DHW Demand Calculator — Hot Water Sizing

A common UK rule of thumb is roughly 35-45 litres of HOT water per person per day for cylinder sizing (Hot Water Association). This calculator uses 45 L/person/day at storage temperature. Note this is hot water only — Building Regulations Part G’s 125 litres/person/day is TOTAL wholesome water (hot AND cold combined), measured by the Part G Water Efficiency Calculator, not a hot-water figure. The BRE SAP 2012 occupancy model gives a similar result: hot water ≈ 25 × occupants + 36 litres/day.

Peak hour demand is the maximum hot water usage in any single hour, typically the morning or evening rush when multiple outlets are used simultaneously. It’s usually 30-40% of total daily demand concentrated in one hour. This drives the sizing of instantaneous heaters and cylinder recovery rates.

For 1–2 occupants with one bathroom, a combi boiler (24–30 kW) is usually sufficient. For 3+ occupants or 2+ bathrooms, stored hot water (cylinder) is recommended as it can meet simultaneous demand. Heat pump installations almost always need a cylinder. See our Hot Water Cylinder Calculator for cylinder sizing, or our Unvented Cylinder Calculator for mains-pressure systems.

This calculator uses a peak-hour-storage method: cylinder ≈ 1.5 × the peak-hour draw-off (rounded up to the next 10 L). For a 4-occupant home that is ~180 L daily, ~72 L in the peak hour, giving a ~110 L cylinder. This is a peak-hour estimate and sits below the full-day-storage rule of thumb (the Hot Water Association suggests ~180–210 L for a 3-bed family). For heat-pump cylinders size larger to allow for slower recovery. Use our Hot Water Cylinder Calculator for a full-day-storage figure.

Store hot water at minimum 60°C to kill Legionella bacteria (which thrive at 20-45°C). Water should reach all outlets at 50°C minimum within one minute of opening. HSE Approved Code of Practice L8 provides detailed guidance for Legionella risk management.