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Solar hot water problems - would love some advice!

 
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Hi Everyone,

I have a few problems with a relatively new solar hot water system I'd love your help and advice on. My old twin-element electric heater died 3 days before Christmas 2018. At that time all the plumber supplies have closed for 2-3 weeks Christmas break (that's pretty typical Down Under - it's summer school holidays so lots of business take some downtime). I was able to source a new solar hot water system from eBay from a local reseller that used 30 x evacuated tubes & a 315 litre stainless steel tank which also came with an electrical booster element (connected to off-peak overnight circuit not mains). I picked it up Christmas Eve and installed it a few days later using a professional plumber for the parts that needed a qualified tradesman (to meet code) - I did the rest myself. Here's a simple diagram of the design logic:



In summer, it works great. The incoming cold water is 25°C, and the entire 315 litre tank reaches 80°C (maximum) so a 55°C temperature increase. We can then have hot showers for >100 minutes. For a family of 4, even allowing for washing up, this is easily 2 days capacity, so if it's cloudy or raining for 2 days, no boosting is needed. Only if 3 days consecutively are cloudy do we need to boost.

In winter, it's woeful, the incoming water is 8°C and the system struggles to raise the temp above 28°C and often less (so a 20°C temperature increase). This means cold-ish showers, and if it's cloudy or raining, very cold showers! So I rigged up a solar reflector to double the amount of sunlight on the evacuated tube heat collector between 10am & 2pm and it was better (around 30°C temperature increase) on good days (but still not quite hot enough) and on cloudy days, terrible performance. So, in winter, we boost all the time. The electric booster element is connected to an off-peak circuit which kicks in between midnight & 6am. It only costs ~15c / kwh for off-peak compared to ~55c / kwh for peak. The off-peak boost though only heats slightly less than half the tank - say 150 litres, and only to 70°C, the limit of the thermostat on the electrical element, a separate thermostat to the solar hot water pump and sensors which can heat the tank to 80°C. This means we end up with 150 litres @ 70°C which is 41.6% of the maximum capacity. However this does not provide 42 minutes of hot showers, because, the cold water is 8°C not 25°C, so you need more hot water in the shower water mix.* We get about 23 minutes of hot water which for 4 people trying to shower and shave and wash up (especially if the wife wants to shampoo her beautiful long hair) just doesn't cut it.

This bad situation (of not enough hot water) is about 7 months (April - October) of the year whilst the other 5 months (November - March) no problem.

So now I'm in a bad situation because in hindsight I should have bought a 500 litre tank with 50 evacuated tubes (but took the advice of the seller, who assured me it would be fine lol), but I have what I have, and I'm trying to make the best of it. That seller has disappeared as well so no advice or help from him or recourse to upgrade or exchange etc.

So, here are my questions based on that information:

1) What kind of performance should I be getting from my system? Is what I've described typical of a 30 evac tube / 315 litre tank system? Or is this possibly some problem I can fix? (I've checked all the tubes, one was broken from a hailstone, replaced it with a spare, all others seem fine)
2) What do you think I should do to improve my hot water production?
a) Add another electric tank say 160 litres connected to off-peak.
b) Add another solar panel / evacuated tube array in series with the existing eg expand to 50x evac tubes
c) Add a second pump to the existing tank utilising the "auxiliary" outlet at the top of the tank that cycles water from the top to the bottom. Run this pump during off-peak (connect it to off-peak) so the entire tank gets boosted to 60°C, provides enough hot water for a day's showers, and allows solar heating if possible during the day.

Thanks all in advance for your thoughts. Stay safe!

* In summer we use 3 litres of 80°C hot water + 7 litres of 25°C cold water per minute to get a 41.5°C shower (total of 10 litres / minute; I'm keeping the math simple and approximate). In winter we use 5.5 litres of 70°C hot water + 4.5 litres of 8°C cold water per minute to get a 42°C shower.
 
rocket scientist
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Hi David;  Big Welcome to Permies!
Solar power is a subject I could help you with, however solar hot water is not something I have ever needed. I use on demand instant propane hot water.

