As the link obove, ... We have a gas instant 8 liter PM heater unit, Puts out nice and hot 40 / 42 deg C shower water with a reasonable flow in winter time .. [ would get much stronger shower flow if we used cooler temperature setting IE: Summer time ]..
We use a 12.5 Lpm pump .. It pays to prime the heater core before you start your shower though , We use 25 ltr containers of cold water for showers and these give roughly 6 minutes of continuos shower time ..