Occurs when you have a simple on-off controls system. When the temperature falls below the set point, the power is turned on and the room heats until it is above the set point, and then the room cools until the temperature is below the set point again, at which point the heat is turned on again, and so on. This may take a little time to happen, but when the temperature is just balanced at the switch-over temperature, this hunting can be frequent. High-frequency switching like this is undesirable because turning things on and off tends to wear them This is true relays as well. One way to minimize this effect is to introduce something called hysteresis, and you may have noticed a variable called hysteresis in the sketch that is set to a value of 0.25 C. Figure 8-3 shows how we use a hysteresis value to prevent high-frequency hunting As the temperature rises with the power on, it approaches the set point. However, it does not turn off the power until it has exceeded the set point