Domestic Hot Water Preparation in District Heating Networks

Domestic hot water (DHW) preparation is the only year-round thermal load in district heating networks and determines the entire heat demand during the summer months. Three systems are available: freshwater stations achieve the lowest return temperatures (< 30 °C) with minimal Legionella risk, storage tanks with external heat exchangers reach < 35 °C, and storage tanks with internal heat exchangers result in approximately 45 °C return temperature. The choice of system significantly influences the required connection capacity, the return temperature and thus the overall efficiency of the network.

Variants of Domestic Hot Water Preparation

Instantaneous Water Heater (Freshwater Station)

In a freshwater station, the drinking water is heated on the instantaneous principle via a plate heat exchanger. There is no storage of hot drinking water — the hot water is produced directly at the time of demand.

Advantages:

• Low primary-side return temperature (< 30 degrees C), as the cold water flows through the heat exchanger at approximately 10 degrees C
• Small space requirement, as no storage tank is needed
• Reduced Legionella risk, as no stagnant hot water is present

Disadvantages:

• High connection capacity required, as the entire draw-off demand must be met in real time
• Sensitive to limescale deposits and impurities in the drinking water that can clog the heat exchanger
• Good and fast control of the primary side necessary to ensure stable hot water temperatures at varying draw-off rates

Storage Water Heater with Internal Heat Exchanger

In a storage water heater with internal heat exchanger, heating surfaces (coils or finned tube registers) are located directly inside the storage tank. The drinking water is heated and stored in the tank until drawn off.

Advantages:

• High draw-off volume available, as the storage tank serves as a buffer
• Low control requirements — the system is sluggish and tolerant of fluctuations

Disadvantages:

• Rising primary-side return temperature during charging, as the temperature in the storage tank continuously increases (typically up to approx. 45 degrees C)
• Heat losses of the storage tank through radiation to the surroundings
• Not ideal for circulation, as the circulation return temperature further raises the return temperature in the network

Hot Water Storage with External Heat Exchanger

This variant combines the instantaneous and storage principle: an external plate heat exchanger heats the drinking water, and a charging pump on the secondary side delivers the heated water into the storage tank. The cold water is drawn from the lower section of the storage tank and heated via the heat exchanger in the instantaneous flow mode.

Advantages:

• Low primary-side return temperature (< 35 degrees C), as the heat exchanger operates in counterflow and the cold water enters from the cold storage zone
• High draw-off volume thanks to the storage tank as a buffer
• High utilisation rate through good stratification in the storage tank

Disadvantages:

• Additional control and charging pump required
• Heat losses of the storage tank to the surroundings
• Higher installation effort compared to the purely internal solution

Comparison Table

Priority of Domestic Hot Water Preparation

The priority switching determines how the available connection capacity is divided between DHW preparation and space heating. It thus directly affects the required connection capacity of the transfer station.

No Priority (Parallel Operation)

DHW preparation and heating circuit operate simultaneously and independently of each other. The connection capacity equals the sum of both individual capacities:

$\dot{Q}_{\text{connection}} = \dot{Q}_{\text{heating}} + \dot{Q}_{\text{DHW}}$

This variant requires the largest connection capacity but offers maximum comfort, as there is no mutual interference.

Absolute Priority (Cylinder Priority Switching)

During DHW preparation, the heating circuit is completely shut down. The connection capacity corresponds to the higher of the two individual values:

$\dot{Q}_{\text{connection}} = \max(\dot{Q}_{\text{heating}}, \dot{Q}_{\text{DHW}})$

This is the most common variant for single-family houses. Since the storage charging times are short (typically 20 to 40 minutes), the comfort loss for space heating remains minor. The connection capacity and thus the dimensioning of the transfer station are significantly reduced.

Reduced Heating Operation

The heating circuit is not shut down during DHW preparation but is reduced to a setback temperature (e.g. reduced mode). DHW preparation has priority; at least one control valve is required to throttle the heating circuit. This variant represents a compromise between comfort and connection capacity.

Legionella Protection

Legionella are rod-shaped bacteria that multiply in stagnant water in the temperature range of 25 to 45 degrees C. The optimum for multiplication is approximately 37 degrees C. In domestic hot water systems, they pose a health risk, particularly when aerosols are inhaled (e.g. during showering).

The standard SIA 385/1 defines the following requirements:

• The discharge temperature at the draw-off point must be at least 50 degrees C after 7 times the discharge volume
• In kept-warm pipes (circulation lines), the water temperature must always be at least 55 degrees C
• No water temperatures in the range of 25 to 45 degrees C may prevail permanently in the system

As an additional measure, thermal disinfection can be carried out, in which the entire hot water system is periodically heated to at least 60 degrees C. This requires that the district heating network can provide the corresponding network temperature — an aspect that must be considered during network planning.

Freshwater stations have a systemic advantage here: since no hot water is stored, the risk of stagnant water volumes in the critical temperature range is eliminated.

Cold Water Preheating

An additional heat exchanger on the cold water side can preheat the cold water using the primary-side return flow. The principle: before the cold water enters the actual water heater, it flows through a preheating heat exchanger that utilises the residual heat of the network return.

This approach further reduces the primary return temperature, as additional heat is extracted from the return flow. At the same time, the energy demand for the actual DHW preparation decreases, as the cold water enters already preheated. Cold water preheating is applicable to all three variants of DHW preparation and is particularly effective when the return temperature of the heating circuit is still significantly above the cold water temperature.

Conclusion

The choice of DHW preparation directly affects the return temperature and thus the overall efficiency of the district heating network. Freshwater stations achieve the lowest return temperatures (< 30 degrees C) and minimise the Legionella risk, but require a higher connection capacity and precise control. Storage solutions with external heat exchangers offer a good compromise between low return temperature and high draw-off capacity. The priority switching determines the required connection capacity and should be carefully selected when dimensioning the transfer station. In VICUS Districts, the various DHW preparation variants including priority switching can be configured directly in the transfer station and their impact on return temperature and network efficiency simulated.

Further reading: Transfer Stations describes the complete design and dimensioning of the heat transfer station, Network Temperatures explains the supply temperatures required for the various DHW preparation variants, and Hydraulic Circuit Types shows the hydraulic integration of DHW preparation into the secondary circuit.

References and Standards

• VDI 6003 — Domestic hot water heating systems — Comfort criteria and design fundamentals
• DVGW W 551 — Technical measures for reducing Legionella growth in drinking water installations
• DIN 1988-200 — Technical rules for drinking water installations — Planning, components, apparatus, materials