Anthony — TSSA-Certified G1 Gas Technician
TSSA G1 licence #TBD · 15+ years in gas and mechanical trades · Service area: Greater Toronto Area, Ontario
Last updated: 2026-08-03
What is flue gas spillage?
Flue gas spillage happens when the combustion by-products from a gas appliance — water heater, furnace, or fireplace — fail to draft properly up the vent or chimney and instead spill back into the mechanical room or living space. It's detected by holding a mirror, smoke pencil, or draft gauge at the draft hood or vent connector while the appliance runs.
Spillage is dangerous because it puts carbon monoxide and moisture-laden combustion gas into the house air rather than outside. It's also common: depressurization-related spillage is the single most frequent reason technicians issue a red tag that doesn't involve a broken appliance component.
What is worst-case depressurization testing?
Worst-case depressurization testing checks whether an appliance will spill under the most negative-pressure conditions the house can realistically create. The technician turns on exhaust fans, the clothes dryer, and closes interior doors to simulate the house at its most depressurized, then checks whether the water heater or furnace still drafts properly.
This test matters because a water heater can vent fine on its own and still spill flue gas the moment a bathroom fan, range hood, or dryer is running and pulling air out of the house faster than it can get back in. CSA B149.1 requires this worst-case check as part of diagnosing spillage, not just a single at-rest reading.
- Baseline draft reading taken with no fans running
- Exhaust fans, dryer, and range hood turned on
- Interior doors closed to simulate worst-case pressure
- Draft and spillage re-checked at the draft hood after 1 minute
- Result documented as pass or fail against CSA B149.1
What causes flue gas spillage?
Spillage usually comes from one of five things: an orphaned appliance sharing an oversized chimney, a blocked or disconnected vent, insufficient combustion air supply to the mechanical room, competing exhaust fans depressurizing the house, or a physically damaged/corroded vent connector.
An orphaned water heater is extremely common in the Greater Toronto Area homes that upgraded their furnace to a high-efficiency condensing model. The furnace switched to its own sealed PVC venting, leaving the water heater venting alone into a chimney that was sized for both appliances — now too large for the water heater's smaller flue gas volume to keep warm enough to draft properly.
| Cause | How it's identified |
|---|---|
| Orphaned water heater | Furnace vents separately (PVC); chimney is oversized for the water heater alone |
| Blocked/disconnected vent | Visual inspection, camera, or smoke test at connector joints |
| Insufficient combustion air | Room volume and vent-opening calculation against CSA B149.1 tables |
| Exhaust fan competition | Worst-case depressurization test with fans running |
| Corroded/damaged connector | Visual inspection for rust-through, gaps, or disconnection |
How is spillage corrected?
The fix depends on the cause. An orphaned water heater is usually corrected by relining the chimney with a properly sized stainless liner, or by converting the water heater to a power-vented or direct-vent model that doesn't rely on chimney draft at all.
Insufficient combustion air is corrected by adding a dedicated combustion air duct or louvre sized per CSA B149.1 tables based on the mechanical room's volume and appliance input. Exhaust fan competition is sometimes solved with a make-up air damper, and blocked or corroded vent connectors are simply replaced.
How much does correcting flue gas spillage cost?
Costs vary widely by cause. A chimney liner for an orphaned water heater typically runs CAD 800 to CAD 1,800; converting to a power-vented water heater runs CAD 1,600 to CAD 2,800 installed; adding a combustion air duct is usually CAD 300 to CAD 900; and a vent connector replacement is often under CAD 300.
Because refrigeration, electrical, and masonry/chimney work fall outside a gas technician's scope, chimney relining and any electrical hookup for a power-vent system are coordinated through vetted partner contractors rather than done in-house.
Can the appliance keep running while spillage is fixed?
No. Once spillage is confirmed under CSA B149.1 testing, the appliance is red-tagged and taken out of service, because it's actively putting combustion gas into the house. It stays off until the underlying cause is corrected and the appliance passes a re-test.
Most spillage corrections can be scheduled within a few days, and because the appliance itself is often undamaged, the timeline is usually faster than a full appliance replacement.
How is a spillage red tag cleared?
A certified gas technician re-runs the same draft and worst-case depressurization test after the correction is made. If the appliance drafts properly under worst-case conditions, the technician documents the pass and clears the red tag, restoring gas service.
Keep the written report from both the original red tag and the clearance — it documents what was wrong and confirms it was properly corrected, which matters for insurance, resale, and any future service calls.
Frequently asked questions
Is flue gas spillage the same as backdrafting?
They're related but not identical. Backdrafting usually refers to combustion gas being pulled back down the entire chimney or vent, while spillage refers to gas escaping at the draft hood or connector even though the vent above it may still be working. Both are tested for during the same inspection.
Can I fix spillage myself by opening a window?
Opening a window can temporarily relieve depressurization, but it is not a permanent fix and a technician will not clear a red tag based on a window being left open. The underlying combustion air or venting deficiency needs to be corrected.
Why did my water heater start spilling after a furnace upgrade?
High-efficiency furnaces vent through sealed PVC pipe instead of the shared chimney, leaving the water heater as the only appliance using a chimney that's now oversized for it. This is called an orphaned water heater and is one of the most common causes of spillage red tags in the Greater Toronto Area.
Does a bathroom fan really cause a gas appliance to spill?
Yes, in a tightly sealed home, exhaust fans, a running clothes dryer, or a range hood can depressurize the house enough to reverse draft on a naturally vented appliance. This is exactly what worst-case depressurization testing is designed to catch.