If your furnace is on but your home isn’t getting warm, start here. This Toronto-focused guide walks you through quick DIY checks, what likely needs a pro (will highlight in orange), and how to keep the heat steady through our long heating season. It also includes links to deeper dives on closely related issues like a furnace that won’t start or blows cold air, so you don’t waste time guessing.

Prefer to skip the troubleshooting and book a licensed tech? Visit our Repair Service page: Furnace Repair in Toronto↗ or Call us at 1-855-795-1199.

First, confirm the basics (fast checks)

1. Thermostat sanity check

Make sure it’s set to Heat, the setpoint is above room temperature by at least 2–3°C, and the schedule/hold isn’t blocking a call for heat. Replace thermostat batteries if the display is dim or blank (where applicable). Many “no heat” calls trace back to settings or a dead battery.

2. Service switch & breaker

Find the furnace service switch (looks like a light switch near the unit) and ensure it’s ON. Then check your electrical panel: reset a tripped furnace breaker by pushing fully OFF, then ON. If it trips again, stop—there’s an electrical fault that needs a pro.

3. Filter & registers

A clogged filter chokes airflow, can trigger limit safety trips, and leaves rooms cold. Replace if dirty; a clear rule of thumb is every ~3 months (more often with pets/allergies/renos). Clear return grilles and open supply registers so air can circulate. Regular filter changes help efficiency and reliability.

If the furnace runs but rooms stay cold

1. Airflow or temperature-rise problems

• Dirty filter / blocked return / closed registers reduce airflow, so the heat exchanger can’t transfer enough warmth into the rooms. Swap the filter, open registers, and check for crushed/fallen flex duct (finished basements/attics).
• Short cycling (unit starts and stops quickly) often ties back to airflow restrictions or a failing limit/pressure control; it leaves the house under-heated and wastes energy. Have a pro verify temperature rise, blower speed, and safety switch behavior.

2. Burner lights but stalls, or supply air feels cool

• Limit switch trips (overheat protection) can shut burners while the blower keeps running, pushing lukewarm air. Common causes: low airflow, a tired blower motor/capacitor, or a faulty limit switch.
• Dirty flame sensor / burners lead to unreliable flame proving, so the gas valve closes and you get tepid air at best. Cleaning/servicing is a pro task; it’s routine maintenance on modern furnaces.

3. High-efficiency condensate or venting issues

Condensing furnaces rely on clear condensate drains and proper intake/exhaust. A clogged trap or frozen/blocked intake can hold off ignition or shut the system mid-cycle, leaving rooms cool. A faulty pressure switch can also inhibit heat if vent pressure isn’t correct; diagnosis/replacement is a technician job.

If the furnace doesn’t produce heat at all (but the fan may run)

1. Ignition system & sensors

• Hot surface igniter or spark system failures stop heating entirely.
• Flame sensor coated with oxide won’t “see” flame and the control board shuts the gas valve. These are classic no-heat culprits best handled by a pro (testing resistance/µA, proper cleaning or replacement).

2. Safety & fuel supply

Tripped limit or rollout switches, blocked flue, or failed pressure switch will inhibit burner operation by design. Don’t bypass safeties; have a licensed contractor correct the underlying fault.

3. High-efficiency drain/pressure issues (again)

If you hear the inducer but burners never light, a condensate blockage or stuck pressure switch diaphragm can be the reason. This is especially common after construction dust or during freeze-thaw.

Not sure whether it’s a start-up failure versus “runs but not heating”? Cross-check with our companion guides:
• Furnace Won’t Start
• Furnace Blowing Cold Air

Toronto-specific safety notes you shouldn’t skip

• In Ontario, safety authorities advise having fuel-burning appliances inspected annually by a TSSA-registered contractor and installing/maintaining certified CO alarms on every level of the home. That’s not just about performance; it’s about family safety.
• Routine maintenance and filter changes are part of that safety/efficiency picture; Canada’s federal guidance also emphasizes regular service to keep systems efficient.

DIY vs. call-a-pro: a quick decision tree

Safe DIY checks (no tools or just a new filter):

• Thermostat mode/setpoint & fresh batteries.
• Service switch ON; breaker not tripped.
• Replace dirty filter; open all supply/return vents.
• Clear snow/debris around outdoor intake/exhaust for high-efficiency furnaces.

Call a pro (licensed gas tech) if you notice:

• Burner won’t light / relights and drops out (igniter/flame sensor/gas valve control).
• Repeated safety trips (limit/rollout/pressure switch), flue issues, scorch marks.
• Persistent short cycling, grinding/screeching, or strong gas odors.
• Any CO alarm activation: evacuate and call your utility/911; do not re-enter until cleared.

Ready for expert help? Call us to book now—same-day service is available even during peak season.

Step-by-step: what most homeowners can try in 15 minutes

Why your furnace “runs” but the house still won’t warm up

• Duct leakage/unbalanced air: Leaky or poorly balanced ducts spill heated air into basements or utility rooms, starving living spaces.
• Closed/covered registers: Rugs or furniture over supplies/returns drastically reduce delivered BTUs.
• Insulation/air sealing gaps: Attic bypasses and thin insulation make rooms feel cold even with a working furnace. (We can assess as part of a whole-home comfort visit.)
• Oversized equipment short cycling: Brief runtimes fail to heat the envelope; a pro can verify sizing and blower speeds.

Again, preventing the next “no heat” day

• Filters: Replace about every three months under normal conditions; more often with pets/allergies/wildfire smoke or if using high-MERV media. Put a calendar reminder on your phone.
• Annual safety inspection & tune-up: Schedule with a licensed, TSSA-registered contractor before deep winter to catch ignition/combustion and condensate issues early.
• Keep the area around the furnace clean: Dust and lint impede airflow and can affect components.

Related reading (keep troubleshooting smart)

• Furnace Won’t Start (Toronto)↗ — no power/ignition lockout scenarios.
• Furnace Blowing Cold Air (Toronto)↗— when the fan runs but supply air isn’t hot.
• Noisy Furnace Operation (Toronto)↗— what different sounds mean and what to check first.
• Uneven Heating (Toronto)↗— balancing, duct/return issues, and room-by-room fixes.

For bookings and same-day support, head to Furnace Repair in Toronto↗.