Dilution air is supplementary air introduced into the venting system of a combustion appliance, such as a natural draft water heater or an older furnace, after the combustion process is finished. This air is not used to burn the fuel, but instead mixes with the hot exhaust gases, or flue products, before they travel up the chimney or vent pipe. Its primary function relates to the safe and reliable removal of these combustion products from the living space, ensuring that the appliance operates correctly under varying conditions.
The Critical Function of Dilution Air
Dilution air serves two main mechanical functions within a combustion venting system, both centered on maintaining safe operation. The first purpose is to stabilize the natural draft, which is the upward flow of exhaust gases created by buoyancy. Natural draft can be highly variable, affected by wind, outdoor temperatures, and changes in indoor air pressure, such as those caused by exhaust fans or clothes dryers. By introducing a consistent volume of room air, dilution air helps regulate the negative pressure within the vent, ensuring a steady flow of exhaust gases and preventing pressure extremes that could extinguish the pilot flame.
The second primary function of dilution air is to cool the hot exhaust gases leaving the appliance heat exchanger. Flue gases from non-condensing appliances are typically well over 300°F and can damage certain vent materials if their temperature is not managed. Mixing cooler room air reduces the overall temperature of the gas mixture before it enters the vent pipe or chimney liner. This cooling protects the physical integrity of the venting materials, which are often limited in their maximum operating temperature.
The cooling must be carefully balanced to prevent overly rapid condensation inside the vent. By regulating both temperature and pressure, dilution air supports the safe and continuous removal of combustion products from the appliance and out of the building envelope.
How Dilution Air Enters the Venting System
For many common residential appliances that rely on natural draft, dilution air is introduced passively through a device known as a draft hood or draft diverter. This component is situated directly above the appliance’s outlet and features a standardized opening that is constantly exposed to the air in the room. The negative pressure created by the rising, hot flue gases pulls air through this opening, effectively mixing the dilution air with the exhaust stream.
The draft hood is engineered to allow room air to be drawn into the flow of combustion gases whenever the appliance is operating. This passive design means the amount of dilution air entering the system is based on the pressure difference between the vent and the surrounding room. The draft hood also protects the burner flame from sudden changes in the venting system, such as downdrafts caused by wind, by acting as a buffer.
In some larger heating systems, such as boilers, a mechanical device called a barometric damper may be used to regulate the draft. A barometric damper utilizes a counterweighted swinging door that opens inward to allow room air to enter the flue pipe. This mechanism provides a more controlled method of regulating draft compared to a fixed draft hood, allowing it to adjust precisely to fluctuations in atmospheric pressure or wind conditions.
Many modern, high-efficiency appliances do not use dilution air at all, instead employing sealed combustion or power-venting systems that manage the exhaust flow mechanically.
Impacts of Improper Dilution
A venting system requires a specific balance of dilution air to operate safely and efficiently; when this balance is disrupted, significant problems can arise.
Insufficient Dilution and Spillage
If the appliance is experiencing insufficient dilution, meaning too little room air is entering the draft hood, the flue gases remain excessively hot. This can lead to thermal stress and potential damage to the vent pipe or chimney liner over time. A lack of stabilization can also cause an unstable draft, increasing the likelihood of exhaust spillage.
Exhaust spillage occurs when combustion products fail to exit the building and instead spill out of the draft hood and into the living space. This backdrafting is a serious safety hazard because combustion gases contain toxic compounds, most notably carbon monoxide. Operating a vent with insufficient dilution air significantly elevates the risk of carbon monoxide exposure.
Excessive Dilution and Condensation
Conversely, excessive dilution air, where too much room air is pulled into the vent, creates operational issues and long-term damage. When the flue gases are cooled too rapidly, the water vapor created during combustion can quickly drop below its dew point. This causes excessive condensation within the vent pipe or chimney, which accelerates the corrosion and deterioration of the venting materials. Furthermore, pulling an excessive amount of heated air from the home and exhausting it outside reduces the overall energy efficiency of the appliance.