My system is around 7000litres with 40mm solvent weld pipe straight to drain, I open the valves and drain 20% to 30% in a couple of minutes, then refill will water from a mixer tap run through a declor unit, that's also done with rigid pipe work. I just have to open and close a few valves, the tanks are heated using heat exchangers from the boiler so adding warm water back to the tanks hardly uses any extra gas. adding slightly warm water also removes condensation from the pipe work as the normal water is very cold when it enters, and even with pipe lagging there used to be lots of puddles from the condensation.