Solar hot water fault

[Chris_909]

All I’ve managed to find was this

I was under the impression the differential was the difference in tank temp vs the solar panel temp
So it does increase maybe about 5° an hour? Once it hits 30° it’ll increase slowly but if I have my boiler on and the tank temp will be maybe 70° the pump will not come on as the solar is less than the tank so the controller does work

 

[Johntheo5]

There's quite a lot of info in that attachment but some, like the output logic, a bit confusing, to me, at any rate.

The cylinder shows two probes (quite normal), one, the "Low" is installed just above the solar heating coil, the other, "Aux" is installed (normally) in the cylinder top with the boiler coil below it may be heating maybe the upper two thirds of the cylinder so you could have the upper two thirds at 70C but the lower as low as 15C or so, the solar system should then heat this one third up as well, OR if no boiler on, the solar will heat the whole cylinder.
The pump shows a start contact L1 but doesn't show what energises this as the logic shows this switched only for controller models 501 & 501D, (yours is 502D), what it does show though is that the pump stop contact L2 is energised when "T.high < T.low+T.diff which means when the solar panel temperature is (or falls) to the store temperature+4C then the pump stops, this means, by inference, that once the solar panel temperature reaches store temperature + 4C + the hysteresis (given as 1C), ie, when the panel temperature increases to store temp + 5C then the L2 is de energised and the contacts will switch over to L1 and pump restart. If this is how it operates then very unusual IMO to use a normally closed (NC) contact and a set switching hysteresis of 1C but it is what it is, maybe?. I would also increase that 50C SP to say 65C.
Its a mystery though why the circ pump runs for hours on end as we can see now that its not variable speed controlled, the schematic also shows a motorised valve, don't know what this does. Your cylinder should heat up in a few hours in this weather using solar only, mine is reaching 65/70C every day for the past 4 or 5 days.

 

[Chris_909]

There's quite a lot of info in that attachment but some, like the output logic, a bit confusing, to me, at any rate.

The cylinder shows two probes (quite normal), one, the "Low" is installed just above the solar heating coil, the other, "Aux" is installed (normally) in the cylinder top with the boiler coil below it may be heating maybe the upper two thirds of the cylinder so you could have the upper two thirds at 70C but the lower as low as 15C or so, the solar system should then heat this one third up as well, OR if no boiler on, the solar will heat the whole cylinder.
The pump shows a start contact L1 but doesn't show what energises this as the logic shows this switched only for controller models 501 & 501D, (yours is 502D), what it does show though is that the pump stop contact L2 is energised when "T.high < T.low+T.diff which means when the solar panel temperature is (or falls) to the store temperature+4C then the pump stops, this means, by inference, that once the solar panel temperature reaches store temperature + 4C + the hysteresis (given as 1C), ie, when the panel temperature increases to store temp + 5C then the L2 is de energised and the contacts will switch over to L1 and pump restart. If this is how it operates then very unusual IMO to use a normally closed (NC) contact and a set switching hysteresis of 1C but it is what it is, maybe?. I would also increase that 50C SP to say 65C.
Its a mystery though why the circ pump runs for hours on end as we can see now that its not variable speed controlled, the schematic also shows a motorised valve, don't know what this does. Your cylinder should heat up in a few hours in this weather using solar only, mine is reaching 65/70C every day for the past 4 or 5 days.

Hmm okay from my research this is a very old controller, 90s electronics inside maybe 80s maybe worth noting the tubes are rather old I don’t have an actual install date but definatky older than 2011 so probably not as efficient as yours, I’ll change the settings as you said and continue to use them under supervision as they do heat the water just slowly, I’m just more concerned the circulation pump is going to burn out (it’s a grundfos ups 15/50) ….there not cheap either lol

 

[Johntheo5]

If L1 was the switched pump contact like its being showed for models 501&501D, switching when the solar panel temp is > the store temp + Tdiff + hysteresis, then that would make real logical sense, thats how mine is done + a much bigger switching hysteresis, there may be no E.tube degradation as the pump, in effect is running all the time as its might be wrongly switched on and the solar panel is, at times actually re cooling the water in the cylinder, maybe worth switching the wiring from L2 to L1,

 

[Chris_909]

If L1 was the switched pump contact like its being showed for models 501&501D, switching when the solar panel temp is > the store temp + Tdiff + hysteresis, then that would make real logical sense, thats how mine is done + a much bigger switching hysteresis, there may be no E.tube degradation as the pump, in effect is running all the time as its might be wrongly switched on and the solar panel is, at times actually re cooling the water in the cylinder, maybe worth switching the wiring from L2 to L1

I dunno as there is a “test” button on the controller (12 on instructions) when I switch that, pump comes on like it says it should?

