David Soede

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.

@ 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.

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 . . .

@ 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

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?

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.

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.

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

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.

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.