💧What is DHW balancing?
DHW (domestic hot water) balancing involves adjusting circulation flow rates in the hot water distribution system of a multi-occupancy building so that every outlet receives water at the right temperature within a reasonable time. It goes hand in hand with DHW secondary circulation, the recirculation loop that keeps hot water constantly moving through the pipework.
Without balancing, the risers closest to the heat source receive too much circulation flow, while those furthest away are starved. The result: very long waiting times in some dwellings, temperatures low enough to encourage legionella growth, and energy waste from overproducing hot water to make up for the system's shortcomings.
The principle of balancing is to add pressure drop on the most favoured circuits (using balancing valves) to redirect flow towards the disadvantaged circuits. This gives an even distribution of circulation flow, in proportion to the heat losses of each branch of the system.
DHW balancing is a fundamental part of HVAC commissioning. It is one of the tasks carried out by the commissioning engineer at the handover of a new building, as well as during the maintenance of existing installations. A well-balanced system ensures water safety, comfort and energy performance.
Why DHW balancing matters
Legionella prevention
A key regulatory obligation: keep water above 55°C at every point in the system to prevent the growth of Legionella pneumophila, which causes serious lung infections.
Shorter waiting times
A well-balanced system delivers hot water in under 30 seconds at every outlet, eliminating wasted cold water and improving everyday comfort.
Energy savings (20 to 40%)
DHW balancing reduces heat losses from the circulation loop, optimises the operation of the circulation pump and cuts overproduction of hot water. Observed savings range from 20 to 40% on DHW energy use.
Occupant comfort
Stable, even temperature on every floor and in every dwelling. No more cold showers or unpleasant temperature swings during use.
Compliance with the Order of 30 November 2005
This French Order sets temperature requirements for the production, storage and distribution of DHW in public-access buildings (ERP) and residential blocks.
Protecting the installation
Correct balancing limits corrosion and scaling caused by temperature differences. It extends the service life of pipework, valves and DHW production equipment.
DHW secondary circulation: principle and operation
How recirculation works
The DHW circulation loop is a closed circuit that keeps hot water constantly moving through the distribution pipework. Without it, hot water stagnates in the pipes when no water is drawn off, gradually cools and reaches temperatures that favour legionella growth (between 25°C and 45°C). The loop prevents this by continuously returning water to the heat source to be reheated.
The circulation pump (or secondary return pump) is the heart of the system. It draws water from the loop return and feeds it back into the production circuit. Its sizing is crucial: too low a flow rate cannot maintain temperatures, while too high a flow rate increases electricity consumption and heat losses. The circulation flow rate is generally calculated to offset pipework heat losses, i.e. about 0.5 to 1 litre per minute per riser depending on insulation.
Reference temperatures
The DHW flow temperature (heat source outlet) must be above 55°C, ideally between 55°C and 60°C. The loop return temperature must stay above 50°C, with a maximum difference of 5°C from the flow. On each riser, the balancing valve adjusts the flow rate so that the temperature at the foot of the riser (return) does not fall below 50°C to 55°C.
Balancing valves on each riser
Each riser (vertical distribution pipe) must be fitted with a balancing valve on the loop return. These valves, whether manual or thermostatic, regulate the circulation flow through each riser. Self-regulating thermostatic valves are increasingly common because they adjust automatically to load variations, providing dynamic balancing without manual intervention.
Circulation loop versus trace heating
DHW secondary circulation and electric trace heating are two ways of maintaining temperature in pipework, but they work differently. Secondary circulation keeps hot water moving in a closed circuit, whereas trace heating uses an electric cable fixed along the pipe to offset heat losses. Trace heating is generally reserved for terminal branches (dead legs) or installations where a circulation loop is technically impossible. Secondary circulation remains the reference solution for communal systems, as it is more reliable, cheaper to run and compliant with regulatory requirements.
DHW balancing method
DHW balancing follows a rigorous 7-step methodology. Each step is essential to achieve a result that meets regulatory requirements and occupants' comfort expectations. The method applies both to initial commissioning of a new building and to rebalancing an existing installation.
Mapping the DHW system and circulation loop
A complete survey of the distribution system: identifying risers, branches, outlets and the circulation loop. Checking that the drawings match the installation as built. Locating the existing balancing valves and assessing their condition.
Checking the circulation pump
Checking the operation of the secondary circulation pump: flow rate, differential pressure, direction of rotation and time schedule. An undersized or faulty pump makes balancing impossible.
Measuring flow/return temperatures
Recording temperatures at the heat source outlet (DHW flow), at the loop return and on each riser. The difference between flow and main return must not exceed 5°C. These readings are the baseline for balancing.
Adjusting the balancing valves on each riser
Methodically adjusting the balancing valves on each riser to distribute the circulation flow rate in proportion to the heat losses of each branch. Start with the index riser, then balance the others progressively.
Checking waiting times at outlets
Measuring the time taken to obtain water at a minimum of 50°C at each representative outlet. The target is a waiting time of less than 30 seconds in every case, a sign of an effective circulation loop.
