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Solar Water Heating - Reverse Flow & Heat Traps


RyanF
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Really enjoy reading all the feedback on this forum, very educational and nice that it is specific to SA conditions.

I have many years of construction experience on larger commercial projects. Doing some smaller jobs at the moment and installed a few solar water heating systems, all purchased from TheSunPays.co.za. Their standard kit has no controller, only a 'dumb' 12v solar powered circulation pump. Of course it makes sense to at least install a timer relay on the geyser element to ensure you get a reasonable amount of solar energy harvested during the day. A timer is uncomplicated, much cheaper than a differential temperature controller and can be serviced by your average electrician. So overall this seems like a good approach for your average customer who has no interest or time to maintain a controller.

On the most recent installation (for a family member), I built an arduino controller to record temp data, and it also has two relays for geyser element and solar pump control. Got the first few weeks of data now and seen a couple of things where I need to make adjustments for a more efficient system.

So after all that blah, blah history, my questions:

  1. The guys at TheSunPays provide both a typical SANS installation diagram, plus their own recommendation which is reverse flow i.e. water from hot side of the geyser goes to the evacuated tubes, and the return water goes in the cold side of the geyser. I spoke to the one guy and they swear from experience that the reverse flow is better.  See their diagram attached below. Anybody that has this 'reverse' installation and some longer term experience with it? Good, bad, ugly?
  2. Their diagram also wisely shows the use of heat traps to prevent reverse thermosyphon. Seems a heat trap is a better long-term maintenance option as opposed to non-return valves. Does anyone have experience on how deep the heat traps on both sides need to be to be effective?

TheSunPays Reverse Flow.JPG

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  • 2 weeks later...

Thank you @phil.g00 

Further to the questions above, I am trying a few modifications on this system and recording the data, which I will post for those who are interested. So first graph below is the following:

  1. 24 Evacuated tubes working with a 150L geyser.
  2. 12v DC Solar pump with head of about 2m.
  3. Reverse flow as recommended by supplier - Water from geyser outlet (hot side) to solar panel, return comes in cold side.
  4. Heat traps of 150mm deep, on both supply and return lines.
  5. No non-return valves.
  6. No electrical heating during this period, with hot water being used by occupants in the late afternoon.
  7. May 2, 3 & 4 was the first cold snap in Joburg, sunny days, but much colder incoming water, hence the larger differentials.
  8. Data has been scrubbed - some days the system was being worked on and data was removed.

1807381641_20210504GeyserData.thumb.jpg.ba55d5790b57855c74844fa0b7caccb0.jpg

Here is what I could pick out the data:

  • Average over the 7 days of heating gain during the day in the geyser was 22.7°C.
  • Average for heat loss in the geyser overnight was 18.4°C.
  • From the above it was clear the heat traps were not working.
  • Noticed too that the geyser would only rise about 25°C each day, even if the panel was considerably hotter.
  • So it seemed that, despite the pump constantly working during daylight hours, the head was too much, resulting in a low flow rate.
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Thank you for the data.

I am not sure about this conclusion.

9 hours ago, RyanF said:

From the above it was clear the heat traps were not working.

In the light of this:

 

9 hours ago, RyanF said:

No electrical heating during this period, with hot water being used by occupants in the late afternoon.

but this does seem to be an issue:

 

9 hours ago, RyanF said:

So it seemed that, despite the pump constantly working during daylight hours, the head was too much, resulting in a low flow rate.

Please can you justify your conclusions.

Edited by phil.g00
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  • 1 month later...

I have a non return valve in my system and I am having problems, the hot water is reverse thermosyphoning, I say this because evening, and late evening I notice the manifold temp sensor is always about 1 or 2 degrees less than the geyser, even early mornings, and the geyser temp is no longer "constant" it drops rather quickly.

I pick this up because I observed a drastic rise in my electricity usage.

I keep daily readings, so I quickly picked this up.

I have a loop in the outlet, but what I see in this drawing is that my inlet into the geyser, cold water, is also the feed to the manifold collector, the total opposite of what is proposed here.

The hot water from the manifold feeds into the hotwater outlet of the geyser.

Now I am uncertain if I should get a plumber to redo all the piping...