 

[Johntheo5]

You mean the Dif Test?, even so, simply put, you want the pump to start and only run when the sun comes out so why should the default pump mode be apparently, running, the sun must go down and the pump contactor energised to switch off.

 

[Johntheo5]

Also just wondering if you have that (a) motorised valve in the solar circuit to prevent thermo sypon circulation and cylinder cooling at night, the normal method is to have a check valve in the circuit.

 

[Chris_909]

I’ll give it a try and report back, and nope no motorised valves in the system

 

[Johntheo5]

Here are are a few screenshots from my system today, the first was taken at 0743hrs after a shower, the store temperature (44C) probe is installed just above the solar coil which heats between 50L and 140L depending on whether the oil fired coil which heats 90L is in service or not, it hasn't been. There would have been some small HW usage up to 1257hrs where the solar coil has now heated all the water up to 52C and is now beginning to heat the whole cylinder, there was little or no usage until 65C was reached at 1532hrs. The two other temperatures are the solar panel and the return from the cylinder coil which are used to calculate the solar output which was ~ 0.97kWh/hr between 1257hrs and 1357hrs and ~ 0.72kWh/hr between 1357hrs and 1532hrs, it was getting slightly cloudy during that hour and 35minutes.

 

[Johntheo5]

Did a few tests today.

Total cylinder contents heated and between 57C & 60C during tests.
First test: Normal setting with pump on at store temp+7C, pump off at store temp + 3C.
Total cycle time of 8mins 20secs, On time 3mins 40secs, Off time 4mins 40secs, Run time 44.0%.
Second test: Settings with pump on at store temp+4C, pump off at store temp + 3C.
Total cycle time of 5mins 42secs, On time 1mins 50secs, Off time 3mins 52secs, Run time 32.1%.

I then ran the pump continuously and after about 10 minutes the panel temperature fell below the cylinder temperature and started to slowly cool down the cylinder as expected, eventually I would expect the cylinder temp to fall to some equilibrium temperature, I might do this test tomorrow but I think its quite obvious that the pump just can't run continuously to get the best performance from the system, it can only run continuously if solar panel/store temp dT control is installed to maintain the choosen dT by means of pump speed control, otherwise the pump must be stopped at or before the dT falls to zero deg C.

 

[Chris_909]

Tried swapping connections but the pump was on at night and the dif test didn’t work so I’ve definitely got the wiring correct,
Im sort of coming to the conclusion that maybe where the tubes are old there just not very efficient maybe, or the controllers just bad. do you get yours serviced or anything like that?

 

[Johntheo5]

No, Chris, the system did develop a slight leak about 3 years ago (after ~ 11 years) so I renewed the solar fluid after repairing the leak but the old fluid looked perfect, probably because the flat plates never went higher than ~ 80/85C.

Is/your pump stopping/starting at any times during the day??, and when its running can you see the collector outlet temperature and the store temperatures?, mine does run continuously for 1.5 to 2.5hrs on very sunny days, (like the past few), and depending on the cylinder temperature but does cycle on/off quite a lot in the morning and evenings.

You could just switch off the pump say now while very sunny and just let the collector temperature rise to say 75C max before switching the pump back in and note how long it takes to rise from whatever its temperature is when the pump was switched off, it shouldn't take longer than 10 mins max IMO.

 

[Chris_909]

I’ll monitor it closely tomorrow but I’m sure the pump constantly runs, the display only shows one temp which I believe is the AUX sensor (on instructions) which I think is the top of the hot water tank , I cannot see the collector temp other than manually read with multimeter, I’ve had it on just over 2hrs and it’s gained about 20° (started on about 15° it’s at 35° ish )
Cheers I’ll try that tomorrow to

 

[Johntheo5]

What is the cylinder capacity and is the solar coil completely below the boiler coil, how many E.tubes?

 

[Chris_909]

144 litres? It’s written on the tank in bad handwriting lol, yes I think
AUX is attached to the top of the tank near the outflow, where it is normally hottest. It does not control the pump, but is usually the one whose temperature is shown in the display;

LOW is attached to the middle of the tank, near the top of the solar coil (which will be below the boiler coil).

HIGH is near the outflow of the collector on the roof -
I have 10 tubes(there rather dirty at the moment which probably isn’t helping things

I think you are right though, the pump being on constantly is not allowing the tubes to heat enough and is in effect cooling the system past a certain point, ill try switching it on for and hour, off for an hour and repeating on a good day and see if the temp climbs then I guess I’ll now if it’s the system or the controller not doing its job