Jules I'm not sure on your physical attributes
I heard a recommendation on the radio about a month back that people over 50 shouldn't climb ladders, which I think is setting the bar pretty low! Perhaps people under 20 should be ruled out too, on grounds of immaturity?
More to the point, I've steered clear of the hot water tank problem for the moment because with the power off it's working fine in the heat of summer [47C last weekend]. Your suggestion is an essential part of sorting out the problem. While I'm not closing off any options I'm less certain that a reverse syphon could be causing the problem. The insulated pipe that would feed back to the roof panels does get warm but I think this is just heat conducted from the top of the tank. The pipe temp. drops off very quickly as the distance from the tank increases, so even a couple of metres away from the tank, it's only mildly warm. The other thing is that the hot water is very hot and I would have thought that if there was a syphon in action, it would be cooling it down. A half dozen counter arguments occur to me but they can wait.
Thanks for the suggestion Helipos. I will find a solution to this before winter and I'll report back here.
Primary system: .8KW Trina panels; Plasmatronics PL40; 1,000Ah VRLA 12X2V battery bank; 1.7KW Selectronic inverter
Veteran system, semi retired: 6 X 55W Solarex 12V panels; Fangpusun CC; 6 X 2V, 35 year old 550Ah lead acid flooded cell batteries.