Edited by Antony
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On 2021/05/20 at 7:00 AM, phil.g00 said:

Please can you justify your conclusions.

Sorry for the long delay in replying @phil.g00. I have data from a few more iterations which I will post as I have time.

Regarding the main conclusion of the 150mm heat traps not working, that seems evident from the panel and geyser declining in temperature at exactly the same rate during the night hours. The two have almost the exact same temperature and downward trend when the ambient temp it is warmer - April 24 to 27. But this becomes even more apparent when the ambient temperature dropped on May 2 to 4. For those nights the differential between the panel and geyser increases to more than 10°C (with the panel being warmer!!!) but the downward trend is exactly the same. But with the correct setup (minimal to no thermo-syphoning), once night comes, the panel temp should drop below the geyser temp fairly quickly - the time for that temp drop depending on the ambient temp. But as you can see from the graph this is not the case - the energy from the geyser is bleeding to the panel - keeping it nice and cosy on the roof! 😝

Regarding the secondary conclusion about the pump head reducing the flow rate and thereby constricting the energy harvesting, that also is most evident once the ambient temperature fell on May 2 to 4. During the day when both the panel and geyser are trending up, you would expect to see a much closer correlation between the panel temp and geyser, not a differential of >25°C . The pump data sheets shows a flow rate of 3L/min with a 2m head - that therefore seems woefully insufficient flow to harvest the available energy. Obviously the panel heats up and cools down much quicker than the geyser, given that it is a much smaller amount of water. However the faster the flow rate the more energy is transferred from the hotter manifold to the colder water. Since the geyser is much better insulated and a larger volume, you obviously want to harvest the energy as soon as possible to get it into the geyser where it should cool slower.

Hope that makes sense? But data can be interpreted different ways, so looking forward to any other insights.

The next sets of data also seem to debunk another couple of myths floating around, so more coming soon...

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On 2021/06/23 at 11:20 AM, Antony said:

I have a non return valve in my system

@AntonySo after trying two depths of heat trap loops (150mm first, then 600mm on both inlet and outlet), they really don't seem to work much (if at all). After the heat traps, the next iteration was to install a flap type NRV (horizontally positioned so the flap would close under reverse flow conditions) on the pipe which flows from the panel to the hot side of the geyser. I expected that ALL of the thermo-syphoning was going out the top of the geyser and the flap-type NRV would sort the problem. To my surprise the thermo-syphoning continued and when I had a close look at the data it was clear that it was going out of the cold side of the geyser up to the panel during the night (by this stage I had installed a temperature probe on both the inlet and outlet of the geyser). Some posts suggested a spring type NRV on the cold side, but I was not convinced because during the day the water has to flow in the same direction as the thermo-syphoning would go at night. But to my surprise it stopped the thermo-syphoning - I guess the spring is just strong enough to stop the little pressure 'bursts' which characterize thermo-syphoning - but not strong enough to prevent the pump flow during the day. For the fist time the panel temperature dropped well below the geyser temp for the whole night!

So my experience is that you need two NRVs - one on each line, but the cold side one has to be the spring type. Probably worth trying before re-piping 😰. Just be careful to get the NRV directions correct so you don't end up with a closed loop 🥵

FYI - Below is a graph of the data when the thermo-syphoning was happening out of the 'cold' side of the geyser. The blue line is panel temp, the grey line is geyser hot side temp and the orange line is geyser cold side temp. The more regular 'up-and-down' of the orange line from around 23:00 is the small bursts of pressure that thermo-syphoning produces as it forces the hot water along and it is replaced by cold water. Overall you can see that the panel is hotter than the geyser during the night and they all have the same downward trend indicating the heat is being bled out of the panel.

image.png.71ef8ca33c0098c32fc7ff4a7a15ec4d.png

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12 hours ago, RyanF said:

FYI - Below is a graph of the data when the thermo-syphoning was happening out of the 'cold' side of the geyser. The blue line is panel temp, the grey line is geyser hot side temp and the orange line is geyser cold side temp. The more regular 'up-and-down' of the orange line from around 23:00 is the small bursts of pressure that thermo-syphoning produces as it forces the hot water along and it is replaced by cold water. Overall you can see that the panel is hotter than the geyser during the night and they all have the same downward trend indicating the heat is being bled out of the panel.

