A furnace blowing cold air usually has its thermostat fan set to ON, a clogged filter, or a blocked vent pipe or condensate drain, all of which you can check. A dirty flame sensor, failed igniter or gas supply fault is gas-side work that Ontario and Quebec rules generally reserve for certified technicians.
Get Your Furnace FixedOne liner: Check the thermostat, filter, vents and drain line, then hand the gas side to a licensed technician.
| Stat | What it measures | Source |
|---|---|---|
| 17 seconds and 7 seconds | Igniter warm-up and ignition | Carrier 59SC6A-03SI, edition 7/26 |
| 25 seconds | Blower-on delay | Carrier 59SC6A-03SI, edition 7/26 |
| 30 seconds | Blower-on delay | Goodman IOG-2020B, 2019 |
| 90 to 180 seconds | Blower-off delay | Carrier 59SC6A-03SI, edition 7/26 |
| 12 in (305 mm) | Vent clearance above grade or snow | CSA B149.1, cited in Carrier 59SC6A-03SI |
| Below 0 °C (32 °F) | Condensate freeze protection needed | Carrier 59SC6A-03SI, edition 7/26 |
| Every two months, or as needed | Disposable filter replacement | Goodman IOG-2020B, 2019 |
| Every year, before the heating season | Heating system verification | RBQ, Heating systems |
| 1-866-763-5427 | Enbridge Gas emergency line | Enbridge Gas, checked November 3, 2026 |
In short: A fan setting, dirty filter, blocked vent or drain, or an ignition fault, after you rule out the normal blower delay.
A furnace blowing cold air on a freezing night feels like an emergency, but the cause is often simple. A fan setting, a clogged filter or a snow-covered vent pipe can each stop the heat, and you can check all three in minutes without touching anything gas-related. Causes like a dirty flame sensor or a failed igniter are different. They sit on the gas side, where Ontario and Quebec rules call for a certified gas technician, with only a narrow exception in Ontario. Here are the seven causes, what each one looks like, and where your own checks should stop.
Before you troubleshoot, rule out normal operation. On a modern gas furnace, the main blower doesn’t start the moment the thermostat calls for heat. Carrier’s installation manual for its 59SC6A furnace gives the timing. The igniter warms for about 17 seconds and ignition takes about 7, then the blower waits 25 seconds after the gas valve opens. Goodman uses a similar 30 second delay.
So you may feel cool air, or none at all, for close to a minute. The delay lets the heat exchanger warm up first. Once the burners light, warm air should reach your vents within about a minute.
The end of a cycle works the same way. After the burners shut off, the blower runs on for 90 to 180 seconds on Carrier’s settings, or 100 or 150 seconds on Goodman’s. It’s pulling leftover heat out of the heat exchanger, so the air cools before the fan stops. That’s normal.
What isn’t normal: a furnace blowing cold air when heat is on for several minutes straight, or air that turns cold mid-cycle while the thermostat still calls for heat. Either one points to one of the seven causes below.
In short: In Ontario and Quebec, gas-side work needs a certified technician; your checks stop at settings, filters, vents and drains.
In Ontario, no one may repair or service a gas furnace without a certificate, under Ontario Regulation 212/01 (Gaseous Fuels). In Quebec, maintenance and repair of a gas appliance require a certificate in gas appliance techniques (TAG), and the Régie du bâtiment du Québec (RBQ) says a licence is always mandatory for work on gas installations. That’s a different picture from the many online guides that tell homeowners to clean the flame sensor, relight the pilot or check the gas line.
The Technical Standards and Safety Authority (TSSA), which enforces Ontario’s fuel rules, puts it plainly: furnaces “must be maintained by TSSA-registered contractors”. Ontario has one narrow exception. Under Ontario Regulation 215/01, the owner of a detached house who lives in it may service their own appliance, and may relight it only by following the manufacturer’s certified lighting instructions. The exception names detached houses only, not townhouses or condos.
Manufacturers draw their own line, too. Goodman’s service manual labels flame sensor cleaning “Qualified Servicer Only”. Carrier’s manual calls for fine 0000 steel wool and warns against sandpaper or emery cloth.
| Safe checks you can do | Leave to a licensed gas technician |
|---|---|
| Thermostat mode and fan setting | Flame sensor cleaning or replacement |
| Furnace filter | Igniter or pilot assembly |
| Snow and ice around the intake and exhaust pipes | Gas valve and gas piping |
| Condensate line and pump, for visible blockage or ice | Burners and anything behind the burner access panel |
| Furnace power switch and breaker | Limit switch, pressure switch and control board repairs |
| Status light code, read through the viewing window | Clearing a frozen condensate trap |
The seven causes below follow that split: four you can check, and three to leave to a technician.
In short: Set the fan to AUTO, replace a clogged filter, clear snow from the vent pipes and look for a blocked drain. 