My thought's about your problem are, if you live in a rural area, or just not in town.  
Can you burn wood?  Do you know about Rocket mass heaters?
There is a safe way to provide hot water for your family during the winter months.
Here is a link to a video by Geoff Lawton  


Here is another link to Matt Walker's Site. http://walkerstoves.com/  
Matt has just designed and built a smaller rmh hot water heater.  He sells plans very reasonably for any of his designs.
 
David Soede
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Hi Thomas, thanks - great site. Been lurking for a while.

I live on 2 hectares but have to buy firewood (not enough to harvest from my own property) - council & government restrictions prevent much local harvesting for free. Any tree felling has to be approved and the council wants to keep as many trees as possible for the environment. Firewood is expensive, often shipped interstate. We pay about AU$300 / 2 cubic metres (half a cord) non-seasoned loose-loaded ie tossed in the back of a box trailer. I do have a slow-combustion fireplace which works well heating the house, but it's design isn't good for adding water heating as it's a "twin box" with forced airflow from a fan between inner and outer layers. I have thought about modifying the flu but it's against code Down Under, have to be careful from house insurance and government fines viewpoints. I am aware of rocket mass heaters and perhaps next house . . .

Thanks for your reply and take care - David.
 
pollinator
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Two things right off the bat that stand out to me (many will disagree with me on these....I won't rebut those arguments while here at Wheaton Labs due to time contraints).

I design my solar hot water systems such that my hot water tank is above my solar collector and in the hottest part of the shelter: delta T is reduced by a minimum of 20F and you can gravity feed your hot water.
I also use two methods of solar gain: the evacuated solar tubes and an insulated box containing the water heater which has a south double pane solar window: giving the hot water tank an insulated box in addition to the tank's own insulation, lowers the delta T for the heat loss of the tank by 10F or more AND you get extra heat during the day (a night time cover on the solar gain double pane drops black body radiation at night -there are many ways to do this without climbing on the roof).
Finally, the long inlet pipe feeding the solar water tank, located at the hottest part of the shelter, is ran through the heated space of the shelter with a one way check valve at the base: This is only good for warming a small column of input water during periods when you haven't used hot water for an hour and a half...works better if you run the input feed through your shelter's heater as long as the inlet pipe doesn't get above 200F -make sure the one way check valve is before it enters your shelter's heater. This seems trivial but I use every temperature bump I can get when heating water and reduce delta T where ever I can.


Yes, water is 8.33 lbs/gallon so your attic would need some serious framing/metal struts; however, your ambient temperature for your hot water tank (insulated or not) will go up by a minimum of 20F degrees immediately. Even if you have the best tank insulation, also dropping your ambient delta T for your tank by 20F is significant.


The cost of re-doing your plumbing may seem like you will never get a return on your investment....but if your water is always hot without electricity and you have no income due to, say a virus outbreak, then the person without hot water during winter can jabber all they want about your construction cost not being cost effective. You can charge them a cost for a hot shower and all their family members too.

good luck!
 
David Soede
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Thanks Orin,

The drop in tank temperature isn't a concern. We have very mild winter temps and very warm summer (air) temps. Never frosts, never snows, typically 20°C / 68°F day temp in winter, minimum overnight can be 6°C. We had a blackout once during a cold winters day, the tank had heated to 70°C (top half) and the bottom half was 10°C, we were out all day & didn't use any hot water, tank temps were the same at night when we got back. (The blackout meant solar hot water heating couldn't happen - no pump - still running on mains electricity sorry).

So for me, it's all about hot water production. Where I am Down Under hot water tanks are stored outside homes and aren't even insulated with a blanket. I think I've read that tank temps don't even drop half a degree so no-one bothers. I totally get your situation though and would do the same if in such a cold climate.

The system was designed to be split, with tank at ground level and evac tubes on roof.

