Furnace Problems in Extreme Cold | Toronto Winter Heating Tips

Extreme Cold in Ontario: 5 Furnace Issues That Cause Winter Shutdowns (And How to Prevent Them)

When temperatures drop below -20°C in Ontario, your furnace shifts from occasional use to nonstop operation.

During extended cold snaps and heavy snowstorms, heating systems are placed under constant stress.

The reality?
Most winter furnace shutdowns are not major mechanical failures. They’re small, preventable issues that escalate under extreme weather conditions.

Here are the 5 most common causes of furnace problems during Ontario’s deep freeze — and what homeowners should check.

1. Always Check the Outside Vents After Heavy Snow

Modern high-efficiency furnaces use two PVC pipes:

Combustion air intake
Exhaust vent

After heavy snowfall, these pipes can become partially or fully blocked.

Why This Matters

Restricted airflow can cause:

Ignition failure
System lockout
Short cycling (turning on and off repeatedly)

What To Do

Clear at least 12 inches (30 cm) of snow around both pipes
Remove visible ice buildup
Ensure openings remain unobstructed

Hidden Risk: Snow Inside the Intake Pipe

During strong Ontario snowstorms, blowing snow can be drawn inside the intake pipe — even if the exterior opening looks clear.

If snow packs inside:

Airflow becomes restricted
The furnace may fail to ignite properly

If your furnace stops immediately after a snowstorm, inspect the intake carefully (if safely reachable).

Do not insert tools deep into the pipe. If ice buildup is significant, professional service is recommended.

2. Check Your Furnace Filter More Often in Winter

Extreme cold means longer runtime.

A filter that appears “moderately dirty” may still restrict airflow under continuous operation.

Consequences:

Overheating
Short cycling
Increased wear on components
Reduced system lifespan

Recommendation:

Check filters monthly during peak winter, especially in homes with pets.

3. Try Not to Let the Temperature Drop Too Much Overnight

Lowering the thermostat drastically overnight may seem energy-efficient.

However, in deep winter conditions, the large morning temperature recovery forces the furnace to run continuously for extended periods.

This can:

Increase strain on the system
Raise gas consumption
Reduce long-term efficiency

Maintaining a steady indoor temperature around 20–22°C is often easier on the system during extreme cold.

4. Pay Attention to the Condensate Drain

High-efficiency furnaces produce condensation water.

If the condensate drain line freezes:

The system will shut down as a safety measure

It may appear to be a major failure, but it’s often simply a frozen drain.

Prevention Tips:

Keep the mechanical room adequately heated
Ensure drain lines are not exposed to cold air

5. Don’t Block Too Many Vents

Modern HVAC systems are engineered to move a specific volume of air.

When supply vents are closed or return vents are blocked by furniture:

Internal pressure increases
Airflow becomes restricted
Components operate under stress

During extreme cold, even minor airflow imbalances can become noticeable.

Final Thought: Small Issues Become Big Problems in Extreme Cold

Ontario winters are demanding.

When temperatures remain below -20°C for extended periods, heating systems operate under continuous load.

Most winter shutdowns are not catastrophic equipment failures.
They’re small, preventable issues amplified by extreme weather.

After a snowstorm, take a few minutes to check:

Outdoor intake and exhaust vents
Furnace filter condition
Airflow and vent obstructions

In extreme cold, a little attention can prevent a major service call.