Quick answer
A furnace was installed correctly if three things line up: the visible work matches the manual (vent pipes sloped back to the furnace, sealed plenum joints, a trapped condensate drain, a shut-off and drip leg on the gas line), the installer left a commissioning sheet with gas pressure, temperature rise and combustion readings, and the furnace runs long, quiet cycles through its first winter.
You can tell whether your furnace was installed correctly without opening the cabinet: look at the pipes around it, read the paperwork that came with it, and pay attention to how it behaves in its first season. A furnace is a factory-built box that performs only as well as the venting, gas line, ducts and set-up done in your basement. Most installation faults leave evidence a homeowner can find with a flashlight in ten minutes.
Visible checks around a correctly installed furnace
- Vent and intake pipes slope back toward the furnace along their whole run, about a quarter inch per foot, with no sags. Condensate forms inside the exhaust pipe and has to drain back to the furnace; a low spot holds water and trips the pressure switch.
- The pipes hang from proper supports every few feet and do not rest on ducts or dangle from wire.
- The plastic vent pipe is marked ULC S636. Ontario's gas code calls for certified vent pipe, fittings and cement from one listed system; plain plumbing pipe or a mix of brands is a defect.
- The outside terminations sit well above the snow line and keep the clearances set by the manual and CSA B149.1 from windows, doors, the gas meter regulator and any air intake, including the neighbour's.
- The joints between the furnace, coil cabinet, supply plenum and return drop are sealed with foil tape or mastic. You should not feel air blowing out or see light through a gap.
- The condensate line has a trap, slopes continuously to a floor drain or a condensate pump, and is dry at every joint.
- The gas line has a manual shut-off valve within reach of the furnace, a union so the furnace can be disconnected, and a drip leg, the short capped stub below the tee that catches debris before it reaches the gas valve.
- There is a power switch at the furnace, a filter you can slide out without tools, and room to take the doors off.
One check applies to the appliance beside the furnace. If the old furnace shared a chimney with a gas water heater and the new one vents through the wall, the water heater is now alone on a chimney sized for two appliances. It may need a liner to draft properly; chimney liners after a furnace replacement explains why.
Paperwork a proper furnace installation leaves behind
- A commissioning or start-up sheet. It records manifold gas pressure (about 3.5 inches of water column on high fire for most natural gas furnaces), temperature rise measured across the furnace and compared with the range on the rating plate, and combustion analyser readings. A static pressure reading is a good sign if it is there too.
- Warranty registration. Most manufacturers give the full parts term only if the furnace is registered within a set window after installation, often 60 to 90 days. Ask for the confirmation; whether you have to register a furnace warranty covers what happens if nobody did.
- An invoice showing the model and serial number, the installation date and the contractor's TSSA registration number. Gas contractors in Ontario must be registered with the TSSA, and the technician who signs off the gas work holds a G2 or G1 certificate. How to verify a TSSA licence shows where to look it up.
A missing commissioning sheet does not prove the furnace was set up badly. It does mean nobody can show you it was set up at all. Ontario does not send an inspector to every furnace replacement either, as whether a new furnace installation needs an inspection explains, so the paperwork is often the only record.
Symptoms of a bad furnace install in the first season
- Short cycling: the furnace runs a few minutes, stops and restarts. The usual causes are a furnace too large for the house, or ducts too restrictive for its blower, which overheats it until the limit opens.
- Pressure switch lockouts on cold or windy days. Look for a sagging vent pipe holding water, a vent run longer or with more elbows than the manual allows, or a termination that ices over.
- Water at the base of the furnace. The trap was never primed, the drain has no slope, or the cabinet was set tilted away from its drain.
- Ducts that boom, whistle or roar. A new blower pushing more air than the old one through the same return shows up as noise at the grille and popping sheet metal.
- Rooms that used to heat and no longer do. A takeoff was left disconnected, a damper was closed during the work, or the new plenum starves one trunk.
- A gas smell at the furnace or soot around the burner compartment. Turn it off and call; neither is a settling-in quirk.
None of these wear off with time. A furnace that short cycles in November will do it all winter.
What to do if the installation looks wrong
Start with the installer, in writing, with photos. Workmanship falls under the installer's labour warranty, and a contractor who stands behind the job will correct a vent slope or a drain at no charge. Give them the chance before anyone else touches the furnace, since a second contractor's changes can muddy who is responsible.
Bring in an independent licensed technician when the installer will not return, when you bought the house with the furnace already in place, or when you want numbers instead of reassurance. That visit repeats the commissioning: gas pressure, temperature rise, combustion analysis, static pressure and a vent inspection against the manual, with a written report you can hand to the installer or the manufacturer. Anything that is an immediate hazard under the gas code gets tagged and shut off, which is the reason to have it checked.
ZK Mechanical does this as an HVAC inspection across Burlington, Oakville, Hamilton, Milton and Mississauga, on furnaces we did not install as well as on our own. A TSSA-licensed technician takes the readings and puts the findings in writing. Book the inspection here and have the model number and the invoice handy.