I'm really trying to figure out if my system performance is expected or significantly sub-substandard, and how to improve hot water production at best cost and least environmental impact.

Regards,

David
 
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couple of thoughts and questions.  how do you pump the water to the tank.  Is the outlet of the tubes slightly higher than the inlet of the 30 tube array.  Have you considered that the 30 tube is just not enough to actually supply?  Do you normally take your shower morning or evening.  Do you have a coil in the tank or an external heat exchanger.  kind of important to know so we actually have more idea about what is going on.  I did notice that your roof is at 20 degree slope so this is ideal for summer heat but if you look at the angle of sun in the winter it is Way to low to produce enough heat.  Where I am 70 degree so almost flat is good for summer and because the sun is low in the sky in winter I need to be much closer to 35 degree from perpendicular.   This in itself may need to be addressed and see where you are.  With evacuated tubes they tend to stay with light fairly perpendicular to the arc of the sun but not the azimuth
 
pollinator
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We built a 700 square foot shower building at my last foster home to deal with the 60 or so kids there for each summer camp.  Apparently parents had complained about their kids not showering for an entire week.

To heat the water we simply placed a 500 gallon black plastic tank on the roof and allowed the sun to heat it.  One would think ohhh yeahhh pretty simple and easy right.... wrong... lol...   The dang tank exploded twice as it was getting too hot turning the water to steam and had no pressure relief valve.

After we got the pressure relief system figured out wow... the only problem then was that the cold water was simply fed by the spring that we had and did not have a lot of pressure and it was hard to get enough cold water to offset the pressurized steaming hot water and it tended to burn people like me who are more sensitive to hot water.

Rather than use a "heater" system maybe try a more direct approach like we did.

For the main house we also added in a system of water tanks around the chimney upstairs on the second floor to preheat water before the water went on into the hot water heaters, worked quite well.
 
pollinator
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I don’t have any answers, but I thank you for the great question! I am considering a solar water heater system myself, and you details will help others like me to translate it to our contexts. Best of luck and thanks for the answers all.
 
steve pailet
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best thing to do before building a solar hot water system is do enough reading and watch enough video that you can actually talk to others.  Doing hot water using the sun is not hard but if you do not understand that you can only get x number of btu of heat out from what you put in you are not going to go very far.
 
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I am also in a mild-winter location, and there just are days in the winter where there won't be hot water.  On days where the water can be tepid, a small supplemental heating source helps, like a tankless electric water heater that brings up the tepid water to whatever temp you want.  While electric is not ideal, it's fast, and you're not heating from scratch if you choose a day where the water is at least somewhat warmed.

Insulation is crucial, really serious insulation on all the pipes and tank (which can be wrapped in an insulation blanket made for tanks), the pipes are out of the wind, out of the shade for most of the sunny hours 10:00 AM to 2:00 PM in winter, helps to keep heat loss on the way to the tank.  Are they going the shortest distance from the roof heater to the tank?  Your drawing may just be a diagram, while the actual pipes don't stick out from the building?

There are also propane camping heaters for supplementing, like a Camp Chef Triton heater.  It has to have the 60 lbs of water pressure to function correctly, which would require a pump if the water doesn't have that high a pressure.   If there's only a few months where you're using propane, even if it's from the small BBQ sized tanks, it's not like having to use it for a full year.
 
Cristo Balete
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You could set up a separate 2-gallon/8 liter shower for the winter, inside the shower or bathtub you now use, by putting an insulated picnic cooler up on a shelf overhead on the wall of the bathtub or shower, and attach a spigot from a camping solar shower bag into it, seal it with aquarium sealer, fill it from the top with hot water from whatever source, like hot water in a large pot heated inside a propane BBQ.  

