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I use an old convection oven. I usually preheat at 180°C for 10 mins. Recently I bought an oven thermometer and found that the oven temperature after the said duration was higher than 180°C, around 210°C-220°C. Is this normal?

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    For how long did you measure? An oven is meant to stabilize its temperature after running for a while, but after a short preheating phase, the internal temperature is still very dependent on whether the heater is on or off in that moment. – rumtscho Mar 17 at 11:22
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    Like many things with a large heat sink & a thermostat, there is a great tendency to overheat the first cycle, as the thermal mass is still not up to temperature. You'll probably find the instructions on your steam iron say don't do anything delicate on the first cycle, for similar reasons. – Tetsujin Mar 17 at 14:59
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    Think about how you'd build an oven (and remember cost is very much an issue). If the user sets the oven to 180°C, the sequence would be: turn on the oven heating element, wait till your temperature reaches 180°C, turn off the oven heating element. As you probably noticed with stove elements, they stay quite hot after you turn them off. Same in the oven, and while it's hot, it's adding heat to the system. Then consider that the manufacturer probably considers overshooting safer than undershooting and that he's doesn't want to spent to much on accurate measurements... – Flydog57 Mar 17 at 17:22
  • @rumtscho i measured it after 10 mins. it was still on – sachin Mar 18 at 7:31
  • @Flydog57 thanks for explaining the details! – sachin Mar 18 at 7:32
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Sadly, yes.

Your oven most likely has been running too hot for a while and now that you are actually measuring it, you noticed it.

We have a lot of Q/As on the site that recommend using a separate oven thermometer whenever an oven behaves strangely. Thermostats can fail or be generally incorrect, like too hot or too cool. Plus many ovens fluctuate quite a bit.

If you noticed that your recipes didn’t turn out the way you expected or were used to, getting that oven thermometer was a smart move. Either adjust the temperature to the desired value, if possible (and note down what setting that corresponds to), or consider having the oven serviced or repaired. In some models, the temperature knob can be adjusted, which would be a super trivial thing and may even explain why the oven is set to a wrong temperature.

If your question was wondering more about the still rising temperature - ten minutes preheat is on the shorter end of preheat time, especially for old ovens. You can be dealing with quite a bit of thermal mass.

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  • thanks! yes i noticed my cookies got burnt from bottom and remained uncooked that's why i bought this thermometer. so as i understand i should note when the temperature reaches 180 C. i didn't get the part about thermal mass though – sachin Mar 18 at 7:38
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    Thermal mass means that ovens usually have a certain amount of material (typically steel), that helps retain the heat once the oven is fully hot and during operation. (The heat usually switches off and on repeatedly.) But you need to get the entire oven up to temperature and that can take a while especially in older oven. – Stephie Mar 18 at 7:46
  • Yup! Inaccuracies can even happen in the other direction: mine runs about 15 degrees cool. I don't have an oven thermometer, but all my baking attempts failed before I started fiddling with raising the temp. – Alex Reinking Mar 18 at 7:58
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    Also, during preheat the oven thermometer is getting direct heat from the elements, but the regulating sensor is usually near a wall or colder corner. The whole point of pre-heating is to get past that big transient stage where the oven walls are still cold but the food is getting blasted with radiant heat from the elements (ie : burnt bottoms configuration). Until the oven is fully pre-heated, thermometers in different areas of the oven will read wildly different temperatures because the heat distribution is extremely uneven during this phase. – J... Mar 18 at 12:22
  • @sachin for cookies keep in mind the type of cookie sheet you are cooking them on makes a very big difference. – eps Mar 18 at 22:16
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It is entirely possible that your oven runs hot, many ovens do. But right now, you cannot say anything about it. A single temperature measurement at the tenth minute is a pretty useless piece of information.

What your oven does is to turn its heater full-blast on, then off, in a predetermined pattern (the whole time, the oven indicates that it is "turned on", that's normal, I'm speaking of the heater inside). The temperature knob determines the ratio of time the heater is on vs. off. This means that the temperature will oscilate a lot in the beginning, meaning that the temperature is quite hot while the heater is on, and quite cold when the heater is off. With time, the oven is supposed to get properly preheated - that means that the whole heavy iron mass of it gets hot, and starts steadily radiating energy into the cavity. With a properly preheated oven, the food isn't getting cooked as much from conduction from the hot air, as from radiation from the oven walls. The heater still goes on and off, but in a smaller amplitude, and its effect is anyway "overtaken" by the effect of the oven's thermal mass.

You may be thinking that you have preheated your oven, but you certainly haven't given it enough time for that "proper preheating" I have described. While it is impossible to define an exact moment when it happens, it takes at least an hour, preferably longer. At the tenth minute, your oven walls are still quite cold, and what you measure is just one random point in the oscilating heating cycle.

This doesn't mean that you cannot use your oven after ten minutes, you just have to be aware that at this point, it is acting more like a toaster oven than like a classic oven. The temperature is even less precise than in a classic oven, and changes quite erratically. Luckily, most foods can tolerate that reasonably well.

If you are bothered that maybe your oven runs hot in general (that is, that the steady-state temperature it reaches after a proper preheating is higher than what the knob shows), then you should preheat it for a long time, maybe two hours to be on the safe side, and preferably with some added thermal mass like a pizza stone, then take a series of readings without opening the door. If it turns out to be hotter than the set temperature then, you can start calibrating.

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