When a gas or propane water heater burns fuel, it consumes air and produces hot exhaust containing water vapor, carbon dioxide and small amounts of other gases, including carbon monoxide if combustion isn't clean. Venting carries that exhaust outside; combustion air makes sure the flame has enough oxygen. Get either wrong and the heater can soot up, shut down or spill exhaust into the home.
The main venting types
Atmospheric (natural draft). The classic garage tank: a flue runs up the center of the tank to a draft hood on top, and a metal vent carries exhaust up through the roof, relying on hot gases rising. Simple and quiet, but sensitive to vent condition, slope and household air pressure.
Power vent. A blower on top of the tank pushes exhaust through a vent, often horizontally through a wall. Useful where a vertical run to the roof isn't practical. Needs electrical power.
Direct vent. A sealed system draws combustion air from outdoors and sends exhaust outdoors, usually through a concentric or two-pipe termination. The heater doesn't use room air, which suits tight spaces.
Power direct vent combines a sealed system with a blower for more routing flexibility.
Condensing units, including many tankless models, extract so much heat that exhaust is cool and wet. They typically vent with plastic pipe and need a condensate drain.
Combustion air: the other half
A heater that draws air from the room it's in needs enough of it. A garage usually has plenty. A small closet may not, which is why closet installations often have louvered doors, openings to other spaces, or ducts to outside. Tighter modern construction and powerful kitchen or bath exhaust fans can starve an appliance of air or even reverse its draft. Direct-vent and sealed-combustion units sidestep much of this by bringing their own air in from outside. How much combustion air a particular installation needs depends on the unit and the jurisdiction, and we don't state those rules.
Warning signs of venting problems
- Soot or scorching around the draft hood or burner access
- A yellow, lazy or noisy flame instead of a crisp blue one
- Condensation dripping from the vent connector
- A vent pipe that's disconnected, sagging, rusted through or crushed
- A burner that lights then shuts off repeatedly
- Heat or fumes felt around the top of the heater
Soot, fumes or a disconnected vent pipe need prompt attention.
California patterns
Garage tanks in the southern metros and the valley are usually atmospheric with vents through the roof. Problems tend to be corroded connectors, vents knocked by stored items, and dust clogging air intakes on sealed-combustion tanks.
Exterior closets in tract housing combine a small space with louvered doors for air. Blocked louvers, painted-over vents and sun-weathered caps are common findings.
Older Bay Area and urban homes often have long-established vent arrangements, sometimes shared with a furnace, and heaters in basements or interior closets. Replacing a heater there can mean reassessing the whole vent path.
Foothill and mountain homes add elevation, which affects combustion in higher communities, and snow and debris that can block terminations.
Seismic movement can pull vent connections apart. That's one reason a secured tank and a well-supported vent go together; the installer confirms what applies at your address.
What not to do
Leave venting to the technician
- Reconnecting, extending, taping or rerouting vent pipes
- Blocking louvers or openings that supply combustion air
- Boxing in or insulating around a vent
- Swapping metal and plastic vent materials
You can help by keeping storage well away from the heater and its vent, and keeping outdoor terminations visibly clear.
How the technician checks and corrects venting
With the heater firing, the technician checks draft at the draft hood or vent, observes the flame, and inspects the vent connector, joints, slope, supports and termination. They check the air supply to the space and whether household exhaust fans affect draft. Corroded or disconnected sections are replaced, supports are added, and air openings are restored. When a heater is replaced or converted, they design venting to the manufacturer's requirements for the new unit and the local rules.
Venting when you switch fuel or type
Changing the kind of water heater usually changes the venting. Moving from an atmospheric tank to a power-vent or tankless unit typically means a new vent route, often through a wall, and the old roof vent is removed or sealed. Moving from gas to electric or heat pump retires the vent entirely, and the installer should close off the old penetration properly. If the old vent was shared with a furnace, removing the water heater can change how that shared vent performs, which the technician checks.
What affects the cost
| Work | Effect |
|---|---|
| Connector replacement | A modest repair on most atmospheric tanks. |
| New vent run | Roof or wall penetrations and materials add cost. |
| Type change | Converting to power-vent, direct-vent or tankless venting. |
| Combustion air | Louvered doors, openings or ducts for closet installations. |
| Shared vents | Reassessing a vent shared with a furnace takes more work. |
How long it takes
Connector repairs and draft checks fit in a normal service visit. New vent runs, type changes and combustion air modifications take longer and are usually planned alongside a replacement or installation.
Get the vent right before the new unit arrives.
Venting questions
The main types are atmospheric or natural-draft venting, where hot exhaust rises through a metal vent on its own; power venting, where a fan pushes exhaust out, often through a wall; direct venting, where a sealed system draws outside air and expels exhaust outside; and power direct venting, which combines both. Condensing units usually vent through plastic pipe and produce condensate.
It's when exhaust from a fuel-burning appliance spills back into the room instead of going up the vent. It can happen with a blocked or disconnected vent, a vent that's too long or poorly sloped, or a house under negative pressure from exhaust fans and dryers. It's a safety concern, and a technician checks draft on every gas heater visit.
Burning fuel consumes oxygen. If a heater in a small closet can't draw enough air, combustion becomes incomplete, which can produce soot, a yellow flame and more carbon monoxide. Closets often rely on louvered doors, openings or ducts to supply air. How much is needed depends on the unit and the jurisdiction; the installer confirms it.
Sometimes, if it's the same type, in good condition and suits the new unit. Often the vent connector is replaced because the old one has corroded or doesn't match the new unit's outlet. Switching to power-vent, direct-vent or tankless almost always means new venting designed to the manufacturer's requirements.
Power-vent heaters have a blower that runs whenever the burner fires, so a hum or whoosh is normal. A new rattle, grinding or a much louder noise may point to a failing blower or debris, and is worth a check.
Please don't enclose, insulate or build around a vent without a technician's advice. Vents need specific clearances from combustible materials, and restricting them can create a hazard. Keep storage well away from the vent and the heater. If a remodel is planned near the heater, involve the installer early so any vent changes are designed in rather than improvised.
Plastic vent pipe is typical of condensing and some power-vent or direct-vent units, whose exhaust is cool enough for plastic. Standard atmospheric tanks use metal vents because their exhaust is much hotter. The two are not interchangeable. Cracks, sagging joints or a disconnected section on a plastic vent deserve a prompt technician visit.
Soot around the draft hood or burner usually means incomplete combustion or back-drafting. It needs a technician soon. Keep the area clear, and if anyone feels unwell near the heater, leave the building and call 911 from outside. Typical causes are a blocked or disconnected vent, a dirty burner or a closet starved of combustion air.
Elevation mainly affects combustion rather than venting itself, and it's relevant in higher foothill and mountain communities. Units may need configuration appropriate to the altitude according to the manufacturer's guidance. The installer checks this where it applies. When buying a unit for a mountain home, mention your location so the installer can confirm the model suits it.