$500–$2,500
Fix the venting
A properly sized chimney liner (especially for orphaned water heaters), flue repairs, correct slope and connections. Often the whole fix.
Energy test guide · verified October 2, 2026
The combustion safety check: tighten the house, keep the chimney honest
Furnaces, water heaters, and fireplaces that vent by natural draft rely on the house giving them air. Air-seal the house — the best energy move there is — and a bathroom fan or range hood can start winning the tug-of-war, pulling exhaust back inside. This check finds out before your CO alarm has to, and it's included in every audit of a home with combustion appliances.
Cost
Included with fuel appliances
When
At audit + after tightening
Checks
Draft, spillage, CO
End-game fix
What's involved
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Set up the worst case
House closed, and every exhaust device running at once — bath fans, range hood, dryer. The test asks: under maximum depressurization your own home can create, do the appliances still vent?
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Test draft at each appliance
Does the water heater's flue establish upward draft promptly when it fires? The orphaned water heater — left alone in a big chimney after a furnace upgrade — is the classic failure here.
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Check for spillage
Smoke or a mirror at the draft hood shows whether combustion gases spill into the room during startup, and for how long. Brief startup spillage differs from sustained failure, and the protocol distinguishes them.
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Measure CO in the flue
High carbon monoxide in the exhaust stream flags a combustion problem — burner, heat exchanger, or venting — worth a licensed gas technician's eyes regardless of draft.
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Verdict, and retest after air sealing
The result either clears your air-sealing plans or sequences them: fix venting first, then tighten. After major envelope work, the check gets repeated — the house that passed at leaky may not pass at tight.
How to get it done
It's included in the EnerGuide assessment whenever you have combustion appliances — your job is just to make sure it happens and to ask for the results plainly. Fixes belong to licensed gas technicians (or WETT-certified specialists for wood). If you're air-sealing without an audit, book a gas tech to run the check anyway; it's the one test on this site with no safe DIY version.
How to read your result
What the number is: Whether flue gases go up the chimney under worst-case house pressure, plus CO in the flue gas.Unit: Pass/fail (spillage) · ppm CO air-free (flue gas)
- PassNo spillage after 2 min (water heater) or 5 min (furnace, boiler); flue CO at or under its limit
What it means: Appliances vent safely even with exhaust fans, dryer and air handler running.
What to do: Retest after air sealing, new exhaust fans or equipment changes, and keep yearly servicing.
- SpillageExhaust spills at the draft hood beyond those times
What it means: House depressurization or a venting problem can pull exhaust, including CO, back indoors.
What to do: Have a pro fix the cause: leaky ducts, too little combustion air or vent faults; consider sealed-combustion or heat pump replacement.
- High flue COOver the appliance limit (e.g., 400 ppm air-free for furnaces and boilers, 200 for water heaters)
What it means: The burner is producing too much CO and needs adjustment or repair.
What to do: Have it serviced right away by a qualified technician; if it also spills, shut it off until fixed.
- StopAmbient CO 70 ppm or more, or a gas leak detected
What it means: An immediate danger, so the test is halted.
What to do: Leave the house, then call emergency services and the gas utility from outside.
NRCan warning signs: soot or staining near the burner or draft hood, melted plastic on the water heater top, unusual odours or frequent headaches. Canada's code references the CAN/CGSB-51.71 depressurization test.
Where these numbers come from · checked 2026-10-26
If it finds a problem: your remediation options
A failed or marginal result has a clear options ladder — and one hard rule: no further tightening until it's resolved.
$300–$1,500
Provide make-up air
A dedicated air supply so exhaust fans stop competing with chimneys — engineered, not a hole in the wall.
At replacement time
Move to sealed combustion
Direct-vent and power-vented equipment (condensing furnaces, power-vent water heaters) bring their own air through a sealed pipe — the whole backdraft question disappears, and the house can be as tight as you like.
The end state
Electrify and retire the chimney
A heat pump and heat-pump water heater remove combustion from the house entirely — the path our planner maps, with the rebates that currently apply.
Meanwhile: working CO alarms near every sleeping area are your backstop. Building codes require them in new homes with fuel-burning appliances, and some provinces (Ontario, for example) require them in existing homes too. The CO guide covers placement and expiry dates. The alarm is the last line; this check is the first.
Common questions
I have CO alarms. Isn't that enough?
Alarms are the backstop that saves you when everything else fails — this check is what keeps them silent. Alarm rules set the floor, not the ceiling: prevention is the annual servicing plus this test, especially in the year you air-seal.
What's an 'orphaned' water heater?
When a furnace gets replaced with a high-efficiency unit that vents out the wall, the old natural-draft water heater is left alone in a chimney sized for two appliances. The oversized, cool flue drafts poorly — it's the single most common spillage finding, and a liner is the usual fix.
Who actually performs the check and the fixes?
The check: your energy advisor during the assessment (it's part of the protocol when combustion appliances are present). The fixes: a licensed gas technician — TSSA-registered contractors in Ontario — or a WETT-certified professional for wood-burning equipment.
Sources
Air-sealing plans? Sequence the safety check
We'll match you with a registered advisor who runs combustion safety as part of the assessment — before and after the house gets tighter.