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Help needed to understand existing solar installation

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  • Help needed to understand existing solar installation

    I built my own house a few years ago and I had it fitted with an air source heat pump and two solar thermal panels to help heat the hot water. I was told that the heat pump will kick in one the host water side if the solar isn't providing enough. The heat pump and cylinder are Daikin and there is a Vaillant control box for the solar.

    This may not have been the right thing to do but after a number of sleepless nights caused by the solar pumps coming on at strange hours, I resorted to putting the mains supply for the Vaillant box on a timer so it can only come on during the day.

    What frustrates me is that I have no idea how any of this stuff is working because there's no way of visualising it. We had quite high electricity bills for about a year before I realised I was better off running the heating during the day than at night (as you would a traditional gas/radiator installation). I wanted to learn more so a few weeks ago I get a Raspberry Pi and started putting together a data logger. I plan to expand the number of sensors but for the time being I just have a temp sensor on the code side and one on the hot side. I also know the outdoor temperature and weather from a Weather API. Here's some typical data:
    Time Hot temp (IN) Cold temp (OUT) Outside temp Weather
    1452756753 26.062 25.187 1.36 Clear
    1452757661 26.125 25.187 1.36 Clear
    1452758568 26.062 25.187 1.36 Clear
    1452759474 7.937 25.187 1 Clear
    1452760379 9.875 25.187 1 Clear
    1452761283 11.75 33.5 1 Clear
    1452762187 11.562 33.875 1 Clear
    1452763091 11.687 33.125 1.73 Clear
    1452763995 11.312 33.25 1.73 Clear
    1452764899 14.125 32.062 1.73 Clear
    1452765803 11.437 32.562 1.73 Clear
    1452766708 12.187 32.375 2.63 Clouds
    1452767612 15 31.25 2.63 Clouds
    1452768515 17 30.312 2.63 Clouds
    1452769419 18.625 30.312 2.63 Clouds
    1452770328 19.937 28.812 3.47 Clear
    1452771232 21 27.687 3.47 Clear
    What appears to happen is that the solar pumps come one when the control unit starts up about 8 am. As the weather is cold and the sun not high enough the panels are very cold. I appear to have cold fluid coming in and I'm losing heat out of the cold side. I just checked it and the solar pump is still running a few hours after it came on. Is this right? Does the Vaillant just keep pumping regardless? Surely I'm losing energy here.

  • #2
    The solar pump should only come on if the panel temp is 8-10 degrees higher than the tank temp. Check your control configuration, or you may have your sensors reversed.


    • #3
      Hi. Sorry for the delay and thanks for the response. I checked the installation manual and discovered that the 'yield' sensor should have been connected to the return but it was connected to the hot side of the coil. I've moved it and hopefully that will make an improvement. Many thanks.


      • #4
        As a follow up, I've now got some data from my logging tool. Here's a chart. The red line marks where I made the correction to the sensor. You can see before that I was getting a lot of cold pumping through the system everyday. Now the cold cloudy days don't have that problem and on the sunny days it kicks in.