Gas venting, combustion air, and carbon monoxide
Draft hoods, sealed combustion, flame-arrestor screens, and what to do when a carbon monoxide alarm sounds.
A gas water heater is a small furnace with a plumbing connection. It burns fuel, it needs air, and it has to send the products of combustion outdoors. When the vent is blocked, disconnected, or asked to draft in a room that has been depressurized, those products spill into the room.
Carbon monoxide is the one you cannot see or smell. The Consumer Product Safety Commission’s guidance on carbon monoxide is the right companion to any gas-heater work: install alarms, know the symptoms, and get out when an alarm sounds.
This page is the heater side of that hazard. It is not a vent-sizing manual. Sizing a flue, a combustion-air opening, or a sealed-combustion termination is a licensed installer’s job under the fuel-gas code your city has adopted.
Emergency. If a carbon monoxide alarm sounds, or people in the house develop a sudden headache, nausea, and confusion that improve when they go outside, leave immediately and call the emergency number from outside. Do not open windows and go back to sleep to test whether the alarm was serious. If you smell gas, leave and call the gas utility from outside. Do not look for the leak with a flame or a switch.
Atmospheric draft, in one picture
An atmospheric tank has a burner under the water, a flue through the tank, and a draft hood on top. Hot flue gas rises into the vent.
The draft hood is an opening that lets room air join that stream and that lets a downdraft spill without blowing straight into the burner. The hood is not a place to set a storage bin, and it is not a place to stuff insulation.
The vent connector rises from the hood to a chimney or a listed vent, with a slope and a support the code and the manual describe. Joints stay together with the screws the listing allows, not with tape as the only fastener.
A loose connector is a spill path. Soot on the hood, melted plastic grommets, or rust streaks running back out of the vent are evidence the draft has failed. Shut the heater off and have the vent rebuilt. Do not wipe the soot off and keep using it.
Other appliances share this story. A kitchen hood, a clothes dryer, or a fireplace that pulls hard can depressurize a tight house and pull the water heater’s draft hood backward. The flame rolls out, the pilot dies, or a spill switch opens.
The pilot article mentions draft as a reason a pilot will not stay. The cure is not a new thermocouple. The cure is makeup air or a vent that is not fighting the house.
A technician with a draft gauge and a carbon monoxide meter can show the failure. A homeowner with a match held near the hood cannot do that test safely or reliably, and should not try.
Combustion air is a sized opening, not a cracked window
The burner needs oxygen. Older rules of thumb talked about room volume per 1,000 Btu/h. Current fuel-gas codes, including the International Residential Code’s fuel-gas chapter and NFPA 54, use specific methods for indoor air and for outdoor-air openings.
The numbers depend on the total input of all appliances in the space, not only the water heater, and on whether the house is unusually tight. This site will not give you an opening size to cut. A louver that someone replaces with a solid door is a common and serious change.
The heater that drafted for years now backdrafts, soots, and can kill. If a door was swapped, put a listed louver back or have the air recalculated. Do not store the combustion-air grille behind the water heater with a bicycle.
Modern flammable-vapor-ignition-resistant heaters have a flame-arrestor screen at the air inlet. The screen is part of the safety design. Lint, pet hair, and a pile of laundry block it.
The control then locks out, which is preferable to a fire, and you have no hot water. Cleaning is model-specific and sometimes involves the burner tray. The gas no-hot-water page covers the lockout.
Do not cut the screen out because it “restricts air.” Gasoline and solvents do not belong in the same room. The screen is the last layer, not a substitute for storing fuel somewhere else.
Sealed combustion is a different architecture
A direct-vent or sealed-combustion heater draws outdoor air through a dedicated pipe and exhausts through another, or through a concentric termination.
The room is no longer the air supply. That is safer in a tight closet, and it is not foolproof.
The termination can be buried in snow, pointed at a window, or extended with unlisted dryer duct by someone who wanted the pipe to reach. Equivalent length still counts. Condensing heaters need this kind of attention because their plastic vent is easy to modify incorrectly and because the exhaust is cooler and wetter.
Clearances to grade, doors, and openings are in the manual and the code. A termination you can see from the ground should be kept clear of leaves and storage. A termination on a roof is not a homeowner cleaning task on a wet day.
Power-vented heaters use a fan and a pressure switch. If the fan runs and the switch will not close, the vent is restricted or the fan is weak. The correct failure is a shutdown.
Resetting the code without clearing the restriction puts the heater back into a condition it already refused. Write the code down and call. Do not jumper a pressure switch.
Alarms and symptoms
Put carbon monoxide alarms outside each sleeping area and on each level, following the alarm’s instructions and the local code. Replace them when they reach the end-of-life date printed on the back, often five to ten years, even if they still light up when you press test.
A test button checks the horn more than it checks the sensor’s accuracy. Do not paint alarms. Do not place them where a supply register blows on them or where a bathroom’s steam will nuisance-trip them, and do not hide them in a closet so you cannot hear them.
Symptoms the CPSC and public-health agencies describe include headache, dizziness, nausea, and flu-like illness that is suspicious because it affects more than one person and eases outdoors. Carbon monoxide does not always produce a dramatic scene. The alarm is there because people are bad at noticing a gradual poisoning.
If the alarm sounds, the response is evacuation and the emergency number, then a technician and often the fire department, not a forum post. After the building is cleared, the heater and the other combustion appliances need a combustion check before anyone sleeps there again. A heater that spilled once will spill again if the vent is unchanged.
Checklist: vent safety
- Alarm on each level and outside sleeping rooms. End-of-life date not past.
- Draft hood uncovered, vent joints intact, no soot streaking back into the room.
- Combustion-air grille present and not blocked. Solid door not substituted for a louver.
- No gasoline or solvents stored beside the heater. Flame-arrestor screen uncut and unclogged.
- Sealed-combustion terminations clear, listed material only, no improvised extensions.
- Spill switch or lockout investigated as a vent problem, not reset as a nuisance.
- Any alarm event: leave, call, and do not sleep there until the source is found.
What this means for a replacement
A new heater has to match a venting category the house can legally provide. Dropping a condensing unit onto a B-vent, or an atmospheric unit into a closet that no longer has air, repeats the hazard with new equipment.
Ask the installer to name the vent category and the air supply. Ask who pulls the permit. Inspectors catch missing draft and missing air more often than they catch a wrong anode.
When to call a pro includes vent work on the licensed side of the table without exception. There is no safe amateur version of “the chimney seems fine.”
Sources
U.S. Consumer Product Safety Commission, carbon monoxide safety guidance and “Avoiding Tap Water Scalds” is a separate hazard; CO alarms and evacuation guidance are published through the CPSC’s carbon monoxide information. NFPA 54, National Fuel Gas Code, and the International Residential Code fuel-gas and water-heater chapters, as adopted locally, for combustion air, venting, and draft.
Manufacturer installation manuals for vent category, equivalent length, and flame-arrestor service (Bradford White, A. O. Smith, Rheem, Navien, Rinnai). International Code Council water-heater safety notes for the related relief-valve and installation rules that show up on the same inspection.