Quick answer
A red tag means a licensed gas technician, or your gas utility, found an unacceptable condition on your gas furnace and recorded it under Ontario’s gas safety rules. Those rules are set by the Technical Standards and Safety Authority (TSSA). If the condition is an immediate hazard, the gas to the furnace is shut off on the spot and the furnace must not be used until a licensed gas technician fixes it. Enbridge calls this a Type A safety violation. If it’s not an immediate hazard, you’ll get a warning (Type B) with a deadline: Enbridge allows up to 42 days to repair or replace before it shuts off the gas. Common causes are a cracked heat exchanger, carbon monoxide, and venting problems. Don’t remove the tag or relight the furnace yourself.
What red and yellow tags mean
Under Ontario’s gaseous fuels regulation (O. Reg. 212/01), a gas technician who finds an unacceptable condition must act on it. What they do depends on how dangerous it is. Homeowners usually call these “red tags” and “yellow tags” or “warning tags”. The colour of the physical tag varies, so the category written on your notice is what counts.
| Category | Common name | What happens | Your deadline |
|---|---|---|---|
| Immediate hazard (Enbridge “Type A”) | Red tag | Gas to the furnace, or to the whole building, is shut off right away. The furnace can’t be operated until the condition is fixed by a licensed gas technician. | Fix it before you use the furnace again. |
| Not an immediate hazard (Enbridge “Type B”) | Warning / yellow tag | Gas stays on for now. The condition is reported to the gas distributor. | Up to 42 days with Enbridge. If it isn’t fixed, the gas to the appliance can be shut off. |
In both cases the technician reports the condition to your gas distributor. In most of Waterloo Region that’s Enbridge Gas. In Kitchener and parts of Wilmot and Woolwich it’s Kitchener Utilities. For Type B notices, Enbridge says it has to visit to verify the repair and may charge a service fee.
Common reasons a furnace gets red tagged
These are the problems we see most often behind a red or warning tag on a furnace in Waterloo Region.
1. Cracked or failed heat exchanger. The heat exchanger keeps combustion gases separate from the air blown through your house. After years of heating and cooling cycles, especially in an oversized furnace that short-cycles, it can crack or rust through. That can let combustion gases, including carbon monoxide, into your living space, so it’s usually treated as an immediate hazard.
2. Carbon monoxide or poor combustion. High CO readings in the flue or at the registers, flame rollout, or a burner flame that lifts or wavers when the blower starts are all red flags. Causes include a dirty burner, wrong gas pressure, a blocked heat exchanger or a venting problem.
3. Venting problems. These include:
- disconnected, sagging, rusted or improperly sloped vent pipes
- a blocked chimney, or blocked PVC intake or exhaust (snow, ice, nests)
- an orphaned water heater: an old mid-efficiency furnace was swapped for a high-efficiency one, and the gas water heater was left alone on an oversized chimney, where it can spill exhaust
- spillage at the draft hood under depressurisation from exhaust fans
4. Gas leaks. A leaking gas valve, fitting or connector is a hazard on its own.
5. Safety controls bypassed or failed. Jumpered limit switches, a failed rollout switch, or missing panels or doors on the blower compartment.
6. Clearances and installation. Combustibles stored against the furnace, missing combustion air, or an installation that didn’t meet code.
What to do if your furnace is red tagged
If your furnace has been red tagged, do these things in this order:
- Leave it off. Don’t relight it, turn the gas valve back on or remove the tag. Operating a tagged appliance before the hazard is fixed is prohibited.
- If you smell gas, or a CO alarm is sounding, get everyone out and call 9-1-1 from outside. For a gas smell, call your gas utility’s emergency line from outside too.
- Read the notice. Note the category (immediate hazard or not), the condition recorded, the date and any deadline. Take a photo.
- Ask for detail. What exactly was found, where, and how it was confirmed? For a suspected cracked heat exchanger, ask whether it was seen on camera, shown by a combustion test, or both.
- Get heat safely. In winter, use safe electric space heaters (never a barbecue, generator or gas range indoors) and protect pipes in cold rooms.
- Call a TSSA-registered contractor to repair or replace the furnace. Enbridge Gas doesn’t repair appliances itself.
- Make sure the repair is reported. Your contractor sends the gas distributor the required information so service isn’t interrupted. For Type B with Enbridge, expect a verification visit.