 

Why is your panel temp higher than the hot water temp at night?

Edited by Pietpower
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12 hours ago, RyanF said:

FYI - Below is a graph of the data when the thermo-syphoning was happening out of the 'cold' side of the geyser. The blue line is panel temp, the grey line is geyser hot side temp and the orange line is geyser cold side temp. The more regular 'up-and-down' of the orange line from around 23:00 is the small bursts of pressure that thermo-syphoning produces as it forces the hot water along and it is replaced by cold water. Overall you can see that the panel is hotter than the geyser during the night and they all have the same downward trend indicating the heat is being bled out of the panel.

"out of the cold side"
I think your water flow is happening out of the hot side. and in the cold side.  Your geyser is at a constant temp and any water flow out the cold side would be at a constant temp over the thermometer. If it flows out the hot side and in the cold side then the water in the pipes can have time to cool down and cause the night time dips in cold temp measurements.

The dips in the panel temp at night time?
What causes it? It seems to follow your cold water temp although 20C higher. Is it maybe when water is used for showering and cold water flows in parallel through the panel?  Then the temp line is 20C too high.

When your pump is running in the day the hot water outlet temp and the panel temp should be similar.  Here is also a 20C difference which might be a temp measurement problem.  Do you measure the temp at the evacuated tubes on the inlet or outlet?

To lose heat at night your geyser must be warmer than the panel. If the panel is warmer than the geyser then you would be gaining heat in the geyser.  If that line was 20C lower then it would make sense.

If you have no flow at night then the panel temp should drop quick to ambient or even just below ambient and stay low. (Depending on panel design and temp measuring point)

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2 hours ago, Pietpower said:

Why is your panel temp higher than the hot water temp at night?

Because for that data set/graph, I didn't have the spring type NRV in yet on the cold side - flow direction from geyser to panel. Once I installed that NRV, this graph looked much better - see below. As you can see, panel temp dropped quickly between 17:00 & 21:00 to below the geyser temp - and I was a much happier man.

image.png.bae9d260d9e154a94e0f06baf1f03c65.png

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13 minutes ago, RyanF said:

Because for that data set/graph, I didn't have the spring type NRV in yet on the cold side - flow direction from geyser to panel. Once I installed that NRV, this graph looked much better - see below. As you can see, panel temp dropped quickly between 17:00 & 21:00 to below the geyser temp - and I was a much happier man.

What I am trying to say is that it should not be possible to have a panel temp higher than your geyser temp at night. What is giving energy to your panel to have a higher temp?  If your geyser is giving energy then it can only go as high as geyser temp. But why higher than geyser temp? Am I understanding something wrong?

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I think it is possible for the panel to be hotter - because it is higher than the geyser. Simple convection law is that the hottest fluid rises to the top and the coolest to the bottom. The pipes in the panel only hold a few litres of water, so if there is no NRV to stop the water flowing up there at night, the hottest water rises there. That is what the data seems to prove - temperature probes at 3 different levels:

  1. Highest probe is in the panel manifold which is approximately 2.5m above the bottom of the geyser - hottest temperature.
  2. Middle probe is at the top of the geyser, hot water outlet. About 2m below the panel manifold - middle temperature, but closer to the panel temperature as the bulk of the hot water is in this 'zone'.
  3. Bottom probe at the cold inlet of the geyser - coolest temperature.

So with incorrect or no NRVs, the panel and the geyser are effectively one system, joined by the piping - hottest water goes to the top, coldest sinks to the bottom.

Edited by RyanF
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  • 1 month later...
On 2021/06/25 at 5:53 PM, RyanF said:

I think it is possible for the panel to be hotter - because it is higher than the geyser. Simple convection law is that the hottest fluid rises to the top and the coolest to the bottom. The pipes in the panel only hold a few litres of water, so if there is no NRV to stop the water flowing up there at night, the hottest water rises there. That is what the data seems to prove 

Ryan. Think about just this part for a bit.  At night time you don't have any energy sources at the panel and you can't gain energy.  If your geyser is at say 50C and you pump the water to the panel then it arrives at 50C.  It can then not get warmer than 50C.  The panel is cold and will lose energy to the environment and your panel temp will be lower that geyser temp.

