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How quickly do ovens lose heat when they are opened?
While it will vary with the construction of the oven, more metal means more thermal mass, and therefore a slower temperature change.
By measuring oven air temperature during operation, we know that opening the door will cause air temperatures to fall 50–100°F in the first few seconds. After the quick air exchange caused by convection currents from an open oven door, closing the door quickly will stop the temperature fall to 50–100°F below the original temperature.
Heat is also absorbed by the metal liner, racks, cookware, baking stones, and food. These objects act as thermal mass, and account for the vast majority of the heat in the oven. The air, by comparison, is much less dense, accounting for a small percentage (my guess is less than 10%) of the heat in the oven.
Leaving the door open longer will allow more air exchange, cooling the metal liner, racks, and cookware. Even with the door wide open and the heat off, it might take 10–30 minutes for the internal surfaces to reach room temperature.
Normal oven operation allows temperatures to vary 10–20°F above and below the set temperature, due to the lag in sensor sensitivity and responsiveness, as well as the time delay between when the electro-mechanical controls call for heat and the delivery of that heat. Opening the door will probably initiate a heat cycle as quickly as the controls can respond, and temperature will not be out of the desired range for very long.
The temperature loss from opening the door can increase cooking times for 2 to 4 minutes. Not a big deal for a beef roast or a turkey, but a much greater effect on cookies or pizza!