Start here. These four are the quickest causes of a furnace blowing cold air to rule out, and none of them involve the gas side of the unit. Bryant, the furnace maker, advises professional service rather than repeated attempts to restart a furnace, and says a breaker that keeps tripping should not be reset again. If a check doesn’t bring the heat back, stop there.
Carrier’s Canadian furnace guide lists a fan set to “on” instead of “auto” as one of the more common reasons for cold air. On ON, the blower runs nonstop, so it keeps pushing air between heating cycles, when the burners are off. Check three things at the thermostat:
Give the furnace a full cycle after any change. If warm air comes back, you’ve found it.
Goodman’s installation manual calls improper filter maintenance “the most common cause of inadequate heating or cooling performance” and lists blocked filters among the causes of an open limit circuit. A clogged filter starves the furnace of air, so the heat exchanger runs too hot. The high limit switch then steps in. Carrier’s manual says the limit shuts off the burners and keeps the blower running when the furnace overheats.
In other words, the flame goes out to protect the furnace, but the fan keeps moving air. That’s why the problem often looks like a furnace running but no heat: you hear the blower, but the air coming out is cool.
Pull the filter and hold it up to a light. If you can’t see light through it, replace it. Goodman’s guidance is every two months for disposable filters, or as needed.
A high-efficiency furnace draws air in and pushes exhaust out through two plastic pipes, usually on an outside wall. Under the Canadian Standards Association (CSA) B149.1 Natural Gas and Propane Installation Code, as cited in Carrier’s manual for Canadian installs, those pipe ends must sit at least 12 in (305 mm) above grade or the expected snow level. A big snowfall, drifting or ice can still cover them.
When either pipe is blocked, a pressure switch stops the furnace from firing. Goodman lists a “blocked flue and/or inlet air pipe” as one cause of that fault. Go outside and clear snow and ice from around both pipe ends by hand. Don’t pull, twist or knock on the pipes.
High-efficiency furnaces make water as they run, and a drain line carries it away. Carrier’s manual is direct: “a plugged condensate drain or a failed pump may cause the furnace to shut down”. It also calls for freeze protection wherever temperatures can drop below 0 °C (32 °F). Goodman lists a blocked drain among the causes of its pressure switch fault.
Look for:
If the pump is unplugged, plug it back in. Leave a frozen line or a clogged trap to a technician.
In short: Burners that light then quit, no igniter glow, or a gas supply problem all need a licensed technician. 
These three sit on the gas side of the furnace. Your job here is to watch and listen, then describe what you saw when you call. That helps the technician start in the right place. Carrier also lists a faulty fan limit switch, which can keep the blower running when the furnace isn’t heating.
You hear the burners light. The furnace runs for a few seconds, clicks off, then tries again. Meanwhile, the air at the vents never warms up.
That pattern often points to the flame sensor, a thin metal rod that sits in the burner flame and tells the control board the gas is burning. Bryant explains that a dirty or faulty sensor can stop the furnace from detecting a flame that’s lit properly, so it shuts the burners down shortly after start-up. Carrier adds that the signs are a cool house and rapid on-off cycling. The burners keep shutting off, so the air coming through the vents stays cold.
Cleaning the sensor means opening the burner compartment, and it calls for the right material. Carrier specifies fine 0000 steel wool and rules out sandpaper and emery cloth. Leave it to a licensed technician.
Most modern furnaces light the burners with a hot surface igniter, a small part that glows bright orange during start-up. In Carrier’s start-up sequence, that warm-up lasts about 17 seconds. If your furnace has a viewing window, watch it during a heat call. No glow and no flame points to the ignition system.
Bryant puts it simply: “A failed pilot light or electronic ignition prevents the burners from igniting,” and it advises professional service over repeated restarts. Older furnaces use a standing pilot flame instead. In Ontario, only the owner of a detached house who lives in it may relight one, and only by the manufacturer’s certified lighting instructions. In every other case, and anywhere in Quebec, call a certified gas technician.
If gas isn’t reaching the burners, the furnace may try to light and fail, or never fire at all. Carrier notes that fuel supply problems can affect natural gas and propane furnaces alike, and it sends natural gas supply problems to a qualified professional.
Signs it may be the supply rather than the furnace:
Don’t touch the gas valve or piping. If you smell gas, skip the troubleshooting: leave the house and call from outside.
In short: Four causes are yours to check, three belong to a technician.
