Controller - Differential thermostatic controller

The system is guided through its various modes by a differential temperature thermostat. The controller is basically a motor-control switch that automatically starts and stops circulating pumps according to temperature measurements. The controller usually has a light to indicate when the pumps are on. Some models display the temperatures on a digital display.

The controller decides to turn on the pump based on temperature readings from sensors located at strategic points within the system. The controller is called a differential temperature controller because it monitors the temperature difference between the water in the bottom of the storage tank and the fluid in the solar heat collectors. When the fluid in the collectors is several degrees warmer than the coolest water in the storage tank, there is usable heat to be collected. The controller starts the pumps. When the temperature difference drops sufficiently, the controller stops the pumps. The pumps remain off until a useful temperature difference exists again. The temperature range for starting and stopping the pumps are usually preset in the controller.

For draindown systems, the controller also initiates and resets the system's drain-down mode when freeze sensors indicate the temperature is near freezing.