Quick answer
Check it in three steps. Hold your hand in front of a supply register: if the air is cool, the furnace is running but not making heat. If the air is hot but weak, airflow is the problem — filter, ducts or blower. If the air is hot and strong and the house is still cold, heat is leaving faster than the furnace can replace it.
When the house is cold even with the furnace on, the fault lies in one of three places: making heat, delivering heat, or holding onto it. The register test above tells you which one in about 10 seconds, and it stops you chasing the wrong thing on the coldest night of the year.
Cool air at the registers: the furnace is not making heat
If the blower runs but the air is room temperature or cooler, the burners are either not lighting or not staying lit. A dirty flame sensor is the usual culprit: the burners light, the sensor fails to prove flame within a few seconds, the gas valve closes, and the blower keeps pushing cool air around the house. The control board will be flashing a fault code through the small window in the lower door while it happens.
Other candidates are a thermostat fan left on with no heat call, a high-limit trip that has cut the burners while the blower keeps running, or a lockout after three failed ignition attempts. Why a furnace clicks but will not ignite walks through the ignition side, and what a flashing furnace light means explains how to count the code before you touch anything.
Hot but weak air: the heat is not reaching the rooms
- A loaded filter. This is the leading cause of a warm furnace with a cold house, and the leading cause of limit trips that cut cycles short.
- Duct leakage into cold space. Untaped joints above a garage ceiling or in an unheated crawlspace dump heated air where it does nothing for you, and leaks on the return pull cold air in and drag supply temperatures down everywhere.
- Dampers still set for summer. Many homes here have manual balancing dampers on the trunk lines, and if they were flipped in June nobody remembers them in December.
- A blower on the wrong speed tap, or a variable-speed motor never configured after installation. The furnace heats fine and delivers poorly.
- Closed or blocked registers. Shutting vents in spare rooms usually makes the rest of the house worse — should I close vents in unused rooms explains why.
A technician settles this by measuring temperature rise: the difference between return air and supply air, which has to fall inside the range printed on the furnace rating plate, usually somewhere around 20–35°C. Above the range means too little air is moving. Below it means the furnace is not producing what it should.
Good heat, still cold: the house is losing it faster
Two-storey homes built through the 1970s to 1990s in Glen Abbey and River Oaks often have a single large return on the main floor and none upstairs. Warm air goes up, cold air has no path back down, and the bedrooms stay behind all winter regardless of how well the furnace runs. Adding an upstairs return, or at minimum jumper ducts and undercut doors, does more than any equipment change — why one room is always cold covers the balancing side of it.
- Attic insulation that has settled below current levels, or was never extended over an addition.
- Bedroom doors closed at night with the only return grille out in the hallway, so upstairs air cannot circulate at all.
- A thermostat mounted on an interior wall near a supply register, reading warmer than the rooms you care about.
- Uninsulated solid-brick exterior walls in pre-war houses in Dundas and Ancaster, where the real fix is on the building side rather than the mechanical side.
- Wind exposure. Lakeshore homes in Bronte take it straight off the water, and a north-west gale pulls heat through every gap far faster than still air at the same temperature.
Cold snaps: when running non-stop is normal
During a deep cold stretch a correctly sized furnace can run nearly continuously and still sit a degree or two below the setpoint. That is expected, not a fault: equipment here is sized for around −18°C with no spare capacity for −22°C plus wind. What is not normal is a house that cannot hold 21°C when it is only −5°C outside. That gap points to airflow, duct losses, or a furnace that has quietly lost output, and it deserves a diagnosis rather than a thicker sweater.
What to check before you call
- Thermostat set to Heat, fan set to Auto, setpoint above room temperature, and fresh batteries in it.
- A filter you can see light through, installed with the arrow pointing toward the furnace.
- Every supply and return grille open and clear of furniture, curtains and rugs.
- The furnace switch on, the blower door fully seated so its safety switch is made, and the breaker fully in.
If the basics check out and the house is still cold, that is a diagnostic rather than a guess: $90–$150, credited toward the repair when you go ahead. ZK Mechanical carries the common sensors, igniters and switches on the van, so most low-heat calls finish in one visit. See furnace repair for what that includes, or book a time that suits you.