A spillage test confirms exhaust is actually leaving through the vent, checked under realistic conditions with other exhaust fans running.
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Quick answer
A spillage test is a diagnostic check performed by a licensed gas fitter to confirm whether an atmospheric-vent gas appliance's combustion exhaust is reliably exiting through the chimney or vent, rather than spilling back into the room. The technician typically uses a smoke source or a combustion analyzer at the draft hood, often while running other exhaust fans in the house such as a range hood or bathroom fan, to simulate realistic worst-case depressurization conditions. A failed spillage test indicates a real venting problem that needs to be corrected before the appliance is safely used, and the test is one of the more important safety checks performed during installation, service, or a home inspection.
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Atmospheric-vent appliances — the traditional style of gas water heaters and older boilers that rely on natural draft through a vertical chimney or vent pipe — depend on a fairly delicate physical balance to work correctly. The exhaust needs to be hot enough and the chimney sized and conditioned correctly enough for natural buoyancy to reliably carry combustion byproducts, including carbon monoxide, up and out of the home. Anything that disrupts this balance, from a chimney that is oversized or damaged to household air pressure changes caused by exhaust fans elsewhere in the house, can cause the draft to weaken or even reverse.
A spillage test exists specifically to check whether this system is actually working under real conditions, rather than assuming it does because the appliance seems to run fine. It is a direct, practical verification rather than a theoretical calculation.
A technician typically starts the appliance and lets it run long enough to establish normal draft conditions, then holds a smoke source, such as a smoke pencil or match, near the draft hood opening where the appliance connects to the vent. If draft is working correctly, the smoke is drawn steadily into the vent opening, following the exhaust path upward and outward. If draft is weak, absent, or reversed, the smoke will hang in the room, drift outward from the draft hood, or fail to be pulled into the vent at all.
A more thorough test also checks conditions under household depressurization, running exhaust appliances like a kitchen range hood, bathroom fans, and a clothes dryer simultaneously to simulate a realistic worst-case scenario, since these appliances can pull air out of a tightly sealed house fast enough to disrupt or reverse a marginal atmospheric vent's draft. Testing only under quiet, no-fan conditions can miss a real problem that only shows up when the house is depressurized this way.
| Situation | Why a Spillage Test Matters |
|---|---|
| New installation of an atmospheric-vent boiler or water heater | Confirms the venting design and chimney are performing correctly before the appliance goes into regular service |
| Replacing one appliance on a shared chimney | Confirms the chimney is still correctly sized once the combined venting load has changed |
| Home renovation adding new exhaust fans, a range hood, or tightening the building envelope | Confirms existing atmospheric venting still performs safely under the home's new air pressure conditions |
| Routine annual service on a boiler or water heater | Catches gradually developing venting problems, such as chimney deterioration, before they become serious |
| Buying a home with existing atmospheric-vent gas appliances | Verifies the venting is functioning safely regardless of the previous owner's maintenance history |
A failed spillage test means combustion exhaust, potentially including carbon monoxide, is not reliably exiting the home through the vent, and it should be treated as an immediate safety concern rather than something to schedule for later. A technician failing a spillage test will typically take the appliance out of service until the underlying cause is identified and corrected, whether that turns out to be a blocked or deteriorated chimney, an oversized flue relative to the current appliance load, a competing exhaust fan effect that needs to be addressed, or a more fundamental venting design issue.
Correcting a failed spillage test result can range from a simple fix, like clearing a blocked chimney cap, to a more substantial project, like installing a properly sized chimney liner or converting the appliance to a power-vent or direct-vent configuration that does not rely on natural draft at all.
Some homeowners are surprised that a technician insists on running exhaust fans during a service visit just to test the water heater, but this step is exactly what distinguishes a thorough safety check from a superficial one. CANRO performs spillage testing as a standard part of atmospheric-vent appliance installation and service in Metro Vancouver, because a passing result under quiet conditions alone does not tell the whole story of how the system behaves in everyday household use, particularly in more tightly sealed or renovated homes where depressurization risk is higher.
Safety note
A failed spillage test means combustion byproducts, including carbon monoxide, may not be reliably venting outdoors, which is an immediate safety concern. If your appliance fails a spillage test, do not continue using it until the issue is corrected. If you smell gas or suspect carbon monoxide exposure, leave the home immediately and call FortisBC at 1-800-663-9911. Spillage testing and any corrective venting work must be performed by a Technical Safety BC licensed gas fitter.
It checks whether an atmospheric-vent gas appliance's combustion exhaust is reliably exiting through the chimney or vent pipe, rather than spilling back into the room, typically using a smoke source at the draft hood while the appliance runs. A more thorough version also checks this under realistic conditions with other household exhaust fans running.
These exhaust fans can depressurize a tightly sealed house enough to disrupt or reverse a marginal atmospheric vent's draft, so testing with them running simulates a realistic worst-case scenario rather than only checking under quiet conditions. This gives a more accurate picture of how the venting actually performs in everyday use.
A licensed technician will typically take the appliance out of service until the cause is identified and corrected, since a failed result means combustion exhaust may not be safely venting outdoors. The fix could range from clearing a blockage to a more involved venting modification, depending on the cause.
It is typically done at installation, during routine annual service, and any time something changes that could affect household air pressure, such as a renovation adding new exhaust fans or tightening the building envelope. It is also a good check to request when buying a home with existing atmospheric-vent gas appliances.
This should be done by a Technical Safety BC licensed gas fitter, who understands how to properly interpret the results and knows what corrective action is needed if the test fails. It also involves handling combustion equipment and, in a thorough test, coordinating multiple exhaust appliances correctly.
Not in the same way — power-vent and direct-vent appliances use fan-assisted or sealed combustion rather than relying on natural draft, so they are not typically subject to the same natural draft spillage test used for atmospheric-vent equipment. Other checks apply to confirm those systems are venting correctly instead.
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