Each person gets a decent shower for about 5 minutes.  It's a lot easier than trying to heat all of that water in a big water tank, and keep it warm.
 
steve pailet
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did not see whether you are running electric in the house from solar PV.  I would say point of use on a 110 outlet to get the preheated water up to 120 degrees.  I like propane fired with an outdoor unit again point of use to also get the water up to usable temp.  As was mentioned a bbq size bottle would be more than enough.  One other comment.  suggest you increase the amount of insulation in and out of your evac tube system.  Even a little bit exposed can drop its output to your storage tank by as much as 50%
 
David Soede
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Thanks all for your replies. Still seeking answers to the questions I raised, so if anyone has knowledge they can pass on, I'd be grateful. Let me clear up a couple of points:

1) The poor performance is not coming from any loss of heat from the system:
a) Hot water pipes are insulated.
b) Climate is mild (never get a frost let alone snow; 20°C / 68°F is a typical winter's day, overnight 5°C is about the minimum)
c) The initial diagram I did was for the logic flow of the system design, not actual literal / physical path of water pipes. I've modified it to show a more accurate representation.
d) We had a blackout one day when we were out all day (so no hot water use, no pumping possible to allow for solar heating) and the water temps were unchanged from the overnight boost (they did not drop even a tenth of a degree - the tank has multiple sensors and display is in 0.1°C increments). This means there is no measurable heat loss from the system.

2) The issue is lack of hot water production, both from the evac tubes in winter, and from the off-peak electrical boosting element only being able to heat the top 42% of the tank (and only to 70°C not 80°C as the solar can).

3) My questions are:
a) Is the performance of the system in winter typical for a 30x evac tube system, or significantly sub-optimal?
b) What can I do to increase hot water production? As per my original post, I am thinking of purchasing a new (160 litre) electrically-heated tank (connected to off-peak), adding more tubes, or adding another pump to circulate water during the off-peak heating cycle.

Thanks to all who can help. Dave.

 
steve pailet
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I have read what you said and makes me wonder if you should check the thermostat and the upper element.. either could keep you from getting adequate heating. Just something to check along with perhaps putting an amp meter on them to see what the actual amp draw is
 
David Soede
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Hi Steve,
The amp draw is 15A, which Down Under @ 240V = 3.6kw, and that's what it draws - easily measured as the hot water system is the only off-peak load, and there's an off-peak meter in the EDB.
It does heat the top 42% to the thermostat temp ie if thermostat on the electrical element is set to 70°C then the top 42% gets heated to 70°C which is verified by the (separate) temp sensor the solar hot water pump & controller use.
I mean I could switch the electrical element heating from off-peak, to peak, and we wouldn't run out of hot water, but it would cost 4x as much and is worse for the environment because the peak electricity usage (evening) is from baseload which Down Under is overwhelmingly coal-fired power station supplied.
Thanks, Dave.
 
steve pailet
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OK next silly question.. sensor thermister  this might sound odd but if the pump does not shut down you may be pumping some of your heat out at night..  just going thru various permutations that might find the problem.  You did not mention if you have an internal or external heat exchanger for the vacuum tubes into the storage tank
 
gardener
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My first thoughts - as I try to remember some details about these contraptions...

1) What sort of controller do you have on it?  I'm wondering if its running the pump when the delta T is too low, or worse if its confused and is sending boosted water to the tubes in the middle of the night.

2) is this an open or closed system?  Your diagram looks like an open system  I'm wondering if the open system is creating too much of a thermal sink.  And is it possible that the water is either flowing the wrong direction through the tubes, or the tank input/output is reversed?  Some detail is niggling at the back of my mind about this but I don't remember the specifics.

3) Your wonderfully detailed diagram shows that your roof is well below an optimum angle, especially in the winter months.

4) Any chance some of the tubes are clogged and aren't flowing?  That would really reduce your collection area.

