Differential Controller Basics

In layman’s terms, a controller acts as the “brains” of a solar hot water system. It senses when there is solar energy to be harvested from the collectors on the roof, and activates the pumps to bring that energy down into a storage tank where it can be used. In more technical terms, the Delta-T controller is an electronic device that in essence, compares the temperature of the hot solar collectors to the colder solar storage tank. A built-in microprocessor will turn a pump on if it detects a certain temperature difference (delta t) between the 2 locations. Electronic temperature sensors are run from the controller to the roof and storage tank.      

This diagram illustrates how a controller is used in a solar hot water system.

A sensor is wired from the controller, to the roof where it is attached to the return piping coming directly out of the collector. A second sensor is wired from the controller to the bottom region of the storage tank. It is usually attached and secured to a metal stud.

10K ohm sensors and UV resistant wire are used to provide temperature information to the controller. The use of sensor wire that is not rated for the high temperatures and exposure to sunlight will result in early system malfunctions. 

There are other added basic functions that come built in with a standard  solar thermal controller (like the Delta-T) such as automated pump recirculation for freeze protection and tank high temperature shutoff. For full details, download the controller data sheet.

Installers needing a more advanced controller with more input options and web enabled system monitoring, should select an advanced controller such as the Delta-T Pro. The Delta-T Pro  performs all the basic functions of a standard differential controller but offers much more. For example, instead of 2 sensor inputs, the Delta-T Pro offers 5. These additional sensor ports can be used for a variety of purposes such as reading the outdoor ambient air temperature, ground temperature or perhaps determining the temperature of the upper region of the tank. The diagram illustrates all the potential sensor inputs that can be utilized by the Delta-T Pro. 

It’s recommended to compare the two types of controllers to see what best matches your particular solar hot water system needs. Feel free to email us if you need advice in determining which one is right for you.