Most thermally-based renewable energy heating systems have controllers measuring and responding to temperature. Common examples are set-point controllers, outdoor reset controllers, mixing controllers and differential-temperature controllers.
In residential and light-commercial systems temperatures are sensed by NTC (negative temperature coefficient) thermistor sensors. These solid-state devices decrease their electrical resistance as their temperature increases and vice versa. Figure 1 shows a temperature vs. resistance chart for a common “10K” NTC thermistor sensor that’s commonly used in the industry. The “10K” designation indicates the sensors resistance of 10,000 ohms at 77° F (25° C).