Use this table to match what you’re seeing to the likely cause, and to see where your part ends.
| Cause | What you’ll notice | Safe check | Who fixes it |
|---|---|---|---|
| Thermostat fan set to ON | Cool air between heating cycles | Set fan to AUTO, mode to Heat | You |
| Dirty filter | Blower runs, but the air stays cool | Check the filter, replace if clogged | You |
| Snow or ice on vent pipes | Furnace won’t fire after a storm | Clear both pipe ends by hand | You |
| Clogged or frozen condensate drain | Water at the base, ice in the line | Look for ice, check the pump plug | You spot it, a technician clears it |
| Dirty flame sensor | Burners light, then shut off within seconds | Watch and note the pattern | Licensed technician |
| Failed igniter or pilot light | No glow and no flame at start-up | Watch through the viewing window | Licensed technician |
| Gas supply or gas valve problem | Other gas appliances are out too | Check other appliances, call your utility | Gas utility or licensed technician |
In short: Cold rooms with warm vents elsewhere point to vents or ducts, not the furnace.
If most vents blow warm air but one or two rooms stay cold, the furnace is probably working. Bryant notes that leaking or restricted ducts can cut the heated air that reaches your rooms, especially where ducts run through attics or crawl spaces, but that duct leaks generally don’t explain truly cold air at the register. Carrier adds that closing too many supply vents can make the furnace overheat. Open any closed registers and move furniture or rugs off vents and return grilles. If the cold rooms stay cold, have the ductwork checked.
If nothing happens at all when the thermostat calls for heat, with no blower and no burners, that’s a different problem. Start with the checks for a furnace not turning on instead.
In short: Gas smell or CO symptoms mean leave and call from outside; failed checks mean book a technician. 
Two situations aren’t troubleshooting at all. If you smell gas (a rotten egg odour), get everyone out first and call from outside.
| Where you live | If you smell gas |
|---|---|
| Ontario (Enbridge Gas area) | Leave right away, then call Enbridge Gas at 1-866-763-5427 or 911 from a safe distance |
| Quebec (Énergir area) | Avoid flames and sparks, including phones and light switches, leave, then call 911, or 1 800 361-8003 where 911 isn’t available |
Carbon monoxide (CO) is the other. Énergir warns that dizziness, nausea or a headache near a gas appliance means you should leave and call 911, since a faulty appliance can cause CO poisoning. In Ontario, the Fire Code requires CO alarms next to each sleeping area, and on each storey without one, in homes with a fuel-burning appliance. In Quebec, the RBQ recommends at least one CO detector in the house, and says never to disconnect one that keeps going off.
For everything else, book a technician when:
When any of these show up, it’s time for professional HVAC repair from a licensed technician who can work on the gas side safely.
In short: Once your checks are done, a licensed technician handles the gas side.
If the thermostat, filter, vent pipes and condensate drain all check out and the air is still cold, your part is done. What’s left sits on the gas side, where a licensed technician takes over.
Superior Plumbing & Heating offers same-day furnace repair across Ontario and Quebec, with 24/7 emergency service when the cold can’t wait. We hold a TSSA licence in Ontario, and an RBQ licence and membership in the Corporation des maîtres mécaniciens en tuyauterie du Québec (CMMTQ) in Quebec. Our work comes with a warranty of up to 90 days on parts and labour.
Call (866) 932-2818 or book online, and tell us what you saw during your checks.
| Term | Meaning |
|---|---|
| Flame sensor | A metal rod in the burner flame that confirms the gas is burning. |
| Hot surface igniter | A part that glows hot to light the burners. |
| High limit switch | A safety switch that shuts off the burners if the furnace overheats. |
| Pressure switch | A switch that stops the furnace firing if a vent or drain is blocked. |
| Heat exchanger | The chamber that transfers burner heat to the house air. |
| Blower-on delay | The wait, 25 to 30 seconds, between the burners lighting and the blower starting. |
| Blower-off delay | The 90 to 180 seconds the blower keeps running after the burners stop. |
| High-efficiency (condensing) furnace | A furnace that makes water as it runs and vents through plastic pipes. |
| Condensate trap and pump | The parts that hold and remove that water. |
| Gas technician certificate | The Ontario certificate needed to service gas appliances. |
| TAG certificate | Quebec’s certificate in gas appliance techniques. |
| TSSA | The Technical Standards and Safety Authority, which enforces Ontario’s fuel rules. |
| RBQ | The Régie du bâtiment du Québec, which licenses construction and gas work. |
| CSA B149.1 | Canada’s Natural Gas and Propane Installation Code. |
| Property | Value |
|---|---|
| Rule | A certificate is needed to service, repair or activate a gas appliance (s. 6(1)) |
| Property | Value |
|---|---|
| Exception | Owner-occupant of a detached house (s. 55(1)) |
| Property | Value |
|---|---|
| Rule | TAG-1, TAG-2 or TAG-3N/3P for gas appliance maintenance and repair (s. 3) |
| Property | Value |
|---|---|
| Vent clearance | 12 in (305 mm) above grade or the expected snow level |
| Property | Value |
|---|---|
| Service area | Ontario and Quebec |
| Service | 24/7 emergency and same-day service |
| Warranty | Up to 90 days, parts and labour |
| Phone | (866) 932-2818 |
