Heating, Cooling & Insulation
Apr 26, 2020 11:26 pm
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 had closed for a 2-3 weeks Christmas break. 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 plumber for the parts that needed a qualified tradesman - I did the rest myself (eg mount & affix evac tube frame on roof, mount solar control panel and pump on wall etc). 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 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 maths 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.
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