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The devil is in the detail and with a simple change to the return pipework between the roof mounted solar panels the system changed from zero to hero. I replaced two short lengths of 10mm plastic pipe with 10mm translucent silicon pipe pipe so I could visually observe water flows. I was surprised to observed air being carried along with water flowing downwards in a 10mm pipe. This air was creating multiple problems in the hot water cylinder. I lowered the pipe to a submerged position to avoid the venturi effect (pulling in air), and I installed a 15mm barrel air valve.
The system now works perfectly with my 500 litre tank reaching over 75 degrees C from top to bottom!
Heat Storage (Day One – sunny all day)
Hot water is stored at the top of the hot water cylinder and on a hot day the entire contents of the tank exceeds 75 deg c. This effectively stores hot water for three days in my home.
Heat Storage (Day Two – sunny intervals)
The hottest of hot water cools by 10 deg c in 24 hours and the system works on the basis that no cooler water returning from the solar panel disturbs the stratification at the top of the hot water cylinder. When the sun is not shining the panels pre heats the cold water which enters the bottom of the hot water cylinder (replacing the hot water leaving the top of the hot water cylinder to satisfy domestic hot water usage). When the sun shines the preheated water is easily raised to over 60 degrees C and this water is added to the top of the hot water cylinder adding to the heat store.
Subsequent Days
The system can easily handle 1.5 litres a minute when the sun is only partial and 6 litres a minute with full sunlight. It is a fully automated system that maximises heat collection and MORE IMPORTANTLY, does not de-stratify the hot water stored at the top of the cylinder so it can be saved for rainy days.
Note: In the winter I do not anticipate the solar panels heating full tanks of water to above 60 degrees c In winter the panels will heat water at maximum efficiency (drawing cold water from the bottom of the cylinder and returning warmed water to the bottom).