Water heater manufacturers use a gap between the top of the water heater flue and the vent connector pipe above it to pull additional air into the upward flow of combustion air to help it exit more efficiently. There is a tapered cone-shape above the gap, called a draft hood, shown in the photo above.
If the draft hood has been bent out of alignment with the flue opening at the top of the water heater (as in the photo below) or if there is any negative air pressure around the water heater—such as from the return air opening of an HVAC system nearby sucking air into it—then some of the combustion gases from the water heater may be drawn out at the draft hood opening, instead of additional air being pulled in. This is called backdrafting. It can also be caused by any obstruction or convoluted turns in the vent connector.
Signs of a backdrafting problem are melting/buckling of the plastic discs around the pipe penetrations of the top of the water heater around the draft hood, as in the photo above, and corrosion around the opening due to moisture and acidity in the combustion gases that have been flowing over the top of the water heater, shown in the photo below.
It is possible to demonstrate backdrafting when the burners are on by putting a mirror next to the draft hood opening and observing moisture from the combustion gases condensing on it, or by holding a match or smoke-generating device next to the draft hood to see the direction of the air flow.
Because backdrafting means that combustion gases, including carbon monoxide, are not properly exiting the home, it is a safety concern should be repaired immediately. But the wrong solution is to simply remove the draft hood altogether, like the homeowner did the in the photo below.
Also, see our blog post What's that powdery crust on the pipe connections at the water heater?
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Here’s links to a collection of more blog posts about WATER HEATERS:
• Are water heaters required to be raised off the floor?
• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?
• Where are gas water heaters not allowed to be installed?
• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?
• Can you use a light switch for a water heater disconnect?
• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?
• Can you wire a 240-volt water heater with 120 volts?
• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?
• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater?
• What is required clearance for access and working space in front of an electric water heater?
• Why is the water heater older than the house?
• Does a water heater need a shut-off valve?
• Why should a tankless water heater have an isolator/service valve kit installed?
• When was a gas water heater first required to be elevated 18 inches above a garage floor?
• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home?
• What is the purpose of a thermostatic mixing valve above a water heater?
• Does a tankless gas water heater still work with no electricity during a power outage?
• Why do water heaters have a sacrificial anode?
• What is the difference between a manufactured/mobile home water heater and a regular water heater?
• Does a tankless water heater require a pressure relief valve?
• When is a water heater drain pan required?
• Why is there water in my water heater drain pan?
• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"?
• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom?
• Where do I find the water heater in a mobile home?
• Does a tankless water heater in an attic require a drain pan?
• Does an electric water heater require a disconnect?
• Is a catch pan and drain piping required for a replacement water heater?
• What is the difference between a single element and dual element electric water heater?
• What is an FVIR water heater?
• What is a heat pump water heater?
• What is a dielectric union?
• What are the most common installation mistakes with water heater replacement?
• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises?
• What can I do to make my water heater last longer?
• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible?
• How does a hydronic heating system work?
• What is the difference between a regular water heater and a direct vent water heater?
• What is the difference between a regular water heater and a power vent water heater?
• How do I determine if a water heater is gas or electric?
• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?
• What's the valve with the flip-up handle on the water heater for?
• Why is an older water heater an insurance problem?
Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.