5) the booster seems really weak.  Yes, that's a large tank but given 6 hours of heating potential it should be doing better.  My calculations show that you should be able to heat the whole tank to 48C using just 14.7 kwh, which a 2500w element should just be able to do in 6 hours.   Given that the element is in the middle of the tank and you're only heating half of it, then it really should be doing better!  Is it properly wired? (not sure how you wire up a water heater there since you're already operating at 230v.  up in 110/120 v land its possible to accidentally send half the current to your water heater)

For reference, we had a 100 ft2 panel collector on a closed system going to an 80g (300l) tank.  In the summer it was too effective and it would regularly release boiling water/steam so we had to upgrade the controller to actually cool the tank at night.  In winter - even in sub freezing temps - that tank would get to 80F/26C.  We had an external booster heater (a natural gas fired Takagi) that was tremendous.

 
pollinator
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1) Angled for maximum winter energy capture in the winter vs summer or at least equal.
2) Buy more solar panels
3) Insulate to cut heat loss
4) Do you keep the hot water tank outside? If so keep it inside
5) Heat loss from the tank into the house during winter isn't really a lost, its just used to heat the house.
 
David Soede
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Hi Steve,
The evac tubes are connected to a heat exchanger on the roof. Basically the evac tubes have a heat pipe in them connected to a manifold through which water passes (pumped by the electric pump from the storage tank).
Pump only pumps if the water in the manifold is +5°C to the storage tank temp. It does not pump at night.
At 6 o'clock in the morning, after the midnight-6am off-peak circuit has run, the top 42% of the tank is 70°C. We verify this by the 2x tank temp sensors connected to the ECU for the solar hot water system. We are not relying on the electrical element thermostat and assuming it has heated and cutout, or relying on the electricity usage we can see in the EDB, we are relying on 2x tank temp sensor probes. So, we know at 6am the (top 42% or top 150 litres) is 70°C.
You and others seem focused on heat loss as the cause of poor performance. It's not heat loss per my previous comment about the blackout day, but does that also mean you and others think a 30x evac tube system should be producing a lot more hot water than it does?
Clearly in summer the system is terrific but in winter atrocious so I am thinking this reflects much lower solar energy received in winter, and therefore the system is working as intended, but just under-sized for my family & situation (in winter / year round).

Is there anyone who has a 30x evac tube system & family of four and also finds serious lack of hot water production? Or is it just me?
 
S Bengi
pollinator
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Lets say that your solar panels are angled such that it gets equal amount of incoming energy in both the summer and winter.
Lets say that you get equal amount of sunlight and cloud cover in both summer and winter
Lets say you lose the same amount of heat in both summer and winter, both inside the house and outside the house.
But maybe folks are washing dishes with more hot water, maybe they are cooking inside more, maybe they are taking more hot showers, maybe they are takjng longer soothing hot showers


 
David Soede
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@ S Bengi:

1) I could increase the angle the evac tubes are at. I have thought about an adjustable frame. The problem is it lifts / changes the height of the manifold (heat exchanger) which in turns requires an expensive and flexible plumbing solution. Currently the manifold is plumbed together with insulated copper pipe.
2) Yes, strongly considering this (or in my case, more evac tubes)
3) I keep repeating myself in subsequent posts. I am sorry I didn't clarify that in the original post. Pipes are insulated and there is no heat loss (cannot measure below 0.1°C which is accurate enough for this project).
4) Yes, outside, and as there is no heat loss, it can stay there.
5) N/A
 
David Soede
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Hi @ Eliot:

1) delta T is +5°C on roof (manifold / heat exchanger) cf storage tank. Is not running at night, see previous posts.
2) Open system: Cold water flows into the bottom of tank, water is circulated around solar manifold on roof and back to tank, raising storage tank temp, hot water leaves tank at top / high point. As hot water rises the hottest water always leaves tank. Whole thing is pressurised to mains pressure (is on mains).
3) Yes, I could increase angle of evac tubes to say 50° incline but this is very expensive as it involves new plumbing to the raised manifold height and a new frame. Would need to re-build solar reflector too  and make it higher, strengthen for wind load.
4) Tubes don't clog, they use a heat pipe to transfer heat to manifold. All evac tubes are working. The pipe from tank to manifold is not clogged, water is flowing freely, monitoring shows that at both ends.
5) I think the booster is designed to be peak / continuous, not whole tank heating (using off-peak). As the element heats the water, it rises. This circulation only happens in the top half of the tank - the part above the element. Water below the element is not heated. This is by design, and desirable (usually). All hot water systems rely on this hot / cold water layering to separate the hot water from cold water (instead of constant mixing in the tank). My thermostat cuts out the electrical element heating when the top 42% of the tank hits the maximum limit (70°C). If the element was at the bottom of the tank, yes, I could easily heat the whole tank in the 6 hours available (but it's not).
- This is why I am considering attaching a secondary pump to the auxiliary port at the top of the tank, and the cold water inlet (or solar return) at the bottom of the tank, to force whole tank circulation, and in turn, force whole tank heating using off-peak. This is my preference the more I consider my options.

