jimbo wrote:BTW today it's 37 degrees and sunny. Usually the pressure relief valve would be dumping water by 1pm and that's without the booster on at night.
Hi Jimbo, do you turn the pump [EDIT: your main water pump supplying the house] off at night?
Many solar HWS pumps are only capable of around 2.5 to 3 metres of head. If air gets into the system it is possible for the pump to not be able to push water to the collectors.
I expect there would be an air bleed valve on the collectors.... or at least a pressure relief valve. If you have turned the pump off overnight, there is a chance that air might have been sucked back in past this valve and the system could need bleeding. Under normal pressure from your main water pump, you could expect any air in the panels to be expelled.... but, who knows. An air relief valve may be fitted to the panels, and it should expel any air, but it could be faulty or not fitted to that system.
With your main water pump running, and in the morning before the panels heat up so you cannot be scalded, I would hop on the roof and operate the overpressure valve to see if any air comes out. If it does, operate the valve until you have a steady flow of water coming out.
Hopefully, this may be all that is required.
EDIT: THinking this through further, it may be necessary to crack the return line down near the HWS circulation pump to also expel any air in the return line.