The truth about preheating your oven
Preheating your oven is a step every manufacturer and recipe suggests, however, there is much misunderstanding about what constitutes a preheat. Some of this seems to be intentional and we are going to give you some insights into this function and hopefully improve your understanding of why this matters and what the best practice is regarding this.
So, what is preheat? If we look up a legal definition we see… nothing! Legal is an important word here because without it each manufacturer can define “preheat” as whatever they want with no consequence if it’s a tad misleading. We servicers often refer to a “true preheat” and we define that as “when all the inner surfaces of the oven have reached the set temperature”. This includes the top, bottom, walls, inner door including glass, and oven rack.
During the preheating portion of any bake cycle, the control board switches the voltage between the Bake (bottom) and Broil (top) elements. They do this because Broil elements are a higher wattage and therefore have higher heat output. Higher heat output means less time to satisfy the oven control’s demands regarding the preheat timer.
If you look inside your oven you will typically see a metal rod poking in from the back side a distance of about 3-6”, usually near the top of the oven cavity. This is the temperature sensor and it is the only spot in that entire oven where the manufacturer monitors temperature. You may also notice the Broil element being rather close to this sensor. So we have put our sole method of determining the temperature within inches of the hottest surface in the oven- it’s hard to believe that accuracy is the true intent here.
Now I’m going to let you in on a trade secret. What follows is the universally accepted servicer guidelines for testing oven temperature to determine if calibration is required. It is important to note that your oven does not maintain a steady temperature during operation- the temperature fluctuates and it’s not uncommon to see temperature swings of 40-50 degrees from the time the element cycles on to when it cycles back off. First, we use a device called a Weighted Thermocouple. This is clipped to the oven rack in the approximate center of the oven. We then set the temperature to 350. While the oven is cycling, we record the highest temperature achieved and the lowest temperature achieved for 3 full cycles of the element. We then take those 6 numbers (3 highs and 3 lows) and average them. If the average temperature is within 10 degrees +/- of the set temp, the oven is within spec and does not need adjustment. This is literally how we test them, approved by all major manufacturers. This process generally takes 30-40 minutes but can vary depending on the oven brand and design.
Without naming any names, I will tell you that I have had MANY conversations with manufacturer’s technical assistance staff regarding preheat and there seems to be a great many instances where the technical staff and the sales staff of a given manufacturer define preheat in different ways-leading to much confusion and warranty calls to “fix” an oven that is operating within the engineering guidelines set forth by that manufacturer. It’s a rather frustrating thing for both us as servicers, and you as the end user.
So what is the best of course of action regarding preheat? As a general rule, ignore the timer and always preheat for 1/2 hour. This will give the most consistent results as it accounts for the time required to satisfy the requirements of a “true preheat” on any residential oven design. Hopefully, this can help some of you to understand why results may vary so much and maybe it can even help some aspiring baker to perfecting their craft!
If you have any further questions, please don’t hesitate to call or comment and we will do our best to clear it all up for you. Thank you all and Happy Holidays!
