From M.M., 31 December 2009, Conventional ovens: why would I ever use them if I have the convection option?
Q: I have a convection oven. It gives you the option to either use the convection feature or not. But why? Doesn’t the convection feature ensure even cooking and browning? When would I ever cook or roast something without the convection feature?
A: Thanks for your question. It gives me the opportunity to discuss oven mechanics, which I enjoy.
Convection ovens work by circulating the heated air in the oven by using a fan. This circulation reduces the incidence of hot spots and effectively heats from all directions, unlike conventional ovens, which heat from below unless you select the top element in an electric oven (broiling). Gas ovens lack a top element, which is why you broil in that bottom drawer. In any case, the multi-directional air flow cuts cooking time by 15 to 25 percent.
So if convection is quicker and more even, as you already have observed, why use conventional settings at all? Two reasons. First, some dishes work because they are heated from below and, as the heat rises, they rise. Soufflés and puff pastry (mille feuille) are the most obvious examples, but breads also sometimes fare better in conventional ovens. Use the conventional setting for soufflé, puff pastry, and baking bread.
Second, circulating air is less gentle than still air. Some delicate items, like small-flake fish (sole, etc.) dry out more quickly in a convection setting.
The Upstart Kitchen 
I have found my convection oven to be a real boon. However, one of the benefits, reduced cooking time, can be difficult to deal with. Almost every recipe out there will list cooking times based on conventional ovens. So whenever I am faced with cooking something where a precise cooking time is required I use the conventional setting. Otherwise, I guess at the reduced cooking time and just start checking the progress early.
When I am cooking complex dinners, I generally use the conventional settings so that I don’t have to check on the oven all the time. But basically, when I can use the convection setting, I do.
Yes, that’s right! You do need to adjust the cooking times – as I think I mentioned, they can be 15-25% shorter. You also can reduce the cooking temperature by about 25F. For larger cuts of meat, I find this results in a moister finished product.