DIY Solar Hot Water – Temperature Sensing

Temperature sensing within a solar hot water system is essential to the efficient operation of the system.  With the proper number of sensors placed in the right locations within the system, you can gather the data needed to optimize and protect the various subsystems.  There are 3 primary subsystems within an indirect solar hot water heating system.

  • Solar panel loop
  • Storage tank
  • Heat exchanger loop

In this article we will list out the temperature sensors.

Temperature Sensor Schematic

We introduced the placement of the temperature sensors in our last article: https://henryranch.net/solar/diy-solar-hot-water-differential-control-system/ and provided this schematic:

Temparature sensor placement in the DIY Solar Hot Water system

Temperature Sensors within a Solar Hot Water System

In this article we will list out the temperature sensors.

  • Solar panel input
    • This sensor measures the temperature at the input port of the solar panel where cold water arrives from the bottom of the storage tank.
  • Solar panel output
    • This sensor measures the temperature at the output port of the solar panel.  It is the point of hottest water in the system, when the panel is gathering heat.
  • Storage tank solar input
    • This sensor measures hot water coming from the solar panel as it enters the storage tank.  It is typically located at the top of the tank.
  • Storage tank solar output
    • This sensor measures cooler water flowing to the solar panel.  This water has settled to the bottom of the tank. The pipe is ussually located at or near the bottom of the tank.
  • Storage tank heat exchanger output
    • This sensor measures the temperature at the point of use output of the solar storage tank.  The pipe which feeds this output port is typically at the top of the tank and gathers the hottest water in the tank which has risen via internal convection.
  • Storage tank heat exchanger input
    • This sensor measures the temperature at the input port of the storage tank where colder water arrives from the heat exchanger, after giving up its heat to the potable water via the heat exchanger.  This pipe is typically located at the top of the tank but extends internally to the bottom of the tank so that it returns the cooled water where the cold water settles in the tank due to internal convection.
  • Heat exchanger solar input
    • This sensor is located at the hot water input to the heat exchanger and measures the hot solar water from the hot output of the storage tank.
  • Heat exchanger solar output
    • This sensor is located at the cool water output of the heat exchanger and measures the cooler solar water from the heat exchanger.  This water is cooler because it has given up its thermal energy through the heat exchanger to the cold potable water.
  • Heat exchanger potable water input
    •   Cold water from the house service (city, municipal, well, etc…) enters the heat exchanger at this point for the purpose of being heated.
  • Heat exchanger potable water output
    • This potable water has been heated by the hot solar water via the heat exchanger.  Because the solar hot water and this potable water are isolated via the heat exchanger, there is no cross contamination.  The sensor measures the temperature of this water.  The difference in heat from the potable water input and this output can be used to calculate the imputed thermal energy.
  • Potable water system input
    • This is the coldest water point in the system.  The temperature sensor at this point provides a baseline for understanding the seasonal variation in the system.
  • Outside ambient air temperature
    • This sensor measures the temperature outdoors.  It can be used to help determine when to engage freeze protection measures.
  • Interior ambient air temperature
    • This senor measures the indoor air temperature of the space in which the storage tank , heat exchanger, pumps and control system are installed.
  • CPU processor temperature
    • This is a good point to measure temperature also, as it provides an indicator of whether the controller is getting to o hot.  Computers don’t like it when their CPU overheats.  The RaspberryPi contains an internal CPU temperature sensor.

What sensors to use in a Solar Hot Water System?

I am using a 1-wire network with all of my sensors.  We will post a page with details on the 1-wire network configuration soon.

The sensors are inexpensive, environmentally sealed and can be ordered via Amazon. These probes are digital units with 3 wires: Power, Data and Ground and make use of the 1-Wire digital protocol.