Domestic Hot Water Plant Sizing for Commercial Buildings

What You Need to Know

Every commercial building needs hot water. Offices need it for basins. Hotels need it for showers. Hospitals need it for both, plus sterilisation. The size of your hot water plant depends on how many fixtures you have, what type of building you run, and how fast people use water at peak times.

AS/NZS 3500.4 sets the rules for heated water services in Australia. It covers storage temperatures, pipe sizing, and delivery limits. Get the plant too small and you run out of hot water. Get it too big and you waste space, money, and energy.

• Store hot water at 60°C minimum. Deliver from the heater at 60°C minimum (AS/NZS 3500.4 Cl 1.9.1)
• Keep circulation return lines at 55°C or above (AS/NZS 3500.4 Cl 1.9.1)
• Cap delivery to personal hygiene outlets at 50°C. For healthcare, aged care, early childhood centres, and schools, cap it at 45°C (NCC Volume Three B2D5)
• Limit pipe velocity to 3.0 m/s for non-circulatory pipes. For circulation loops, limit flow pipes to 1.2 m/s (copper) or 1.0 m/s (other materials), and return pipes to 1.0 m/s (NCC Volume Three B2P3)
• Route hot water pipes by the shortest path. Use the smallest diameter that meets demand (AS/NZS 3500.4 Cl 1.8)
• Insulate all heated water pipes to the R-values in NCC 2025 Table J6D9a. Protect insulation from weather and sunlight (NCC 2025 J6D9)
• Design heated water supply systems to comply with NCC Volume Three, Part B2 (NCC 2025 J8D2)

Start with demand. Work out how much hot water your building needs each day, and how fast people draw it during the busiest hour. An office uses about 22 litres per person per day. A hotel uses 90–160 litres per guest per day. A hospital uses up to 160 litres per patient per day. These figures assume delivery at 60°C.

Peak hour sets the plant size. An office worker draws about 9 litres per hour at peak. A hotel guest draws about 45 litres per hour. For a 200-person office, peak demand is roughly 1,800 litres per hour. For a 100-room hotel, peak demand can reach 4,500 litres per hour. Size your heater to recover at or above the peak hourly rate.

Use the heater sizing formula. To find the heater input power: multiply the litres per hour by 4.2, then by the temperature rise in degrees Celsius, then divide by 3,600. For that 200-person office (1,800 L/hr, 40°C rise from 20°C inlet to 60°C storage): 1,800 × 4.2 × 40 / 3,600 = 84 kW. That is the minimum heater input. Add 10–15% for pipe losses and standby.

Storage absorbs peaks. You do not need the heater to match every spike in demand. A storage tank smooths out the peaks. Typical storage allowance is 5 litres per person for offices, 27–30 litres per person for hotels and hospitals. Usable storage is about 70% of the tank volume because cold inlet water mixes with stored hot water during draw-off.

Circulation loops keep the system hot. For pipe runs longer than about 12 metres, fit a circulation pump and return loop. Without circulation, water sits in the pipes, cools below 55°C, and creates both a Legionella risk and user complaints. Industry practice suggests a minimum circulation velocity of 0.5 m/s to prevent biofilm buildup. The code caps circulation flow velocity at 1.0–1.2 m/s depending on pipe material (NCC Volume Three B2P3). Install balancing valves on each riser to keep temperatures even across the building. Use AS/NZS 3500.4 Appendix Q to size branch connections from the circulation loop.