Quick answer
A like-for-like furnace replacement takes 4 to 8 hours — one working day for a two-person crew. Add new PVC venting for a high-efficiency upgrade, a custom plenum, a chimney liner for the water heater, or a furnace-and-AC swap and it becomes a 1–2 day job. The house is only without heat during the working hours; the new furnace fires before the crew leaves.
For most GTA homes, installing a new furnace is a one-day job: a crew arrives in the morning, has the old unit out by mid-morning, and is running the new furnace through its commissioning checks by mid-afternoon. How long it takes to install a new furnace in your house depends less on the furnace than on what it connects to — the ductwork, the venting, the gas line and the chimney — so the timeline below starts with the typical day and then lists what stretches it.
A typical furnace replacement, hour by hour
- Hour 1: Floor protection down, gas and power shut off, thermostat wires labelled, old furnace disconnected from the plenum, gas drop, venting and condensate line.
- Hours 2–3: Old unit out of the house; plenum and return drop trimmed or rebuilt to fit the new cabinet; new furnace set, levelled and sealed to the ductwork.
- Hours 4–6: Gas piping reconnected with a new drip leg and shut-off where needed, venting run, condensate drain and trap installed, 120 V circuit and thermostat wired.
- Hours 7–8: Commissioning — leak test on the gas line, manifold pressure set, temperature rise measured against the rating plate, safety controls tested, CO checked at the flue — then the walkthrough on filters and the thermostat.
What turns a one-day furnace install into two
- Mid-efficiency to high-efficiency conversion — a condensing furnace vents through two PVC pipes to the side wall instead of the chimney and needs a condensate drain, or a pump if the floor drain is far away. Coring the wall and running pipe adds several hours.
- The orphaned water heater — when the furnace leaves the masonry chimney, a natural-draft water heater is left venting alone into a flue sized for both, which can condense and spill. The gas code often calls for a chimney liner, a separate few hours of work.
- Ductwork transitions — 1950s and 60s bungalows across the GTA often have plenum and return openings cut for a much larger original furnace, so a custom sheet-metal transition gets fabricated on site.
- Combined installs — swapping the air conditioner or adding a heat pump at the same time means a new evaporator coil, line set and outdoor unit, and usually a second day.
- Gas line upsizing, a new electrical circuit, or moving the furnace to free up floor space, any of which can involve an electrician and a separate inspection.
- Tight access — a furnace in a low crawl space, a closet in a 1970s–90s townhouse, or a condo mechanical room that needs an elevator booking.
What happens before the crew arrives
A proper install starts with a site visit rather than a phone quote. The installer measures the house for a heat-loss calculation so the new furnace is sized for the building instead of matched to the old nameplate — most replacements come out smaller than the furnace they replace — checks the ductwork static pressure, notes the venting path and confirms the gas meter has the capacity. That visit also settles the model, so the truck arrives with the right cabinet width and the transitions are planned. How to choose an HVAC contractor in the GTA covers what to expect from that assessment.
Permits, licences and inspections in Ontario
Gas furnace work in Ontario must be done by a technician holding a TSSA gas licence, to the CSA B149.1 gas code. Many GTA municipalities also require an HVAC permit under the Ontario Building Code for a furnace replacement; the contractor should pull it and arrange the inspection, and a company that suggests skipping it is a red flag. Whether you need a permit to replace a furnace in Toronto walks through the paperwork.
Will the house be cold overnight?
Only during the working hours. Crews sequence a replacement so the old furnace comes out and the new one fires in the same visit; nobody plans a winter install that leaves a house unheated overnight. If the old furnace has already died in January, ask the contractor about temporary electric heaters for the day, keep interior doors open and a tap dripping if the house is dropping toward freezing, and use the no-heat checklist while you wait.
The last hour: what commissioning should include
Do not let a crew pack up the moment the burners light. A finished install has the gas line pressure-tested, manifold pressure set to the rating plate, temperature rise inside the printed range, static pressure checked so the blower is not fighting the ducts, the condensate drain running, and a CO reading taken. You should be shown the filter, the power switch and the thermostat, and handed the paperwork: permit number, warranty registration and the rebate forms if a heat pump or smart thermostat was part of the job.
Pricing is covered in the furnace installation cost guide for the GTA. ZK Mechanical schedules replacements as single-day jobs wherever the house allows it and tells you up front when it will take two — book a free in-home assessment or call (647) 801-1252.