DIY Solar Hot Water – The Differential Control System

Solar Hot Water Temperature control

The Differential Controller

A differential controller is often required for more complex solar hot water heating systems.  While it sounds complicated a differential controller is little more than a control system which monitors various aspects of 2 or more environmental systems, compares the values from each system and then takes action based on those values.

In the case of a solar hot water heating system, such as the DIY indirect system that I built, there are 3 interacting environmental systems to be monitored.

  • The solar panel hot water loop
  • The solar storage tank
  • The heat exchanger for heating incoming cold water from the well

Temperature sensors provide data to the differential controller so that it can make decisions based upon comparisons between sensors in each system, and preset thresholds for taking action.  Actions that the differential controller can take may include turning on pumps, opening valves, engaging over-heat or freeze protection, alerting the user to a situation and recording data.

The key to building an efficient solar hot water heating system is having enough data available to the controller to make good decisions based upon constantly changing conditions.

Each sub-system requires multiple temperature sensors

Each of the sub-systems within the solar hot water heating system requires multiple temperature sensors.

  • The solar panel loop
    • Panel input temperature
    • Panel output temperature
    • External ambient air temperature
    • Storage tank input from panel (hot)
    • Storage tank output to panel (cold)
  • The solar storage tank
    • Bottom of tank (where the cold water settles)
    • Top of tank (where the hot water rises)
  • The heat exchanger (Hx) loop
    • Storage tank loop:
      • Hot solar water input from solar storage tank
      • Cold solar water output from the heat exchanger
      • Hot water output from storage tank to Hx
      • Cold water input from Hx back to storage tank
    • Potable water loop:
      • Cold water input from the well (or service)
      • Heated water output from the exchanger (feeds the main domestic hot water heater)

Basic Layout of the Solar Hot Water System with Temperature Sensors

The following system diagram indicates where temperature sensors have been placed, in order to enable both control and monitoring of the efficiency of the system.

One might notice that in each subsystem, there are temp sensors on each input and output.  This is because we not only want to control the system, but also understand the efficiency of the system.  Part of understanding efficiency is determining heat loss from one component to the next.  For example, in the installed system, the cold water pipe originating from the bottom of the solar storage tank and running to the cold input of the solar panel loses approximately 1.2 degrees F.