How much hot water does a person use per day?

Building Regulations Part G uses a benchmark of approximately 125 litres per person per day (at 60°C storage temperature). This includes all hot water usage: showers (~40L each), baths (~80L each), hand washing (~5L), kitchen use (~20L), and cleaning. Actual usage varies significantly by household.

What is peak hour demand?

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.

Should I choose a combi or a cylinder?

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.

How do I size a hot water cylinder?

Size the cylinder at 1.5 times the peak hour demand. For a 4-person household with ~180L peak hour demand, specify a 270L cylinder (round up to 300L standard size). For heat pump cylinders, size slightly larger to compensate for slower recovery times.

What storage temperature prevents Legionella?

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.

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DHW Demand Calculator — Hot Water Sizing

Calculate daily hot water demand by occupancy per Part G guidance (125L/person/day). Determine peak hour demand for storage vs instantaneous sizing.

Property TypeSelect property or custom occupancy

DHW System TypeStored or instantaneous hot water

Baths

Number of baths in property

Showers

Number of showers in property

How We Calculate This

This calculator estimates domestic hot water demand based on occupancy and property type, following Building Regulations Part G guidance at approximately 125 litres per person per day.