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I recently renovated my kitchen and have a new Bosch wall oven. I've noticed many baking recipes that used to be reliable are no longer so. I have a ThermoWorks ChefAlarm and calibrated my oven according to the procedure on their website. The accuracy of the oven is pretty good - usually only 5-8 degrees lower than the display says. But the temperature swings are surprising: they're +/- 30 degrees. So if I set my oven for 350, the temperature ranges back and forth from 320 to 380 over time.

I've never checked the range on previous ovens so I can't say if this is normal. It seems to me to likely be a problem, especially when a cookie might only be in the oven for 10-12 minutes. Does anyone know? And a follow on: I'm tempted to reach out to Bosch customer service to see if there is a field modification to change this behavior. Anyone know if that's realistic?

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    Lob a baking stone into the bottom of the oven to act as a thermal ballast. – thrig Feb 9 '16 at 21:01
  • @thrig: that made sense to me at first but as I think about it more, that doesn't change the control circuitry that allows the +/- 30 spread. So I think all it will do is slow down the cycle between 320 and 380? Dunno - just thinking out loud here. It's still an interesting experiment to do with the thermometer. – DaveBurns Feb 12 '16 at 15:32
  • The oven will take longer to heat, as some energy is heating the stone. When the thermometer trips off, and the temperature in the oven decays, the stone will release heat into the oven, thus slowing the rate of heat loss, meaning that when the thermometer trips back on, the heat source has less work to do to bring the oven back up to temperature. Thus, the expected result of adding a battery to the system should be to dampen the temperature swings. – thrig Feb 12 '16 at 17:08
  • @thrig: Still thinking out loud and getting nerdy with this. The control circuitry of the oven is temperature-based, not time-based, yes? If so, the oven will decay from high to low (e.g. 380 to 320) more slowly but the oven will still wait until 320 to turn the heat back on. Then it's the same slow rate to heat things back up. If that's the case, isn't it true that the problem is now worse, since whatever is cooking will spend more time away from the requested temp than at it? That is, your average temperature over time hasn't changed, but you're now at the average much less. Am I off here? – DaveBurns Feb 12 '16 at 17:31
  • The rate of change is different; without the stone the oven will fall faster past the trip point for the thermometer, which is not at the observed peaks, but somewhere between them. With a stone, one would expect to see less spread, e.g. 330 and 370, depending on the size of the stone, etc. – thrig Feb 12 '16 at 18:32
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This is fully normal. Ovens are not stable, temperature-wise, and I have frequently seen such large amplitudes in temperature. Of course, it is much nicer if your oven can hold a constant temperature, that's why some people will accept the expense of an Aga. But in principle, baking recipes can handle that. Note that from a historical point of view, people used ovens with solid fuel, which had much larger temperature amplitudes and no temperature display, and their baking goods were still tasty. Ovens are not temperature-accurate, and recipes are robust for that.

As for the recipes which are no longer reliable, the most likely explanation is that your old oven was also inaccurate, but in a different pattern. Another possibility would be a different mix of radiant, convective and conductive heat, which means that the same recipe in the same pan can require a different time. In general, a recipe which specifies time is reliable for a specific combination of pan, oven and amount. If you bake until ready instead of waiting for a time given in the recipe, the problem disappears.

  • Thanks. I haven't run my calibration test with convection mode on but am thinking of doing so. Would it be your expectation that the temperature swings are less or about the same? – DaveBurns Feb 9 '16 at 18:05
  • There are ovens where at least an oven thermometer will be spot-on all the time... but they have their own thermal mass. Such a wide range does sound like a potential problem though, when drying or proofing things in it... – rackandboneman Feb 9 '16 at 23:38

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