Help needed to understand existing solar installation

[stuart_brierley]

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.

[LucMan]

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.

[stuart_brierley]

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.

[stuart_brierley]

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.