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Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.
We are designing a central domestic hot water system with a storage calorifier and a circulation loop for a hotel. The client wants to reduce energy use by lowering the storage temperature.
- What temperatures should the storage, the flow leaving the calorifier, the circulation return and the outlets reach for Legionella control?
- How should the circulation flow rate be calculated?
- Where do the European and US guidelines differ?
We would also like to know the usual design mistakes that leave parts of a loop too cold.
Store at 60 °C or above, keep the circulating loop so that no part falls below about 55 °C (50 °C is the absolute minimum in the UK guidance), and get at least 50 °C at every outlet within one minute. Keep cold water below 20 °C. Size the circulation flow from the heat loss of the loop, not by rule of thumb.
Typical target temperatures
- Storage and calorifier outlet: ≥ 60 °C. UK HSG274, German DVGW W 551 and ASHRAE Guideline 12 all use about 60 °C (140 °F) for storage.
- Circulation return: DVGW W 551 limits the temperature drop round the loop to 5 K, i.e. return ≥ 55 °C. HSG274 requires ≥ 50 °C, preferably 55 °C. ASHRAE Guideline 12 recommends about 51 °C (124 °F) or more.
- Outlets: ≥ 50 °C within one minute of opening (55 °C in healthcare under UK guidance).
- Cold water: below 20 °C within two minutes at outlets.
- Dead legs: DVGW limits the volume between the circulated pipe and the tap to 3 litres; keep unrecirculated branches as short as possible everywhere.
Lowering the storage temperature is only acceptable with a recognised alternative control method (for example continuous treatment) and the agreement of the authority; it is not a free energy saving.
Scald protection
Fit thermostatic mixing valves where scalding is a risk (showers, baths, basins for vulnerable users), as close to the outlet as possible so the pipe carrying mixed water stays short.
Worked example: circulation flow
The heat lost by the flow pipes of the loop must be replaced by the circulating water within the allowed temperature drop:
q = Φ / (c · ΔT)
- Insulated flow pipework: 120 m at an assumed 8 W/m → Φ = 960 W.
- Allowed drop: ΔT = 5 K (60 → 55 °C).
- q = 0.96 kW / (4.19 kJ/kg·K × 5 K) = 0.046 kg/s ≈ 165 l/h.
Size the circulation pipes for this flow at a low velocity (typically 0.2 to 0.5 m/s) and select the pump for the index loop resistance including the balancing valves. In a building with many risers, calculate the flow of each riser from its own losses and balance with thermostatic circulation valves set to the return target, with a test point on each riser return.
Common mistakes
- One large pump flow without balancing: the nearest risers take the flow and the far risers drop to 40 °C.
- Uninsulated or poorly insulated return pipes and valves.
- Long unrecirculated branches to showers or kitchens.
- Calorifier outlet set to 60 °C with a return that is never measured or monitored.
- Large storage that stratifies and leaves a cool volume at the bottom; provide destratification or a periodic heat-up of the full volume.
Related: Hot Water Recirculation and Legionella Temperatures, Hot Water Storage Cylinder Sizing.
Check the edition of the code that applies to your project.
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