A true convection oven is a type of oven that uses a fan to forcibly circulate air inside the oven enclosure, allowing foods to cook faster and more evenly and to brown better than in conventional ovens, which lack fans. The term "true" when applied to convection ovens is somewhat misleading. Many convection ovens have two heating elements, much like conventional ovens. Some finer, more expensive convection ovens add a third element, mounted near the fan, which heats the air as it is circulated directly from the fan. Some people believe and insist that only convection ovens with this third element are true convection ovens, although any oven with a fan that circulates the air in the oven enclosure is technically a true convection oven.
In physics, the term convection refers to the motion of gas or liquids within a system due to temperature differences, resulting in heat transfer within the system. When applied to convection ovens, this word takes on a slightly different meaning, referring to the transfer of heat to foods by the circulating hot air. Air that is in motion transfers heat more efficiently than still air. For example, blowing on a bite of hot food cools it more quickly. A convection oven uses the same principle but to add heat to food rather than to take it away.
Modern ovens generally use either electric elements or gas burners to generate heat. Convection ovens may use either of these sources of heat but rarely, if ever, use both. A true convection oven, however, even if powered by gas, uses considerably more electricity than an otherwise comparable conventional oven because of the fan. This is ameliorated somewhat by the fact that convection ovens generally cook considerably faster than conventional ovens. Convection ovens with a third heating element mounted near the fan will work slightly better than those with elements in the traditional top and bottom arrangement.
Cooking times in a convection oven, as compared to a conventional oven, may be reduced and the food cooked at a similar temperature. The temperature can also be lowered and the food cooked for a similar amount of time. The level of the reduction varies depending on the oven, the food being cooked, and the type of pans or dishes being used to cook the food, but the temperature can generally be reduced by approximately 10% or the cooking time by about 20%. Most convection ovens will have an automatic shut off for the fan that engages when the oven door is opened and a manual shut off that allows the oven to be used as a conventional oven.
Foods prepared in a true convection oven, especially roast meats, will develop a brown crust faster than in a conventional oven, sealing in juices and preventing the meat from drying out. Convection ovens also tend to cook more evenly than conventional ovens, reducing the need to rotate dishes from shelf to shelf when cooking several dishes at once. While true convection ovens have many advantages, they also have some drawbacks. They are more expensive than similar conventional ovens and have more components, which may make them more expensive to repair. Convection ovens are also usually noisier because of the electric fan.