Thermostat Fallacy

Thermostat Fallacy or Thermostat Error is a common beliefbased on a flawed understanding of the function of thermostatic control. This belief manifests itself in a number of common ways:
* The belief that on a cold day, the thermostat on a heater should be turned up; or
* The belief that a space will cool more quickly when the thermostat set point is lowered.
The Function of Thermostats
The Thermostat Fallacy applies only to the type of thermostat that turns a heating or cooling function on or according to the input from one or more temperature sensors, a set point, and, usually implicitly, a hysteresis value.
Example: Heating Thermostat
A heating thermostat will turn a heating function (such as an element or a heat-pump) on if the temperature reported by the temperature sensor is less than the set-point. When the temperature reaches the set-point, the thermostat will turn the heating function off.
In order to prevent chatter (where the element is continually and rapidly turned on and off again as the temperature cycles around the set point) thermostats allow a range of temperature around the set point. This is called hysteresis.
A heating system with a set point of 20C and a hysteresis of 1C might turn on and remain on until the temperature reaches 20.5C, whereupon it turns off until the temperature drops to 19.5C. There is no rule as to whether hysteresis bands straddle the set point symmetrically or are biased on the high or low side.
Other Types of Thermostat
While there are several different types of thermostat, the type described is by far the most common, especially in a day-to-day context that includes typical household or industrial heating and cooling devices, many automotive climate-control systems, the function of common items such as coffee urns, irons, and things such as spa and swimming pool heaters. Thus, although it does not apply to all thermostats, it applies to the great majority of thermostats encountered in day-to-day life.
The Logical Flaw Underlying the Thermostat Fallacy
The underlying logical flaw arises from the fact that the thermostat turns the heating or cooling function on or off. There is no middle ground. Those falling into the Thermostat Fallacy suffer from the belief that the function of the thermostat is more nuanced.
If, for example, in a heating system, the thermostat has the heating function switched on because the temperature is below the set point, then raising the set point will have no impact. The temperature was below the set point; when the set point is raised it will still be below the set point. The heating function was turned on; it will continue to run. The system (e.g. room, car, etc) will heat no faster.
Similarly, take the situation where an airconditioner is attempting to cool a house to 23C (the set point), and due to the very hot outdoor temperature of 35C, the system cannot cool the house below 28C. Because the temperature is above the set point, the cooling function will be on. Lowering the set point will simply result in the cooling function remaining on. It will not increase the power of the overall system (which is what is required to lower the temperature).

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