Second opinions are reasonable. A tag is a serious safety call, and most technicians write them for good reason. If the tag comes with pressure to buy or rent a furnace on the spot, though, it’s fair to have a second licensed technician confirm the condition before you commit to replacement. Leave the gas off to that appliance while you wait. Never sign a rental or purchase agreement at your door. Ontario’s Consumer Protection Act bans uninvited door-to-door contracts for furnaces.
Who can clear a red tag
Only a licensed gas technician working for a TSSA-registered contractor should correct the condition, test the appliance, return it to service and report the repair to the gas distributor. The homeowner can’t clear a tag, and neither can a handyman or a general contractor without gas certification.
When we clear a tag, the visit typically covers:
- confirming the recorded condition
- repairing or replacing the faulty part, or replacing the furnace if that’s the safer and more sensible choice
- testing combustion, gas pressure, venting draft and safety controls
- reporting the completed work to Enbridge Gas or Kitchener Utilities as required
If the fix is a repair, our furnace repair page explains what that involves. If the heat exchanger is cracked on an older unit, replacement is usually the better choice. See our furnace replacement cost guide for typical Waterloo Region ranges, and our furnace installation page for what install day looks like.
Repair or replace after a red tag?
Whether to repair or replace a red-tagged furnace depends on the part, the furnace’s age and the cost.
| Condition found | Usual outcome |
|---|---|
| Blocked vent, disconnected vent pipe, nest or ice in the termination | Repair, often the same day |
| Dirty burners, wrong gas pressure, flame sensor or ignition issues | Repair |
| Failed rollout or limit switch | Repair (replace the switch and find out why it tripped) |
| Gas leak at a fitting or connector | Repair |
| Orphaned water heater spilling into the chimney | Chimney liner or venting change, or a new water heater |
| Cracked heat exchanger, furnace over about 12 to 15 years old | Usually replace the furnace |
| Cracked heat exchanger, newer furnace | Check the manufacturer’s heat exchanger warranty; labour may still be significant |
If you’re replacing the furnace anyway, it’s a good moment to think about a dual-fuel heat pump. Our heat pump vs furnace guide covers the trade-offs for local winters.
Questions to ask the technician who tagged it
The technician who wrote the tag should be able to explain it clearly. Before they leave, ask:
- Which category is it, immediate hazard or not, and what deadline applies?
- What exactly is the condition, and where on the furnace or venting is it?
- How was it confirmed? By camera, combustion analyzer, CO reading, visual inspection, or a pressure or leak test?
- Can you show me or send photos?
- Is the hazard limited to the furnace, or does it affect the water heater or other gas appliances too?
- Has the gas distributor been notified, and what do they need from my contractor to close it out?
Good technicians welcome these questions. Written answers make it far easier for the next contractor to fix the right problem, and to give you an honest second opinion if you want one.
Red tags when buying or selling a home
A tag on a furnace or water heater can surface during a home inspection or right after closing. If you’re buying, ask the seller to disclose any open safety notices, and have a licensed gas technician look at the heating equipment before your conditions expire. If you’re selling with an open tag, clearing it first avoids a last-minute renegotiation.
Carbon monoxide alarms: the rules and the basics
Carbon monoxide is colourless and odourless. Early symptoms (headache, nausea, dizziness, confusion and drowsiness) are easy to mistake for the flu, and high exposure can cause loss of consciousness and death.
Ontario’s rules. Under the Ontario Fire Code, a house with a fuel-burning appliance, a fireplace or an attached garage must have a CO alarm next to each sleeping area. Since January 1, 2026, a CO alarm is also required on every storey, including storeys without bedrooms. Hardwired, battery and plug-in alarms are all allowed.
Good practice:
- Test alarms monthly, and replace them by the date printed on the unit.
- Put one near the furnace room as well, as an early warning.
- If an alarm sounds, get everyone out right away and call 9-1-1 from outside. Don’t wait to see whether symptoms improve.
- Have your furnace and venting inspected once a year. Many tag-worthy problems, like a failing inducer, a rusting vent or a cracked heat exchanger, show up on a routine tune-up before they become an emergency.
A yearly furnace tune-up is the best way to avoid a red tag in mid-January. If yours is tagged now, call a TSSA-registered contractor, leave the appliance off, and keep your CO alarms working.