I wish I had upsized to the 50x tubes & 500 litre tank at the beginning . . . bugger that sales guy who sold it to me . . .
 
David Soede
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@ S Bengi,

See my maths (math) in the first post that explains why and how we get >100 minutes of hot water in summer vs <23 minutes of hot water in winter.

That situation is bad enough without worrying or speculating about other things (like hotter showers, more washing up etc). Even if you eliminate that stuff (which we did on the blackout day I posted previously) completely 23 minutes of hot water with 4 people showering including a woman with long hair trying to shampoo & condition isn't cutting it.

I need a lot more hot water production, would be grateful for advice on how to achieve it.

I'm leaning to my last suggestion option, the secondary off-peak storage tank internal recirculating / mixing pump.
 
Eliot Mason
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David, I think this community might be short on experience with evac tubes specifically.  For ... reasons? ... evac tubes never seemed as popular as panels in the States... and that's where most of the comments are coming from.  I for one have no experience with the capacity of an evac tube - or even the surface collection area of one.

That said, you've just shot down some of my trial balloons.  Let's take a different approach... I found a 30 tube collector than has an area of about 48 ft2.  Some rough wandering around the internet shows that Australia over the last month had a rough average of 27MJ/m2 per day of solar irradiance (http://www.bom.gov.au/jsp/awap/solar/index.jsp?colour=colour&time=latest&step=0&map=solarave&period=week&area=nat).  Converted to watts that's about 7.5 kwhr/m2 a day which for my 30 tube collector would be about 33 kw/hr per day.  Not bad!  But wait, that's just irradiance ... really good evac tubes seem to be about 50% efficient at best (https://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm) which would give you about 16.5 kwhr per day of heating - which I established should be good to heat the tank to useable hot water.

There's a LOT of hand waving and magic there.  My takeaway would be that, given an optimistic average on irradiance the system just barely makes shower water.  That 27 MJ/m2/day figure is super rough ... it might be as little as a third which would totally explain why its not working!
 
David Soede
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Hi Eliot,

Your link to the BOM is great (new resource for me - thanks) but unfortunately your calculations are bad news, for me.

1) My evacuated tubes are 1.8m long but only 1.7m exposed (one end in a plastic cap, the other goes into the manifold, no exposure to the sun). They are 0.055 diameter and separated slightly to allow the sunlight between tubes when the sun isn't directly overhead. So, although the overall frame size is 1.8 x 2.35m (4.23m2) which correlates nicely with your 48 ft2 measurement, the actual evac tube exposure to the sun is only 2.8 m2.

2) I'm at latitude 33°S, just north of Sydney, which has received between 12 and 15 MJ/m2 over the last 2 weeks, about half your estimate for Australia as a whole.

3) Combined, that means I would only be getting a third of your estimate above, which dovetails perfectly into your comment "it might be as little as a third which would totally explain why its not working!".

4) If you then assume my tubes aren't top quality, which means <50% efficiency, and that the tubes aren't square to the sun, well performance drops even further.

So, I should have gone the 50 tube system (although infallible, the retrospectoscope quite annoys me sometimes) and built a frame up to maximise winter hot water production by squaring the tubes to the winter sun.

