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Safety · CombustionCarbon Monoxide Testing & Combustion Analysis
Carbon monoxide testing and combustion analysis is a measurement visit: a TSSA-licensed technician puts a calibrated analyzer in the flue of each gas appliance, reads CO in the room air and at the registers, and checks that every vent is drafting. It is the only way to know whether an appliance is making CO, because a household alarm is built to stay silent at low levels. Book it after any alarm event, when a gas appliance is over 15 years old, or when headaches and nausea ease every time you leave the house.
Three kinds of reading make up the test. The first is combustion analysis: a probe in the flue measures carbon monoxide in parts per million along with oxygen and flue temperature, which shows how cleanly the burner is firing. A healthy furnace reads well under 100 ppm in the flue gas, and a number that keeps climbing as the burner runs matters more than any single reading. The second is ambient CO, taken in the air beside each appliance and at the supply registers, where anything above zero has to be explained.
The third is draft. A natural-draft water heater has an open draft hood on top, and if the chimney is not pulling, exhaust spills out of that hood into the basement. We test it with the burner running, using a smoke source or a mirror at the hood and a draft gauge in the vent. It is a frequent finding in 1970s–90s houses where a new furnace now vents out the side wall and the water heater has been left alone on a chimney that is too large and too cold for it — the fix is water heater venting repair or a liner.
Appliances are tested in a set order: furnace, water heater, boiler if there is one, gas fireplace, then the gas range and oven. Then we repeat the draft test under worst-case conditions, with interior doors closed and the dryer, bathroom fans and range hood all running. Those fans pull air out of the house, and in a tight or renovated home they can drop the basement to a lower pressure than the chimney and reverse the flow. An appliance that passes on a quiet afternoon and spills with the dryer on is still a CO source.
After an alarm, the order of events is fixed. Everyone leaves, and the fire department or Enbridge Gas checks the house and shuts off or tags any appliance they suspect; the steps are in what to do if a carbon monoxide alarm goes off. Our work starts once the house is cleared: find the source with instruments, repair it, and prove it with readings before anything is relit. If the appliance has been tagged, that is red tag furnace repair and clearance.
Book combustion testing when
- A CO alarm has sounded and the house has been cleared, but nobody found the source
- Soot or scorch marks appear around the furnace burner area, the water heater draft hood or a fireplace
- The furnace flame is yellow or lifting instead of steady blue, or the gas range burns orange
- You have replaced the furnace, added a powerful range hood or tightened the house, and the old water heater still vents into the chimney
- People or pets feel unwell at home and better away from it — treat that as urgent and get outside
How it works
If an alarm is sounding now, go outside and call 911 or Enbridge Gas; do not wait for a contractor. Call us after the house is cleared, or straight away if there has been no alarm and you want the appliances tested.
Flue-gas CO, ambient CO at the appliance and registers, draft and spillage at every draft hood, then the same draft checks with exhaust fans and the dryer running. Each reading is written down against the appliance it came from.
Common repairs are a blocked or disconnected vent, a mis-set gas pressure, dirty burners or a cracked heat exchanger. Work on the furnace itself is quoted as furnace repair, and the appliance stays off until the retest numbers are clean — book a test.
Common questions
My CO alarm never goes off. Do I still need testing?
Possibly. Residential alarms are designed to ignore low concentrations for long periods so they do not nuisance-trip, which means an appliance can produce CO for months without setting one off. Testing measures the appliance itself. The alarm remains essential, and Ontario requires them in homes with fuel-burning appliances; carbon monoxide detectors covers placement and replacement dates.
Which appliance is usually the source?
A natural-draft water heater spilling at its draft hood and an older furnace with a cracked or blocked heat exchanger are the two we look at hardest. Gas ranges and ovens produce some CO by design and have no vent of their own, which is why a range hood ducted outside matters. An attached garage with a car idling in it is the non-appliance source to rule out.
Can a brand-new high-efficiency furnace leak carbon monoxide?
It is far less likely, because a condensing furnace has a sealed combustion chamber and pushes its exhaust out through plastic pipe under fan pressure. The failures are installation faults: a vent joint never glued, an exhaust pipe sloped the wrong way and full of water, or an intake and exhaust placed so the furnace re-breathes its own exhaust. A combustion test at commissioning catches all three.
How long does a combustion analysis take?
A single furnace takes about as long as a tune-up, since the appliance has to reach steady operating temperature before the readings mean anything. A house with a furnace, water heater, fireplace and range, plus the worst-case fan test, is a longer visit. We give you the expected duration with the quote and the readings in writing when we finish.
Do home inspectors do this test?
Not normally. A home inspection is a visual check, and most inspectors do not carry a combustion analyzer or take appliances apart. If you are buying a house with a furnace or water heater past 15 years, ask for a combustion test by a licensed gas technician as a condition of the offer. The heat exchanger inspection that goes with it is something a visual walk-through cannot replace.