Why does a "rapid" preheating mode require no more than one oven rack?
I have a Whirlpool wall oven with (optional) convection settings. It has three racks. The obvious place to store oven racks is, of course, in the oven.
The oven has two preheating modes, "rapid" and "standard". The documentation says to use the "rapid" mode with only one rack (removing the other two) and the "standard" mode otherwise. Both modes run the fan audibly.
Why does the presence of more or fewer racks make a difference in preheating? I've found explanations online about using only one rack to improve circulation when cooking -- sure, that's plausible, but we're talking about preheating here, not cooking. Doesn't it use a thermostat to determine when the oven is up to temperature?
I would naturally prefer faster preheating to slower, but I want to keep all the racks in the oven. What can go wrong in using the "rapid" mode with all racks in place?
1 answer
This is a thermodynamics issue: the heat capacity of steel is about 460 times the heat capacity of an equal mass of air -- and there's not a large mass of air in an oven, and each rack weighs a couple of kilograms. So each additional rack is a major factor in the preheat time.
The rapid heating mode turns on the broiler and burner and convection fan to get the air temperature up as fast as possible. To aid the marketing number, they tell you to only put in one rack. If the marketing department could convince engineering to say that no racks should be put in, the time to pre-heat would be even faster.
If you are depending on a stable temperature in your oven, adding thermal mass will slow down the preheat but smooth out fluctuations. Broil a steak? Use the pre-heat and one rack. Baking a cake? Assume that the rapid preheat is only a little faster than normal and put more thermal capacity in your oven.
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