Given I didn't, and now I know there's no (probable) problem with the system, but it's working as designed just not good enough for my needs, I'm still back to my 3 choices, although if I went down 2b) I'd lift up the new evac tubes to square them with the winter sun.

Thanks for your help, hopefully it can also serve as a warning to others considering evacuated tubes.
 
Eliot Mason
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David - glad we made some progress in understanding the problem!  Sorry its bad news but I'm glad that my rough sleuthing (and really, those are estimates, not calculations) has given you some tools to repair the situation.

Given your existing investment, the ability of the equipment to scale ... I agree 2b may be the solution. But do get them lifted up to the optimal winter angle this time! And plan on doing something with all that hot water in the summer!

Don't give yourself a hard time for getting something too small.  It happens but fortunately there's a fairly simple fix.

Do let us know how the fix fares!

p.s.  getting those collectors lifted seems like a simple thing - but don't underestimate the desire of Mother Nature to see that lift as a dare for her to blow them off.  Commercially available racking is seriously engineered, assembled and attached to your roof.  Do the same.
 
S Bengi
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It sounds like your system collects and store more heat in the summer (delta T =55C) compared to the winter (delta T =20C).

So the only real solution is to double the solar collectors in the winter/year-round.
 
pollinator
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Please do give an update after this mystery is solved, I sense it could be useful to a lot of people.

Regarding rocket stoves, to clarify, you wouldn't need to buy or harvest any wood for your situation, you could just use fallen branches from your yard (since it's just for showers and not for home heating it should be more than enough).

There's also Pauls one minute shower route...not suggesting you mention that to the shampoo and conditioner member of the family, but if you wanted to take one for the team until you get your new system in place.

Good luck, hope you find the answer and let us know what you find out when it's working.

 
pollinator
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David

It sounds like you have zeroed in on the solar deficiencies.

Now, if I understand your comments correctly, there are actually two problems with your system - the other one is that "make up" heat during off peak hours cuts out before it heats your tank fully. You have indicated that is by design and I am familiar with the layering concept.  But it doesn't sound like that design works for you. Time to move on? Since, as you have said, the heater does not run all the time during the off-peak period,  that means your little booster heater _could_ heat the tank  more during the off-peak hours if you allowed it to.

You can dip your toe into this pond, you don't have to jump up to your neck right off. You have said that it only ever heats the top half of your tank and it does this with convection currents. What do you think the path of those currents inside the tank looks like? What happens to that thermocline between hot and cold in the tank when the heater just keeps heating?  IOW, what happens if you just hot wire thing one cold night and see what temps you get out of it for the full off-peak period?

Workable? But, if it does something you don't like, _then_ you can look at dispersing your thermocline by pumping it as you describe. Just cut into the feed and outlet pipes, one top, one bottom, make a loop with a tiny little circ pump and done. Depending on what you spend on the pump, you could choose to skip any controls on it at all and let it run  24/7 - or you could get a cheap 24 hour timer from a box store. The more you spend on the tiny pump, the more efficient it gets (cheaper to run) and the longer it runs before replacement or rebuild. Hydronic heating guys use these pumps everyday; that's the place I would look for them. I'd guess you can do with just about the smallest pump you can find. But that's a guess. The concept seems clear, whatever calculations look necessary to you remain to be done.

One last thing. It's been my repeated and continuous experience over 50 years of getting things to do what I want and expect them to that you _cannot_  trust any one sensor or measurement. You really cannot. You cannot trust any calculation until you run it backwards and have your honest critic do the same - and then you cannot trust it because you're betting on all it's input(S) being correct and that's never a very good bet. You mention two sensors in your system. Are you _absolutely_ _certain_ those sensors are telling the truth? Are they located where they have a chance of doing their job and are they installed properly ditto, they have a chance of doing their job? Perhaps your measurements and calculations are spot on and your system is doing just what you think it is. But you quote the service meter for your pump's electrical usage (if I understand correctly) and that's a pretty good bet - but it _is_ a bet!

