The Furnace Itself
A furnace cleaning and a furnace service are two different things, and the second one is the one that matters in a Toronto winter.
Short answer
Cleaning means the blower wheel, the cabinet, the burners and the heat exchanger surfaces. Service adds the combustion check, the heat exchanger inspection, the safety controls, the venting and the gas pressure. In Ontario the gas work has to be done by a licensed gas technician, which is a fair thing to confirm.
The cleaning part
The blower wheel, removed and cleaned, which is the dirtiest component and the one that affects airflow most.
The blower compartment and the cabinet interior.
The burners, cleaned and checked for proper flame.
The flame sensor and the igniter, cleaned, which is the most common no-heat call in January.
The heat exchanger surfaces, and the secondary heat exchanger on a condensing furnace.
And the condensate trap and drain on a condensing furnace, which blocks and shuts the furnace down — another common January call.
The heat exchanger inspection
A cracked heat exchanger puts combustion products into the supply air, which is a carbon monoxide risk. Inspecting it properly means looking at it with a camera or a mirror and a light, not glancing into the cabinet. On an older furnace this is the single most important part of the visit and it is the part most often skipped on a cheap "cleaning".
The service part
Combustion analysis, which measures what the burner is actually doing rather than inferring it from the flame colour.
Carbon monoxide measured in the flue and in the ambient air.
Heat exchanger inspection as above.
Venting checked end to end, including the termination outside and the condensate drain on a high-efficiency unit.
Gas pressure and gas train checked, which is licensed work in Ontario.
And the safety controls tested: the limit switch, the pressure switch, the roll-out switch and the flame proving.
When to have it done
Fall, before the heating season, rather than in January when everybody needs a technician.
Annually on a gas furnace, which is both the manufacturer position and the sensible one.
Before the first cold snap, because the no-heat calls cluster in the first week of real cold.
After any duct cleaning, so the blower and the coil are done as part of the same visit.
Immediately if there is any carbon monoxide alarm activation, any soot, any smell of combustion products, or any symptom in the household.
And a carbon monoxide alarm on every level regardless, which in Ontario is required where there are fuel-burning appliances.
Common questions
What is the most important part of the service?
The heat exchanger inspection and the combustion check.
When should it be done?
Fall, before the heating season — not in January.
Does it need a licensed technician?
In Ontario the gas work does. Confirm the licence.