Legionella control check (>55°C at every point)
Checking that the temperature stays above 55°C at every point in the distribution and circulation system. Measurements at the most critical points (top floors, long branches). Any point below 50°C is a major health risk.
Writing the DHW balancing report
Producing a detailed report covering: the system schematic, flow rates and temperatures measured before and after balancing, valve settings, waiting times, and maintenance recommendations. This document is essential for regulatory compliance.
Tools and equipment
DHW balancing requires a set of specific measuring instruments and components. Here are the commissioning engineer's essential tools for working on domestic hot water systems.
Contact and infrared thermometer
Accurate surface temperature measurement on pipes, valves and outlets. A contact thermometer (Pt100 probe or thermocouple) offers accuracy of +/- 0.5°C. An infrared thermometer allows quick non-contact readings to locate cold spots.
Ultrasonic flow meter
Non-intrusive flow measurement in DHW circulation pipework. Essential for checking flow distribution between risers without shutting down the system. Typical accuracy of 1 to 3% depending on conditions.
Thermostatic balancing valves
Self-regulating valves that automatically maintain the loop return temperature. Thermostatic models (TA-Therm, Caleffi 116, Oventrop Aquastrom type) adjust continuously to compensate for load variations in the system.
Temperature data logger
A stand-alone sensor (data logger) fitted to pipework to record temperatures over several days or weeks. Reveals overnight variations and temperature drops during off-peak periods, and builds a record for legionella compliance.
Rapid legionella test kit
A test kit that detects Legionella pneumophila in water within a few hours (versus 10 days for conventional culture). Useful for rapid diagnosis when contamination is suspected or to confirm the effectiveness of thermal disinfection.
DHW regulations
Order of 30 November 2005
The French Order of 30 November 2005 on fixed installations for heating and domestic hot water supply in residential buildings, workplaces and public-access buildings sets the following temperature requirements:
- ✓ Production: temperature above 50°C at the heat source outlet
- ✓ Storage: temperature above 55°C in storage cylinders (volume > 400 litres)
- ✓ Distribution: temperature above 50°C at every point in systems with circulation loops
- ✓ Outlets: temperature limited to 50°C in rooms used for washing, 60°C in kitchens and laundries
DTU 60.11
DTU 60.11 (Document Technique Unifié, French standard) sets out the design and sizing rules for domestic hot water distribution installations. It specifies requirements for circulation loops, pipe insulation, circulation pump sizing and the positioning of balancing valves. Compliance with DTU 60.11 is essential for a compliant, high-performing installation.
Thermal disinfection against legionella
In addition to balancing, the regulations call for periodic thermal disinfection (raising the temperature above 70°C for 30 minutes throughout the system) to kill any legionella present. These treatments must be recorded in the installation's water safety logbook. They do not replace balancing but complement it: a well-balanced system needs fewer thermal disinfections because conditions are permanently unfavourable to legionella.
Water safety logbook
Every public-access building must keep a water safety logbook (carnet sanitaire) recording monitoring and maintenance of the DHW system. It includes monthly temperature readings, bacteriological test results, balancing work, thermal disinfections carried out and any non-conformities detected. The logbook is the proof that regulatory obligations have been met in the event of an inspection or dispute.
Frequently asked questions about DHW balancing
Q. What temperature should domestic hot water be?
Under the French Order of 30 November 2005, domestic hot water must be produced at more than 50°C and stored at more than 55°C in cylinders. In distribution, the temperature must not fall below 50°C in systems with circulation loops. At outlets, it is limited to 50°C in bathrooms and 60°C elsewhere to prevent scalding. These values form the minimum regulatory framework for communal installations.
Q. How do you detect an unbalanced DHW system?
The main indicators are: excessive waiting times at certain taps (more than 30 seconds), large temperature differences between risers (more than 5°C), a loop return temperature below 50°C, recurring occupant complaints, or unexplained energy overconsumption on hot water production. A temperature survey across the entire system confirms the diagnosis.
Q. How often should DHW balancing be checked?
At least an annual check is recommended as part of preventive maintenance. Additional checks are needed after any modification to the system, after work on DHW production, or in the event of complaints. Public-access buildings (ERP) require monthly temperature monitoring. A water safety logbook must record all readings and interventions.
Q. Who is responsible for DHW balancing?
Responsibility lies with the owner or operator of the installation. Initial balancing is carried out by the commissioning engineer during commissioning. Ongoing balance is then covered by the maintenance contract. In the event of a legionella outbreak, the owner or operator may face criminal liability if monitoring and maintenance obligations have not been met.
Q. What is the link between DHW balancing and legionella?
Legionella pneumophila bacteria multiply in water between 25°C and 45°C. A poorly balanced system inevitably has zones where the temperature drops into this critical range, encouraging bacterial colonisation. DHW balancing keeps temperatures above 55°C at every point, removing the conditions legionella needs. It is the first line of defence, supplemented by periodic thermal disinfection and microbiological monitoring.
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