My furnace keeps shutting off when it hits minus 25 degrees Celsius outside. What should I check first?

Your furnace is likely shutting down due to a blocked air intake or exhaust vent, which is extremely common during Calgary's brutal cold snaps when temperatures hit -25°C or below. This is a safety feature that prevents dangerous combustion issues, but it needs immediate attention.

Start with the exterior vents — go outside and check your furnace's air intake and exhaust pipes (usually white PVC pipes coming through your exterior wall or roof). During extreme cold, these vents can become blocked by ice buildup, snow drifts, or frost accumulation. The intake vent draws combustion air from outside, while the exhaust vent removes combustion gases. If either becomes blocked, your high-efficiency furnace will shut down as a safety measure. Clear any visible snow, ice, or debris from both vents, but be gentle — don't force anything that might damage the pipes.

Check your furnace filter next — a dirty filter restricts airflow and forces your furnace to work harder during extreme cold. When outdoor temperatures drop to -25°C, your furnace runs almost continuously, and a clogged filter can cause overheating and automatic shutdown. Replace the filter if it's dirty or hasn't been changed in the past 2-3 months. In Calgary's dusty conditions and during heavy heating seasons, filters often need monthly replacement.

Calgary's extreme temperature swings make this problem worse — our famous chinooks can cause rapid freeze-thaw cycles that create ice dams around vents, and the extreme cold puts maximum stress on your heating system. High-efficiency furnaces (96%+ AFUE), which are standard in Calgary homes, are more sensitive to blocked vents because they rely on precise air-to-fuel ratios for safe combustion.

When to call a professional immediately: If clearing the vents and changing the filter doesn't solve the problem, you likely have a more serious issue — possibly a failing pressure switch, heat exchanger problem, or gas valve issue. Don't attempt to bypass safety features or ignore repeated shutdowns. A furnace that can't handle -25°C weather puts your family at risk of freezing pipes and carbon monoxide exposure.

Next steps: Clear the exterior vents and replace the filter first. If the problem persists after these basic checks, contact a licensed HVAC contractor immediately. In Calgary's extreme cold, furnace failures are emergencies — most reputable contractors offer 24/7 emergency service during cold snaps. Find verified HVAC professionals in our Calgary Construction Network directory who understand the unique challenges of heating Calgary homes in extreme weather.