If you are not pumping at night and it is only unwanted flow the temp in the panel should be much lower than geyser temp as the water loses energy all the way to the panel and in the panel.

Thus if you blocked the flow completely the panel temperature should settle at the outdoor ambient temperature.  Best case scenario.

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On 2021/08/06 at 12:05 PM, Pietpower said:

Thus if you blocked the flow completely the panel temperature should settle at the outdoor ambient temperature. 

@PietpowerAgree on this point - and that is what the graph in the June 25 post shows very clearly. Once convection flow was stopped with two non-return valves on supply and return legs from the geyser, the panel temp drops below the geyser temp early in the evening. The panel temp starts going down at 5pm and was cooler than the geyser by 8pm. I guess the panel doesn't reach ambient temperature immediately on this installation because the pipe and manifold are very well insulated.

Edited by RyanF
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  • 9 months later...

Hi Ryan. Sorry for replying on an old post. I’m busy with the same installation and I’m still not convinced about either of the water flow directions. I also bought the Sun Pays unit but decided to get a Geyserwise controller. Geyserwise suggests feeding from cold water side and Sun Pays suggests from hot water side with no controller. I notice in your diagram you show the pump on the return side from panel which is contrary to what Sun Pays suggest. Is this not part of your problem? 
geyserwise switches the pump on cold side only if there is a temp differential of 7deg. What does my head in is if this cold water will be extracted from the geyser or the main supply which is under higher pressure. Any thoughts on this? 

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1 hour ago, Derekvdm said:

What does my head in is if this cold water will be extracted from the geyser or the main supply which is under higher pressure. Any thoughts on this? 

As long as you are not opening a tap, the entire system is under pressure and more water cannot flow into the system, so the water cannot come from the supply. Care must be taken to restrict flow enough that when you open a tap, the flow does not bypass the geyser and flow through the collector only. This can be done by adding a valve to the return path and only opening it partially. In the graphs above, you can see that the flow is not fast enough; the collector is always hotter than the geyser, which means that you lose a lot of efficiency (a lot might be an understatement). So it can take some time to find the correct position for the valve.

Now if you choose to take water from the top of the geyser and feed it back to the cold side - you do two things:

Firstly, you break the "geyser" part of your hot water system. It is designed so that the hot water and cold water stay separated for a reasonable amount of time. Which is why when you take a shower the temperature stays more or less stable (until you have used all the water and then it's suddenly cold) and does not gradually decrease like the average temperature of the geyser. Basically, you take hot water at the hot end, make it even warmer and add it to the cold side.

The second thing is that you decrease the system efficiency - if you take the cold water from the intake side, and heat it until it is hotter than the middle of the geyser(where the probe should be), the average temperature of the water in the loop to the collector is lower. Lower average temperature means lower losses.

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Hi Derek. Some thoughts:

On 2022/06/11 at 7:55 AM, Derekvdm said:

Geyserwise suggests feeding from cold water side and Sun Pays suggests from hot water side with no controller.

I know the SunPays swears by their suggestion, but my testing showed that heat traps do not work, you have to have NRV's on both lines. Keep in mind this is a pumped (not thermo-syphon) system. If you feed from the hot side, at night the temp differential between top of geyser and solar panel is greater and therefore reverse thermo-syphoning is more likely to overcome the spring type NRV.

On 2022/06/11 at 7:55 AM, Derekvdm said:

I notice in your diagram you show the pump on the return side from panel which is contrary to what Sun Pays suggest. Is this not part of your problem?

I don't have any problems with my system now, as shown in the diagram - running for over a year and no thermo-syphoning up to now. I do agree though that having the pump on the 'cold' side is better and I plan to move my pump to that side. The main reason for this is that if your panel overheats due to a circulation failure, the steam is forced thru the pump. And a friend had his pump burn out in this situation. So I plan to move my pump to the cold side and put another flap-type NRV between it and the panel to further protect it.

 

On 2022/06/11 at 7:55 AM, Derekvdm said:

What does my head in is if this cold water will be extracted from the geyser or the main supply which is under higher pressure.

