Quick answer
A furnace warranty is voided or cut short by how the unit was installed and how it has been looked after: installation by someone without a gas licence, an internet purchase with no installing dealer, wrong venting or gas pressure, non-original parts, or moving the furnace to another address. Calling a different licensed company for service does not void it.
What voids a furnace warranty is almost always something done to the furnace, not something wrong with it. A manufacturer's certificate covers defects in materials and factory workmanship. It excludes damage traced to how the unit was installed, what it was connected to and how it was maintained, and most disputes are a single declined claim, not a cancelled warranty. The causes fall into two groups: the installation, and the years of ownership after it.
Installation causes: coverage lost on day one
- Not installed by a licensed contractor: certificates require installation to code by a qualified installer, and in Ontario that means a TSSA-registered contractor and a G2 or G1 technician. Installing a furnace yourself fails both tests.
- Bought online: several manufacturers exclude equipment sold over the internet unless the seller is also the installing dealer. A discounted furnace in a crate may have no parts coverage at all.
- Wrong venting: a vent longer than the manual's table allows, the wrong pipe material, or pipe without slope back to the furnace lets condensate pool. The pressure switch faults and the inducer corrodes, and neither is a factory defect.
- Wrong gas pressure: manifold pressure has to be set and recorded at start-up. An over-fired furnace overheats its heat exchanger; so does a propane conversion done without the manufacturer's kit, a real risk on rural lots beyond the gas main.
- Oversized unit short cycling: a furnace too large for the house heats up fast, trips its high limit and cycles off, over and over. The evidence a manufacturer looks for is a temperature rise outside the range printed on the rating plate.
- Used as construction heat: running a new furnace through drywall sanding packs the blower and secondary heat exchanger with dust, and many certificates exclude it by name.
Ownership causes: what happens after the installer leaves
- Unregistered unit: this shortens the parts term but does not void it; see registering a furnace warranty.
- No maintenance records: certificates call for maintenance as the manual describes, and a heat exchanger claim is when a manufacturer asks for proof. Annual maintenance and your warranty explains what counts.
- Starved airflow: a filter left in for a year, or half the registers closed, overheats the furnace, and the resulting damage is classed as improper maintenance.
- Non-OEM parts: a universal igniter or aftermarket control board is not covered itself, and neither is anything it damages.
- Moving the furnace: the warranty belongs to the original installation address. A furnace relocated to a cottage or another house loses it.
- A missing or altered rating plate: without a legible serial number there is no claim.
- Water, power surges and corrosive air: a flooded basement, a lightning strike or pool chemicals and bleach stored beside the furnace are excluded causes, and belong on a home insurance claim instead.
What does not void a furnace warranty
Calling a different licensed company than the installer for service does not void the manufacturer's warranty. Parts coverage follows the serial number, not the dealer, and any licensed contractor can source a warranty part through the brand's distributor. Changing your own filter, replacing the thermostat or selling the house does not void it either, though a sale can change the term. One late tune-up does not cancel a certificate; years with no record give the manufacturer grounds to decline.
The labour warranty is a separate promise. It is set by the installer, written into the installer's contract and can carry stricter conditions, such as requiring that the installer performs the repair. Read it alongside the manufacturer's certificate, because a covered part with no labour coverage is still an invoice.
What a declined warranty claim costs
- Heat exchanger replacement: $1,500–$3,000 in parts and labour
- Draft inducer replacement after condensate damage: $400–$900
- Hot surface igniter: $200–$450
- Annual tune-up that produces the maintenance record: $120–$250
On a furnace past 15 years, a declined heat exchanger claim usually ends in replacement, not repair. Other repair ranges are listed on our pricing page.
How to protect a future claim
- Keep the invoice showing the install date, the installer's name and TSSA registration number.
- Ask for the commissioning sheet: gas pressure, temperature rise and combustion readings taken at start-up.
- File the registration confirmation and a photo of the rating plate.
- Keep each tune-up invoice, dated, with the readings the technician recorded.
Ask for the readings in writing at every tune-up; that report is the record a claim asks for. Book a furnace tune-up with a TSSA-licensed technician, or send us your warranty question with the model and serial number.