Domestic hot water (DHW) is a public health issue. Legionnaires' disease, caused by the bacterium Legionella pneumophila, develops in poorly balanced hot water systems where the temperature drops below 50°C. Balancing the return loop is the first line of defence against this risk.
Regulatory framework
- • Order of 30 November 2005 (French regulation): production temperature ≥ 55°C, distribution ≥ 50°C at every point in the system.
- • DGS circular 2002: thermal shock (70°C) required where contamination is confirmed.
- • Water safety log book: mandatory temperature monitoring in ERPs (French public-access buildings).
- • DTU 60.11 (French standard): sizing and design of DHW systems.
How the return loop works
The return loop (or secondary circulation loop) keeps hot water circulating permanently through the distribution system, even when no one is drawing water. This stops water stagnating and cooling in the risers. The circulation flow rate must compensate for the heat losses of the system.
Balancing methodology
- 1. Record the temperature on each return riser before any work (thermal mapping).
- 2. Calculate the theoretical circulation flow rates based on the heat losses of each branch.
- 3. Set the balancing valves (e.g. TA-THERM) on each return riser.
- 4. Check: the return temperature of each riser must be ≥ 50°C, with a maximum ΔT of 5°C relative to the flow.
- 5. Record the results in the DHW balancing report.
FIELD TIP
Use thermostatic circulation valves (e.g. Caleffi 116): they automatically keep the return temperature above the anti-Legionella threshold, even when conditions change.
Mistakes to avoid
- • Switching off circulation at night to save energy → risk of bacterial growth.
- • Failing to flush dead legs (unused risers) → ideal stagnation zones for Legionella.
- • Setting the cylinder below 55°C → growth zone (25-45°C).
- • Overlooking central mixing valves → they can mask an upstream temperature problem.