How Thermostats Work, Or Why Not Bake When the A/C Is On?

electric fanIn my tips for using conventional ovens, I wrote to avoid baking with the air-conditioner on. Commenter Mimi asked why.

The lion’s share of our energy bill goes to heating and cooling. The refrigerator and freezer, and sometimes the heater or air-conditioner, run all day and night. We use additional heating energy for cooking, water, and  drying clothes.

Electrical appliances that heat or cool all contain heating or cooling elements, a thermostat, and possibly a fan to distribute the hot or cold air. The hotplate, electric kettle or frying pan, crockpot, and even the microwave all use some type of heating element. These heating and cooling elements use large amounts of energy. Sometimes it’s a motor, but the principle is the same.

Electric heating and cooling elements or motors are not smart. They only have two settings, off and on. We can’t set them at some middle range to ensure a constant temperature.

The smart part of the appliance is the thermostat, which has a thermometer to measure the temperature of the surrounding air or water. When the temperature gets too high or low,  the thermostat turns the element on or off. Without a thermostat, an oven could turn your kitchen into a sauna, and an air conditioner could create a freezer in your bedroom.

When you turn on your oven or air-conditioner you set the temperature, but that is not the main factor in your energy usage. What counts is the difference between the room temperature and the target temperature. The bigger the difference, the longer the element will need to operate to reach the target temperature.

In the summer, an air-conditioned kitchen means losing the benefit of the warm air that was already inside your oven. And opening the refrigerator and oven doors at the same times sends a blast of cold air toward the oven and hot air to the fridge. This makes the elements in both appliances operate longer to keep the temperature you have set.

When the heating or cooling elements are off, the appliances are still “on,” but use only small amounts of energy. Understanding this distinction can lead to lower energy costs.


Getting the Most out of Your Conventional Oven.

Do You Need a Second Freezer?

Are Slow Cookers Really Energy Efficient?

Photo credit: macinate


  1. Thanks for the great explanation!

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