I live on the Gold coast in a 3 story house on a hill, with solar hart hot water system on the roof.Hot water pressure is terrible and getting worse. Cold water pressure is fine. The plumber has serviced the unit, inspected all filters, removed the pressure limiting valve and still same.The plumber wants to remove the half inch copper feed to the unit and replace with three quarter piping. This seems OK to me, any other suggestions before this happens

I'm not all that familiar with the plumbing in Australia. Is the hot water pressured by a pump or is it gravity fed from a tank on the roof?

.The plumber wants to remove the half inch copper feed to the unit and replace with three quarter piping. This seems OK to me, If the hot water supply to the house then it's undersized. A 3/4 feed would give you more volume but wouldn't increase the pressure.
Tell me more about your system. Do you have a hot water heater as a back up for the solar panels?
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The plumber has serviced the unit, inspected all filters, removed the pressure limiting valve Please explain in detail. If you had poor pressure why the pressure limiting valve?
What am I missing here? Cheers, Tom