All hydronic systems designed around renewable energy heat sources — as well as those designed around conventional boilers — have at least one controller that measures and responds to temperature. Common examples are temperature setpoint controllers, outdoor reset controllers, mixing controllers and differential temperature controllers. Complex systems that operate in multiple modes, or use multiple heat sources, often have several temperature-based controllers.
Most of these devices use thermistor sensors, or more specifically, negative temperature coefficient (NTC) thermistor sensors. These solid-state devices lower their electrical resistance as their temperature increases and vice versa. Figure 1 shows a temperature versus resistance chart for a “10K” NTC thermistor sensor that’s commonly used in the hydronics industry. The “10K” designation indicates the sensor’s resistance of 10,000 ohms at 25° C (77° F).