Fan or conventional: the twenty degree rule and when it misleads

Everything below about fan versus conventional oven settings comes from what actually happens rather than from what is supposed to.

What the fan changes

Still air is a poor conductor, and food in a conventional oven sits inside a thin insulating layer of cooler air it has chilled. A fan strips that layer away continuously, so hot air is always in contact with the surface and heat transfers much faster. The effective cooking rate rises substantially even though the air temperature is identical.

That is why the standard advice is to reduce the setting by around twenty degrees when converting a recipe. The number is a rule of thumb, and different ovens and different foods sit either side of it.

Where fan helps

Multiple trays of biscuits, buns or small items bake far more evenly with a fan than without one. Roasting and anything where you want a dry, crisp surface benefits from the increased evaporation. Pastry generally does well, since crisping is helped by moving air and the shapes are usually shallow.

Fan also shortens preheat times and recovers faster after the door has been opened. For a kitchen baking several trays at once, the fan is the reason the oven is usable at all.

Where fan hurts

Bread wants a moist environment early in the bake, and a fan actively removes the steam you have gone to trouble to create. Delicate cakes can dry at the edges or develop a skin before the centre has finished rising.

The dough tells you first: meringues and slow-dried items sometimes crack or colour unevenly under direct airflow. Very light batters can be physically disturbed by the moving air, particularly near the fan itself. In these cases conventional heat, or a covered vessel inside a fan oven, gives better results.

Converting in both directions

Going from conventional to fan, subtract about twenty degrees and expect the bake to finish slightly sooner. Going the other way, add twenty degrees and expect a longer bake with a less even result across the tray. Times usually shift by around ten per cent, so check earlier rather than trusting the stated duration.

Recipes rarely say which they were written for, though European recipes often quote both.

When a recipe gives only one figure and no label, assume conventional unless the context suggests otherwise.

Combination and specialist settings

Many ovens offer a fan-assisted mode with the bottom element and fan running together, which behaves between the two. Some have a dedicated bread or steam setting that injects moisture and holds the fan off early in the bake.

Top-only and bottom-only elements are useful for browning a surface or crisping a base respectively. Reading the manual for what each symbol actually switches on is worth twenty minutes of anyone's time. Most people use two of the eight settings their oven offers and never find out what the others do.

Practical defaults

For bread in a covered pot, the setting matters less because the pot controls the microclimate around the loaf. For open-tray bread, conventional with steam usually beats fan, unless your fan oven has a bread mode. For biscuits, tray bakes and pastry, fan is generally the better choice and the more even one.

The dough tells you first: for cakes, either works, and the deciding factor is usually whether your oven's fan is aggressive. Test one recipe both ways and you will know which your particular oven prefers.

The takeaway

Drop twenty degrees for fan, but switch it off when you need moisture rather than speed.

Temperature and time are the same argument held at opposite ends.