Solar hot water system not working

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I hope this is the right forum to post my question.

I have a problem with my solar hot water system. It does not seem to be supplying adequate hot water. Any helpful comments, please!

I have (inherited) an Open solar hot water system (No coils inside tank, the fluid in the collector is potable water and directly connected to the house hot water system) using one vacuum tube panel of 15 tubes, make unknown but I think sourced out of China. The connected tank has 120 l capacity.

The (AC) pump circulating water through the collector to the tank has failed. I have replaced it with a DC pump. For testing purposes I have connected it to a timer which is set to run continuously from 9.00 am to 5 pm each day (I live in Spain where the sun (nearly) always shines).

The system has three temperature sensors; a Pt1000 at the top of the collector (T1), a NTC 10k at the bottom of the tank (T2) and the same kind at the top exit (T3). I took some measurements as follows, with the circulation pump continuously on; ambient day temperature was 31ºC (88ºF):

Resistance value of sensor T1 was 1150 Ohm, equating to 40ºC (104ºF)
(Laboratory) thermometer temperature measured at T1, under the insulation was 39ºC (102ºF)
Resistance value of sensor T2 was 4000 Ohm, equating to 47ºC (117ºF)
Resistance value of sensor T3 was 16000 Ohm, equating to less than 20ºC (68ºF)
Delivered water temperature at exit T3 by thermometer was 40ºC

1. Only 40ºC at the collector top seems far too low.
2. Either the T2 sensor is not working properly or the wiring is bad. To be investigated further.
3. Something is really wrong with sensor T3 and / or the wiring. To be investigated further.
4. The collector is working (somewhat) . The DC pump is doing its job. The exit water temperature is the same as the collector water temperature.
5. Sensor T1 is working well.

What could be the cause of the low circulating water temperature at the top of the collector? (40ºC with ambient temperature of 31ºC !) Circulation pump continuously on? Collector panel not functioning? (How to test that?).

Anybody have any ideas?


The wife is still training me.
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Orlando, Florida
I know nothing about solar systems but my first thought is it is air bound at the top part of the panels assuming they are on a roof. Is there an air bleeder up there? Possible that since you're using potable water supply that the panels tubes are getting plugged up with sediment.
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