DIY Solar Heater Thermal Controllers

Written by richard belmondo
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DIY Solar Heater Thermal Controllers
A thermal control unit is constructed on a printed circuit board. (computer card image by Steve Mann from

A closed loop solar heating system generally uses a mechanical pump to circulate heated water through the system. When integrating the pump into the circuit you must also add a temperature controlled switching unit, which will measure the water at two specific points within the system. Whenever the solar panel is hotter than the stored water, the controller switches on the pump to send hot water from the panel into the hot water tank. Conversely, when the stored water is hotter than the solar panel, the control unit switches the pump off.

Skill level:

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Things you need

  • Printed circuit board
  • Thermistors
  • Variable resistors
  • Microcontroller
  • Mosfet bidirectional relay
  • Temperature differential control
  • Pump
  • Cable

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  1. 1

    Attach one thermistor probe to the top of a previously installed solar panel near to the water exit pipe (the hottest part of the system) and the other thermistor next to the storage tank. You will need enough length of cable to connect the thermistors to the circuit board in the control unit box.

  2. 2

    Place a couple of variable resistors to the circuit board to balance the voltage output, ensuring that each thermistor will give the same reading when they are at the same temperature.

  3. 3

    Add a microcontroller to the board. This component can sense the differences of the two temperatures and initiate the pump when prerequisite conditions are met. A mosfet bidirectional relay (a type of transistor) is used to turn on the pump when triggered by the output from the microcontroller.

  4. 4

    Include an adjustable temperature differential control that goes from approximately 1 to 20 degrees Celsius. If this control is set on 10, the pump will only start when the panel temperature is hotter than the storage tank by 10 degrees Celsius.

  5. 5

    Run all of the wires (thermistors, solar panel, pump and battery) into the control box unit then place it close to the storage tank on a nearby wall.

Tips and warnings

  • Make sure that the control box will not get damp or wet from water leaks in the system. You will also need a space nearby to put the battery.
  • Too large a pump may cause damage to the circuit board by overheating of the output transistor. To use a large or heavy-duty pump it may be necessary to use an external relay and power supply.

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