As @P1000 says, the system is closed and under equal pressure throughout, until a tap is opened. So when the circulation pump starts, it pulls from whatever side of the geyser you have chosen (cold in my case) and pushes to the other side.

Regarding control of the system, that is where I like the SunPays approach. No controls = no electronics = less things that can go wrong! It is true that a controller might introduce some small efficiencies, for example allowing the geyser to settle for the hot and cold water to separate as P1000 describes. But from observation of my solar panel temps in a Joburg environment with ample sunshine, the pump would switch off for 2 or 3 minutes at most and then go straight back on. The last couple of weeks I have been experimenting with only starting the circulation pump in the morning when the panel temp gets to 40 and switching off just before sunset. I have noted very little  overall improvement in efficiency.

BTW I have a Wemos D1 running Tasmota which controls a contactor to switch the element on if the geyser temperature requires it. This same little controller reads the temperatures and lets me keep record - screenshot below of what I see on my network for this controller. This is fun for me because I like tinkering with electronics and data. But if I do an installation commercially like this, unless the Client wants the wifi control, I would just use a standard DIN rail mount timer with a 2-pole contactor to switch the geyser element on at the time(s) the client wants. This way components are simple and easily replaced - doesn't require an electronics technician or programmer.

image.png.46cbf503b3e07b8a43d54bcac84722cf.png

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3 hours ago, RyanF said:

Hi Derek. Some thoughts:

I know the SunPays swears by their suggestion, but my testing showed that heat traps do not work, you have to have NRV's on both lines. Keep in mind this is a pumped (not thermo-syphon) system. If you feed from the hot side, at night the temp differential between top of geyser and solar panel is greater and therefore reverse thermo-syphoning is more likely to overcome the spring type NRV.

I don't have any problems with my system now, as shown in the diagram - running for over a year and no thermo-syphoning up to now. I do agree though that having the pump on the 'cold' side is better and I plan to move my pump to that side. The main reason for this is that if your panel overheats due to a circulation failure, the steam is forced thru the pump. And a friend had his pump burn out in this situation. So I plan to move my pump to the cold side and put another flap-type NRV between it and the panel to further protect it.

 

As @P1000 says, the system is closed and under equal pressure throughout, until a tap is opened. So when the circulation pump starts, it pulls from whatever side of the geyser you have chosen (cold in my case) and pushes to the other side.

Regarding control of the system, that is where I like the SunPays approach. No controls = no electronics = less things that can go wrong! It is true that a controller might introduce some small efficiencies, for example allowing the geyser to settle for the hot and cold water to separate as P1000 describes. But from observation of my solar panel temps in a Joburg environment with ample sunshine, the pump would switch off for 2 or 3 minutes at most and then go straight back on. The last couple of weeks I have been experimenting with only starting the circulation pump in the morning when the panel temp gets to 40 and switching off just before sunset. I have noted very little  overall improvement in efficiency.

BTW I have a Wemos D1 running Tasmota which controls a contactor to switch the element on if the geyser temperature requires it. This same little controller reads the temperatures and lets me keep record - screenshot below of what I see on my network for this controller. This is fun for me because I like tinkering with electronics and data. But if I do an installation commercially like this, unless the Client wants the wifi control, I would just use a standard DIN rail mount timer with a 2-pole contactor to switch the geyser element on at the time(s) the client wants. This way components are simple and easily replaced - doesn't require an electronics technician or programmer.

image.png.46cbf503b3e07b8a43d54bcac84722cf.png

Hi @RyanF interesting , I see you have three temperature probes, if I may , where have you fitted them ? 

I have incidentally connected my own ds18b20 to the element flange last week and I'm getting surprisingly good result .  

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Hi BritishRacingGreen,

1 hour ago, BritishRacingGreen said:

where have you fitted them

I have them (also DS18B20) straight on the copper pipes - at the geyser inlet, at the geyser outlet and one on the solar panel inlet. Previously I had the solar thermometer in the pocket provided, but found out after some time that it was over-reading significantly. I guess it must have been in contact with metal inside the pocket that was heated by the sun, but wasn't influenced by the water circulation. Now it is straight on the copper pipe and readings are more beleivable.

I just tape them on with electrical tape and cover with elastomeric insulation.

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