When it’s -25°C outside and your furnace suddenly starts blowing cold air, it’s more than inconvenient, it’s stressful.
But don’t panic just yet.
At A1 Heating & Cooling, we’ve helped countless St. Albert homeowners troubleshoot this exact issue. And while some cases do require a professional repair, many can be traced back to simple, fixable causes.
Here are five common reasons your furnace might be blowing cold air, along with practical steps and insights from our team.
One of the first things to check is your thermostat. If the fan is set to "On", the blower will run constantly—even when the furnace isn't heating. That means it may be circulating cool air between heating cycles.
Fix: Switch the fan setting to "Auto" so it only runs during active heating.
This is an easy mistake to make and often the first issue we recommend troubleshooting before scheduling service.
When a furnace filter becomes clogged, it restricts airflow and can cause the system to overheat internally. To protect itself, the furnace will shut off the burner while leaving the fan running. That’s when cold air starts flowing through your vents.
This is by far one of the most common causes we see, especially during the heating season or after home renovations.
Fix: Turn off your furnace, remove and inspect the filter. If it’s visibly dirty, replace it. Once reinstalled, the furnace may need to reset itself. This usually happens automatically after a short time.
Pro tip: If you’ve recently completed drywall work or a renovation, your furnace may need a filter change every 24–48 hours. Post-construction duct and furnace cleaning is also a smart follow-up step.
Modern furnaces have built-in safety systems. If there are multiple failed attempts to ignite, or if overheating occurs repeatedly, the system may enter lockout mode. This shuts off the burners and keeps the blower running to cool the cabinet.
Homeowners may not even realize the unit is locked out, especially if the system resets itself after a few hours. However, if it happens more than once, it likely points to an unresolved issue with airflow, ignition, or internal components.
St. Albert homeowners often turn their thermostats down overnight and back up in the morning. But when outdoor temperatures are very low (–20°C or colder), this sudden change can overwhelm your furnace, especially older or undersized units.
The system may run on high fire for too long, causing overheating and safety shutdowns.
Fix: If possible, increase your thermostat gradually during cold spells or program a smart thermostat to warm the home slowly before peak hours. These systems are designed to stage heating more efficiently and reduce stress on the furnace.
If basic checks don’t solve the issue, your furnace may have a faulty sensor, blower motor problem, or control board failure. These issues often cause the system to run the fan without heat or shut down unexpectedly.
Next Step: If you're seeing error codes or the furnace keeps restarting, it’s time to call in a technician. Prompt service can prevent bigger problems and get the heat back on quickly.
While some furnace issues are safe to troubleshoot (like checking your thermostat or replacing a filter), others require a trained technician. Modern systems have diagnostic codes, safety switches, and sensitive electronics that should only be handled by certified professionals.
If your furnace is:
…it’s time to book a service call.
Serving St. Albert since 1979, our team knows exactly what winter throws at your heating system and how to keep your home warm and safe.
Call us today or book online to schedule your furnace checkup.
Whether it’s a quick fix or something more serious, we’ll get you sorted with honest service and local expertise you can count on.
