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This is a silly question, but my wife will turn on the oven to preheat while she prepares stuff, but sometimes it takes her 15 minutes to 45 minutes before she puts things into the oven.

Generally, how long does one need to preheat the oven before it's to the desired temp? I know that this can easily be solved with a thermometer, but we don't have one.

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Is your oven electric, gas or solid fuel? –  Orbling Dec 10 '10 at 13:59
    
When I read "preheat the oven for 10 minutes at 180 degrees", I preheat it for 2-3 minutes at 360 degrees. It's not like I'm following the rest of the recipe anyway... :P –  Camilo Martin Aug 20 at 0:57

3 Answers 3

up vote 10 down vote accepted

This depends on a lot of things.

The idea of preheating is that you want to get all the surfaces inside your oven (walls, floor, door, racks) up to the desired cooking temperature. This makes for more even temperatures throughout the oven, and gives a little thermal mass so you don't lose ALL your heat when you open the door for a few seconds or put something cold in there.

Then there's the question of what you're putting in the oven. An aluminum sheet with a few room temperature cookies on it won't pull the temperature in the oven down like a 25 pound turkey that's 40 degrees inside. You want to be more careful to do a complete preheat if you're going to be soaking up a lot of your starting heat.

Our oven, which has a large baking stone in the bottom all the time, takes a while to get uniformly up to temperature, even after the oven says it's preheated, because the stone doesn't heat up as fast as the rest of the surfaces. It takes at least 20 minutes after the "I'm fully heated" beep before the stone is fully up to temp. We have problems with things baking poorly if we don't preheat for quite a while, but on the upside, if we put a cold roast in or open the door a lot, the temperature in the oven stays pretty high.

If your oven is lightweight, flimsy or drafty, it may be as hot as it's going to get the moment the preheat alert goes off.

45 minutes is probably a lot more preheat than you'll need in almost any case. In some cases even 15 minutes is more than you need. It really depends on your oven and what you're putting in.

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Absolutely correct. The preheat beeper goes off when the air has reached temp, and opening the door of the oven will kill that right away. The more thermal mass (baking stone, bricks, I've even seen a full clay oven liner(found it hearthkitchen.com)) you have the longer it takes to heat up, BUT, the less fluctuation in temperature as you put food into it. –  Doug Johnson-Cookloose Dec 10 '10 at 4:53
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Add to that... consider the degree you need to maintain constant temperature. When baking bread, you don't want that initial drop in temp from improper preheating (so preheat longer). But if you're warming up yesterdays casserole, who cares? –  Robert Cartaino Dec 10 '10 at 15:37
    
As a person from "the rest of the world", 40 degrees is pretty hot. –  Camilo Martin Aug 20 at 0:53

Most ovens I have used take from 15 to 20 minutes to get to 180C (350F). Many ovens have an indicator light that glows while it is below temperature (i.e. when the elements are on) so just watch for that to go out, and your are at your desired temperature

The key thing is that most modern ovens are well insulated and will not use much energy once at temperature and with nothing else to heat up, so going a bit longer isn't a big energy waster

If you want to save energy have a queue of things to bake one after the other, as a significant amount of energy if used just getting the oven up to temperature. Electric ovens are from 3000 to 5000 Watts, so each warm up cycle uses approximately one kW

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My sister-in-law is conplaining about the same thing. Her oven takes 20 minutes to heat up to 350 degrees. I timed our oven last night and it took 7-1/2 minutes. I was suprised it heated up so fast.

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