On tank water only and we're having troubles with our Rinnai 20 instant HWS. The Rinnai is fed by the pump via a 60L 'Pressure Wave' pressure tank. It seems as the pressure tank delivers its 20 or so litres before the pump kicks in, the Rinnai is sensing the slow drop in pressure and I can hear its booster fan start and stop and we get moments of hot than warm, than the correct temp again. And when the pump does finally kick in after 4-5 mins the Rinnai seems to "drop out" suddenly then "adjust" to the new pressure. The Rinnai has a temp control panel in the bathroom set at 44C.
Our plumber has checked the regulator and gas pressure at the HWS while running and it's reading around 2.8kpa so apparently fine for the Rinnai. He recommends to delete the pressure pump or get a smaller 20L tank so when showering it has little effect but we still get 8L or so without the pump coming on when brushing our teeth or flushing the toilet.
We went with the 60L pressure tank to minimise pump starts but the varying pressure seems to be upsetting the Rinnai.