When you burn your finger you blow on it to cool it down so it is counter intuitive that putting a fan in an oven will make it hotter. This article is a beginner's guide to fan assisted ovens. It does not include product recommendations but should help you understand how these devices work and provide insight into whether they are right for you.
How it works
Convection Ovens
When you cook on a hob heat is transferred from an energy source (the flame if you are cooking with gas) via conduction. This means that the hobs gets hot which makes the pan get hot which in turn makes the food get hot. When you cook in an oven you are working in a different way. The heat source is actually warming the food by transferring energy to the air and it is the air which warms the food.
Warm air will naturally rise, this is known as convection. Conventional ovens rely on convection to move hot air around and keep a constant flow of heat going into food. The problem with this is that hot air will pool in the top of the cavity and end up heating the roof of the oven. This means that in order for the whole of the oven to be filled with hot air you first need to pre-heat the metal sides in order to stop them from soaking the energy out of the air.
A fan-assisted oven (which is confusingly also known as a convection oven) forces air from the heat source directly into the centre of the cooking space so that anything placed there gets hot. Energy is still transferred from the air to the walls but because the fan is constantly forcing hot air in and around the cavity it maintains a steady temperature.
What this means
1. Fan assisted ovens do not need preheating - Because the fan forces hot air directly into the cooking area a fan assisted oven either takes no, or very little, time to preheat.
2. Fan assisted units provide even cooking - As both hot and cold air are pushed around the cavity the difference between the warmest and coldest places is reduced.
3. Fan assisted cooking is quicker - Because they transfer energy more efficiently than regular machines they give the same results at a lower temperature and in less time. This is can be useful but it is also the reason why so many people end up burning their food when using a fan assisted oven.