Quick Answer
A straightforward like-for-like furnace replacement takes 4 to 8 hours — one working day. Installations involving ductwork modifications, venting changes for a high-efficiency upgrade, or a simultaneous AC or heat pump install typically take 1 to 2 days. Your home is rarely without heat overnight, because the old unit is removed and the new one commissioned the same day.
For most GTA homes, furnace installation is a one-day job. A two-person crew arrives in the morning, removes the old unit, sets and connects the new furnace, and has heat running by late afternoon. Here's how the day actually breaks down, and what can extend it.
Typical installation timeline
- Hour 1: Protect floors, shut off gas and power, drain and disconnect the old furnace
- Hours 2–3: Remove the old unit, prep the plenum and gas line, set the new furnace
- Hours 4–6: Connect ductwork, gas, venting, condensate drain, and electrical; install the new thermostat if included
- Hours 7–8: Commission the system — gas pressure, temperature rise, safety controls — and walk you through operation and filter changes
What makes an install take longer
- Mid-efficiency to high-efficiency conversion — new PVC venting through the side wall and a condensate drain add several hours
- Ductwork transitions — older GTA homes often need a custom sheet-metal plenum fabricated on site
- Furnace relocation or tight access — moving the unit or working in a low crawl space adds time
- Combined installs — replacing the air conditioner or adding a heat pump at the same time usually makes it a 2-day project
- Gas line upsizing or electrical panel work, which may require a separate visit and inspection
Permits and inspections in Ontario
In Ontario, gas furnace work must be performed by TSSA-licensed gas technicians, and municipalities may require an HVAC permit for replacement. The contractor should pull the permit and arrange any inspection — if a company suggests skipping it, that's a red flag. Our answer on choosing an HVAC contractor in the GTA covers what credentials to verify.
Will I be without heat?
Only for the working hours of the install itself. Crews schedule removals and installs in a single visit precisely so the house isn't cold overnight. In an emergency no-heat situation in winter, reputable companies prioritize replacement installs within 24–48 hours and can provide temporary heaters.
Wondering about cost? See our full furnace installation cost guide for the GTA. When you're ready for a quote, ZK Mechanical offers free in-home assessments across Toronto, Mississauga, Hamilton and the GTA — book yours here.
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