Why do my potlights flicker when the furnace or AC kicks on?

This is almost always caused by a brief voltage dip on the circuit when your furnace blower or air conditioner compressor starts up and draws a large surge of current — it's common in GTA homes and usually not dangerous, but it can indicate an undersized panel or shared circuit that's worth investigating. Large motor-driven appliances like furnaces, central air conditioners, and sump pumps draw 3 to 7 times their normal running current for a fraction of a second when they start. This momentary surge pulls voltage down across your home's electrical system, and LED potlights — which are extremely sensitive to voltage changes — respond with a visible flicker.

In most cases, the flicker is a nuisance rather than a hazard, especially if it happens only at the moment the appliance starts and lasts less than a second. This is normal behaviour in homes where the furnace or AC shares a circuit with other loads, or where the panel is a 100A unit running near its capacity. It's particularly common in post-war GTA homes across Scarborough, North York, and Etobicoke where the original 100A panel is now supporting far more electrical load than it was designed for — central air conditioning, multiple bathroom fans, kitchen appliances, and LED lighting that the 1960s electrical design never anticipated.

However, persistent or severe flickering deserves professional attention because it can point to underlying issues. A loose connection at the panel, a deteriorating breaker, or an undersized neutral wire can all amplify voltage fluctuations beyond what's normal. Loose connections are especially concerning because they cause arcing — tiny electrical sparks at the connection point that generate heat and are a leading cause of electrical fires in residential properties. If your lights dim noticeably (not just a brief flicker) when the furnace runs, or if the flickering has gotten worse over time, have a licensed electrician inspect your panel and the circuits involved.

There are several solutions depending on the root cause. If the furnace or AC is on the same circuit as your lighting, your electrician can run a dedicated circuit for the appliance so its startup surge doesn't affect other loads — this costs $300 to $800 depending on the distance from the panel. If your panel is at or near capacity, a panel upgrade from 100A to 200A ($2,500 to $5,000 in the GTA) solves the problem by providing enough capacity that individual appliance startups don't create noticeable voltage dips across the system. For air conditioners specifically, a hard-start kit ($100 to $200 installed by your HVAC technician) reduces the compressor's startup surge by up to 50 percent, often eliminating the flicker without any electrical panel work.

If you're experiencing this in a condo, the situation is different. Condo electrical distribution means your unit panel draws from the building's main switchgear, and voltage dips are sometimes caused by loads on other units or the building's common systems. Document the flickering pattern and report it to your property management — the issue may require the building's electrical contractor to investigate the distribution system. For house-related flickering concerns, get matched with a licensed electrician through Toronto Electrical Repair for a proper assessment of your panel and wiring.