Quick answer
Yes, you can buy a house with a 20-year-old furnace, but price it as a furnace you will replace. Gas furnaces in Ontario typically last 15 to 20 years, so budget $4,500–$7,500 for a high-efficiency replacement within a few years. Have a licensed technician check the heat exchanger and venting before you waive conditions, and use the result to negotiate.
Buying a house with a 20-year-old furnace is fine as long as you treat the furnace as a known cost and not a surprise. Twenty years is the far end of a normal service life, not proof of a problem: some furnaces that age run cleanly for several more winters, and some are one inspection away from being shut off. The age tells you to budget. An inspection tells you when.
How to date the furnace from its serial number
The rating plate inside the furnace's upper door gives the model and serial number, and often a manufacture date printed outright. When it does not, the serial number carries the date:
- Carrier and Bryant: the first four digits are the week and year, so a serial starting 2306 was built in week 23 of 2006.
- Lennox: the third and fourth characters are the year, followed by a letter for the month.
- Goodman and Amana: the first two digits are the year and the next two the month.
- Other brands: look up the serial format on the manufacturer's site, or send a photo of the plate to a contractor.
A furnace is usually installed within months of being built, so the plate date is a fair stand-in for its age. If the listing gives a newer date than the plate, ask for the invoice. Against the typical 15 to 20 years covered in how long a furnace lasts in Ontario, a unit built around 2006 has little time left even if it is running well today.
Mid-efficiency or condensing: the detail that changes the replacement
Look at how it vents. A furnace with a metal flue pipe rising into a chimney is mid-efficiency, about 80%. A furnace with plastic pipe running out through the side wall is a condensing model, 90% or better. A furnace from the mid-2000s could be either.
The difference matters to a buyer because a mid-efficiency furnace cannot be replaced like for like. New gas furnaces have to meet a 95% AFUE minimum, so the replacement will be a condensing unit that needs plastic venting through an outside wall, a condensate drain, and often a chimney liner if a gas water heater stays behind on the old flue. That work is normal and belongs in a proper quote, but it is why the first replacement of a chimney-vented furnace sits toward the upper part of the price band. See replacing a mid-efficiency furnace.
An early condensing furnace has its own tell. Ontario's gas code now calls for vent pipe certified to ULC S636, and furnaces installed before that requirement often vent through black ABS or plain white plumbing pipe. Expect that pipe to be replaced along with the furnace.
What to check on a 20-year-old furnace before you waive conditions
- Heat exchanger condition. This is the part that ends a furnace's life, and it cannot be judged from outside the cabinet. It takes a licensed technician with a combustion analyser and an inspection camera or mirror.
- Venting. Rust streaks or holes in a metal flue, white residue at the joints, or plastic pipe that sags or has pulled apart.
- Signs of a previous red tag. A warning tag, or the wire from one, on the gas line or cabinet; a gas valve to the furnace left closed; a recent service sticker with no explanation. Ask the seller what was found and what was done about it.
- Rust or water marks at the base, scorch marks around the burner compartment, and a blower that squeals on start-up.
- Service records. At 20 years, a furnace that has been cleaned and tested regularly is a better bet than one nobody has opened.
A home inspector runs the furnace from the thermostat and reports its age and visible condition; taking it apart and testing combustion are outside that job. Whether a home inspection is enough for the furnace and AC explains the limits.
How buyers use an old furnace in the negotiation
- Price reduction. A new high-efficiency furnace runs $4,500–$7,500 installed. Buyers ask for some or all of that off the price, with a written quote for that house as evidence.
- Inspection condition. Make the offer conditional on a licensed HVAC inspection, separate from the home inspection. A clean report lets you buy with a budget; a failed heat exchanger turns the request into a replacement before closing, or a credit.
- Replacement by the seller. Acceptable if you get the invoice, the model you agreed to and the warranty registered in your name. A furnace chosen by a seller on the way out is rarely the one you would pick.
Two other parties care about the age. Home insurers may ask how old the heating system is and what fuel it burns when you apply, and some want an inspection on older equipment; our guide to furnace inspections for home insurance covers what they look for. If the furnace turns out to be rented and not owned, that is a separate question with its own math.
ZK Mechanical inspects furnaces for buyers during the conditional period: a TSSA-licensed technician dates the unit, tests combustion, checks the heat exchanger and venting, and writes up the findings with a replacement price for that house. Read what the HVAC inspection covers or book one before your condition date.