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Combustion air enters the bottom, combusts with gas, then rises through the flue which is surrounded by water. The gases heat the water as they rise through the tank. Dilution air enters through the draft diverter.

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The draft inducer fan controls the draft and reduces excess air to a minimum, increasing the efficiency. These improved water heaters are vented through a sidewall.

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These water heats have lower excess air and no dilution air, giving the units a higher recovery efficiency. The sidewalk venting eliminates the need for a vertical chimney. Outdoor combustion air and sealed combustion eliminate dilution air losses. Drawings and captions used with permission from Residential Energy ©1994 Saturn Resource Management.

Gas Water Heaters and Backdraft Interactions

The addition of combustion equipment to a home needs to be done with careful consideration for the location of the heater and the proximity of other appliances and heating equipment. Installing the wrong type of gas heater in an unfortunate combination of circumstances can result in the backdrafting of poisonous combustion gases into the home. This problem results in unnecessary deaths and illnesses every year in the United States.

There are basically three types of gas water heaters: atmospheric draft, induced draft, and sealed combustion.

Atmospheric draft water heaters are the least efficient and potentially most dangerous of the three types. They are also the most commonly seen gas water heater. Atmospheric draft water heaters can easily interact with other appliances in the home to create backdraft hazards. Their efficiency is poor compared to the other available types for four reasons:

1. Standby loss; when the burner is off, air can still flow up the flue and remove heat from the heater tank.
2. Excess air flowing past the burner and up the flue during combustion. Because the burner is open to the air, there is no way to effectively control this air flow, and every molecule of air escaping up the chimney carries wasted heat with it.
3. Dilution air entering the chimney at the draft diverter. This dilution air is usually heated air being drawn from the house.
4. The very common pilot light ignition wastes fuel. Using an electronic ignition system on this type heater improves its efficiency slightly, but does not improve its other characteristics. Atmospheric draft heaters have Energy Factors (EF) of .51 to .56.

A second type of gas water heater is the induced draft, also called the direct vent heater. Induced draft heaters have a fan which induces a controlled draft past the burner during combustion. Standby losses, excess air, and dilution air are all reduced. Since the fan forces a positive flow of combustion gases out of the home, the potential for backdraft interactions is significantly reduced. Induced draft heaters can achieve an EF of .60 to .80.

A third type of gas water heater is the sealed combustion heater. In this type of heater, the combustion and venting system are entirely separated from the air in the home. Combustion air is drawn directly from outside, through a sealed pipe into the burner area, and exhaust gases are vented back out through a sealed pipe, often in a concentric vent arrangement. Since combustion is sealed from the home, the possibility of backdraft interactions is nearly eliminated. Sealed combustion heaters can also achieve an EF of .60 to .80.

Backdraft Interactions

Atmospheric draft water heaters can be induced to backdraft by other common appliances, and in common installation locations. All that is required to create combustion appliance backdrafting is a condition of negative pressure in the area of the burner and venting arrangements sufficient to overcome any stack effect which may be present for the appliance. Another result of negative pressure in the vicinity of atmospheric draft water heaters is flame rollout, which can create serious fire hazards if flammable materials are stored too close to the heater. Negative pressures can be created by: Forced air heating system return leaks or single return system pressure differentials, clothes dryers, ventilation fans, range hoods, and house stack effect. Common locations for water heaters are laundry rooms, furnace rooms, and garages. Picture a gas water heater installed next to a clothes dryer in a laundry room, or next to a forced air furnace with some major return leaks in a utility room or garage. (And sharing the flue with a gas furnace.) Combustion air supply grilles located in an outside wall can make the situation worse if a strong wind is creating a zone of low pressure along that wall. A whole house fan or attic fan can depressurize a home significantly.

Backdraft interactions are more common than we would like to think. Preventing them may be quite difficult, and require trained personnel to diagnose and resolve. The choice of which type of gas water heater to install or recommend should be informed by answers to some basic questions as to location, what other appliances are in the vicinity, what type of heating system does the house have, and where are its major plenums and blower located, the presence of whole house or attic fans, and what activities take place in the vicinity of the heater. When in doubt as to the safety of a particular installation, an induced draft or sealed combustion gas water heater may be the wise choice. The additional energy savings from an efficient water heater should pay the extra initial expense of the higher quality, safer appliance over its operating lifetime