A gas water heater's draft hood and vent are what keep its normal combustion byproducts, including carbon monoxide, safely outside the home.
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Quick answer
A gas water heater produces carbon monoxide during normal combustion, and an atmospheric-vent unit relies on natural draft through a draft hood and chimney or vent pipe to carry that exhaust outside. CO becomes a hazard inside the home when the draft hood spills, when the vent is blocked or disconnected, or when negative pressure from exhaust fans overpowers the water heater's weak natural draft. Because a water heater is often located in a closet, garage, or utility space near living areas, a working CO alarm nearby matters as much as it does near a boiler or furnace.
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Unlike a boiler, which is usually tucked away in a basement mechanical room, a gas water heater is often located somewhere closer to daily living, such as a closet off a hallway, an attached garage, or a small utility room. This proximity means a venting failure at the water heater can put CO closer to occupied space more quickly than a similar failure at a boiler in a more isolated location.
Many atmospheric water heaters in Metro Vancouver homes vent through a draft hood at the top of the tank into a vent connector and then into a shared or dedicated chimney. This is a simple, passive system with no fan, which makes it more sensitive to competing pressure from exhaust fans, dryers, or a poorly performing chimney than a fan-assisted venting system would be.
A spillage test at the draft hood, checking whether exhaust is escaping into the room instead of drawing up the vent, is one of the more telling checks a technician performs. Visible clues for a homeowner include soot staining around the draft hood or the top of the water heater, rust or heavy condensation forming near the flue collar, or a faint exhaust smell noticeable when standing close to the unit while it's firing.
Some homes vent a water heater and a furnace or boiler into the same chimney. This shared venting has to be sized correctly for both appliances' combined output; if one appliance is replaced with a different type, such as swapping a furnace for a high-efficiency model that no longer uses the chimney, the water heater's shared vent may become oversized for its exhaust alone, which can reduce draft and increase backdraft risk. Any time one appliance sharing a vent is replaced, the impact on the other appliance's venting should be checked, not assumed to be fine.
| Situation | Why It Matters |
|---|---|
| Water heater shares a chimney with a furnace or boiler | Vent sizing must suit combined or remaining appliances |
| One shared appliance is replaced with high-efficiency equipment | Remaining atmospheric appliance's draft can weaken |
| Water heater located in a small closet | Combustion air supply can be marginal |
| Water heater near a powerful exhaust fan or dryer | Increased backdraft risk from negative pressure |
A water heater installed in a small closet needs enough combustion air, either from louvred doors, vents to an adjacent space, or a dedicated duct to outdoors, depending on the closet's size and code requirements. Homeowners who have sealed up a water heater closet during a renovation, for example replacing louvred doors with solid ones for appearance, should have combustion air adequacy checked, since this is an easy detail to overlook.
If a CO alarm sounds or you suspect spillage, leave the home and call for help from outside rather than opening the closet door to investigate. Do not block louvred doors or vents intended for combustion air, and don't store boxes or laundry against a water heater in ways that restrict airflow around it. A working CO alarm near the water heater's location, in addition to ones near sleeping areas, adds an important layer of protection given how close these units often sit to daily living space.
Safety note
If you smell gas, hear hissing near a gas line, or your carbon monoxide alarm sounds, leave the home immediately with everyone inside, do not operate switches or light anything, and call FortisBC at 1-800-663-9911 from outside the house. Gas and combustion work must be carried out by a Technical Safety BC licensed gas fitter.
It can be, since atmospheric water heaters rely on a relatively weak natural draft and are sometimes installed in smaller, more enclosed spaces with limited combustion air.
Yes, if it removes or blocks the combustion air openings the water heater relies on. Any changes to a water heater closet's doors or vents should be checked against combustion air requirements.
Shared venting needs to be sized for whichever appliances are actually using it. If one appliance is replaced with different venting needs, the shared vent's performance for the remaining appliance should be reassessed.
Yes, in addition to alarms near sleeping areas. Because water heaters are often close to living space, a nearby alarm can catch a venting problem early.
A yellow, flickering flame instead of a steady blue one can indicate incomplete combustion, which increases carbon monoxide production. It's worth having a technician inspect the burner.
In some homes, yes. A powerful dryer or range hood running at the same time can create enough negative pressure to interfere with an atmospheric water heater's natural draft.
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