Size it, pick the unit, and flag every install issue before it blows the quote. Based on AS/NZS 3500.4, AS 4032.2 & 4032.3.
Hot water sizing depends on the system type. For instantaneous gas, the key figure is flow rate (L/min) at the required temperature rise. Climate zone determines the cold water inlet temperature — from 10°C in alpine areas to 25°C in tropical Queensland — so the same unit performs differently depending on location. For storage systems, the key figure is daily demand in litres matched to tank recovery rate.
The installation flags are where this tool earns its place on a quote. Every instantaneous gas recommendation links directly to the gas line sizing tool pre-filled with the unit's MJ/h rating. The tool also flags tempering valve and TMV requirements, flue clearance rules, condensate drain needs for condensing units, PRV requirements, and current state rebates for heat pump systems.
Use this when replacing a dead hot water unit or quoting an upgrade to instantaneous or heat pump. The tool covers major models from Rinnai, Rheem, Bosch, Thermann, Reclaim Energy, Sanden, iStore, and Chromagen with current flow rates, capacities, and indicative pricing.
Full bathrooms with shower or bath.
Ask the customer: "Think about a normal morning shower — not a quick one, a normal one. Are we talking 5 minutes, or closer to 10–15 with a proper hair wash?"