My lights flicker when the furnace kicks on - is this dangerous?

Flickering lights when your furnace starts is typically not dangerous, but it indicates your electrical system is being overloaded and needs attention from a licensed electrician.

When your furnace motor starts up, it draws a large surge of power (called "inrush current") that can temporarily drop the voltage in your home's electrical system. This voltage drop causes lights to dim or flicker momentarily. While this won't immediately harm you, it signals that your electrical system may be undersized for your home's current demands.

Common causes include:

• Undersized electrical panel - Many Ottawa homes still have 100-amp panels that struggle with modern electrical loads


• Shared circuits - Your lights and furnace may be on the same circuit, which violates current Ontario Electrical Safety Code requirements


• Voltage drop from utility connection - Loose connections at your meter or service entrance


• Aging wiring - Older homes with knob and tube or early aluminum wiring may not handle modern loads efficiently

In Ottawa's older neighborhoods like the Glebe, Westboro, or Sandy Hill, this issue is particularly common in homes built before 1980. These homes often have 100-amp electrical services that were adequate when installed but now struggle with modern appliances, computers, and heating systems.

When flickering becomes concerning:
If lights flicker severely (dimming by 50% or more), stay dim for several seconds, or if you notice other electrical issues like outlets not working or breakers tripping frequently, this requires immediate attention. These symptoms could indicate dangerous loose connections that pose fire risks.

ESA requirements mandate that major appliances like furnaces have dedicated circuits. If your furnace shares a circuit with lighting, this violates current code and should be corrected. While existing installations may be grandfathered, it's worth upgrading for safety and performance.

Next steps: Have a licensed electrician evaluate your electrical panel capacity and check for proper circuit separation. In many cases, upgrading from a 100-amp to 200-amp panel solves flickering issues permanently. This typically costs $2,000-$3,500 in Ottawa and often pays for itself through improved efficiency and home value.

For a professional assessment of your electrical system's capacity, Electrical Ottawa offers free consultations to help determine if a panel upgrade or circuit modifications would resolve your flickering lights safely and permanently.

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