Let me ask you this: How hot is that heat exchanger on the roof at the solar tubes? How hot is the cold side pipe coming from your storage tank at the point where it enters your exchanger on the evac-tubes? How hot is the hot side pipe exiting the heat exchanger  on the roof and heading down to the tank? At what rate does water flow through the exchanger? How hot is that "hot pipe" coming down from the roof, at the point where it enters your storage tank? Have you measured the temperature of the cold make-up water entering your hot water storage tank (sorry I don't recall seeing if you mentioned this)? What flow rate do you get from your kitchen sink? Bathroom basin? Tub spout? Shower? Laundry (actually, how many gallons water does the laundry use)?  What temperature is the hot water delivered to each of these outlets AND what, at that same moment, is the temperature of the water leaving the storage tank headed to your running fixture? As a Wild Ass Guess, how much not water gets mixed with the cold water for various usages in your house?   The DIFFERENCES in these temperatures throughout the system will speak volumes about what is going on with your hot water.

There is one tool which is hugely helpful when figuring out a hydronic system. The box stores sell "reasonably priced" IR laser guided thermometers. The laser is just so you have no excuses about where you're pointing the thing. The actual area measured is an average over what starts at a couple inches square and grows geometrically as you "shoot it" from farther and farther away. This is important to keep in mind when using these tools because it means that measuring the exact same spot the laser hits from 6" and then from 30" will yield completely different results. The cheap IR thermometers claim some decent margin of error, but their real value doesn't depend on whether two different tools will return the same measurement w/in 1 degree.  It's far more important that one particulaer tool be _consistent_  with itself. This is what will allow you to compare differences which you measure w/in some small window of time and conditions. And the differences are what will really help you.

At a guess, the above interrogation is really and truly nothing you ever signed up for when you carefully sorted through, selected and paid for a new hot water system to fix a simple problem that needed fixing. It would be nice if new complex systems could be simply bolted in by work-a-day guys selling them for a living  - and work like you expected them to. To my (admittedly slightly limited) knowledge that cannot be counted on. Even with good and virtuous workmen, when a system is complex and site-dependent, there are no guarantees - even if/when in writing. When you find a trademan that does what he says, when he says and for about what he asked... Pay him!!! See if he might marry your daughter... <G>

Best luck.
Rufus
 
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We use a wood stove for heating. We live in the West Kootenays and firewood is around 225-250/cord.  I bought a John Woods range boiler that failed and had to buy another one, no warranty.  We put a copper coil on the wood stove and the range boiler is elevated so the hot water is circulating.  It work ok but it did not circulate through the hot water tank.  When we burn firewood 24/7 we often got enough hot water but not always.  A couple of years ago I bought 2 flat plate solar panel and a solar hot water tank from China.  The tank had a separate intake outtake so the water from the range boiler circulated inside tank.  After burning firewood 24/7 for a couple of weeks we could turn of HWT off for the winter until we burned only at night.  We are in that heat dome and are getting so much hot water, sometimes the water in the tank reaches 80 C.  I'm watching it now and will run the hot water in the afternoon to cool it down.  Too much hot water, I'm amazed.
 
pioneer
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If at all possible, I would optimize the angle which your collector faces the sun in the winter.  Optimize it for Solstice day.   Insulate your lines as much as is reasonable.
 
pollinator
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I have a shortage of solar water in winter, but I wrapped 18m of 1/2 inch copper pipe around the flue of the wood heater, just above the firebox and it works verry well.
 
Rocket Scientist
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Just be aware that heating water by cooling the woodstove flue will cool the chimney and make creosote condensation more likely. I would hate for someone to have a chimney fire because they tried a homemade water heating experiment.
 
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You say that the incoming water temp is 8C in winter and 25C summer.  Seems like that is adding a big load just when you don't need it.  Maybe you can find an affordable way preheat the incoming water, maybe an uninsulated tank located indoors that would warm up to room temp.    I've also seen schemes for DIY flat panel collectors located on a south facing wall and so better optimized for winter use.  
 
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