entire house and affects neighbours too, the issue may be on NB Power's side — a failing transformer, loose connection at the pole, or overloaded neighbourhood circuit. Contact NB Power at 1-800-663-6272 to report voltage fluctuations. They'll investigate at no charge.

What to Do

A diagnostic visit from a licensed electrician in New Brunswick typically costs $80–$150 and can identify whether you need a panel upgrade, a dedicated circuit, or just a connection tightening.

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