Network Operating Modes

The network operating mode determines whether the supply temperature of a district heating network is adjusted continuously with outdoor temperature (sliding), held constant, or operated as a combination of both (sliding-constant). Sliding-constant operation is the standard for most thermal networks because it simultaneously serves weather-dependent consumers (space heating) and weather-independent consumers (domestic hot water at a minimum of 60-65 degC). The choice of operating mode directly affects heat losses, generation efficiency, and security of supply, and is specified in the Technical Connection Regulations (TAV) of each network operator.

Sliding Operation

In sliding operation, the supply temperature is continuously adjusted along the heating curve as a function of outdoor temperature:

• As outdoor temperature drops, the supply temperature rises gradually to the maximum value
• As outdoor temperature rises, the supply temperature decreases gradually until the heating limit is reached and heat supply is discontinued

Advantages:

• Minimum heat losses, as the supply temperature is always matched to the current demand
• High efficiency for heat pumps and condensing boilers due to low return temperatures (see return temperature optimization)

Limitation: Purely sliding operation is only suitable for space heating and other weather-dependent consumers. It is unsuitable for domestic hot water preparation or process heat that requires a constant minimum temperature level.

Sliding-Constant Operation

Sliding-constant operation is the most common operating mode for thermal networks, as it enables simultaneous supply of different consumer types:

• As outdoor temperature drops, the supply temperature rises gradually to the maximum value
• As outdoor temperature rises, the supply temperature decreases gradually, but only to a minimum value that is not undercut

The minimum value is determined by the requirements of weather-independent consumers, typically domestic hot water preparation (at least 60 — 65 °C supply temperature) or process heat.

Advantages:

• Good compromise between efficiency and security of supply
• Weather-dependent and weather-independent consumers can be supplied simultaneously

Constant Operation

In constant operation, the supply temperature is kept constant regardless of outdoor temperature:

• The supply temperature corresponds to the maximum required by all consumers
• Flow control is achieved via the volume flow rates at the transfer stations

Advantages:

• Simple control
• All consumers can be supplied with the full supply temperature at any time

Disadvantages:

• Higher heat losses, as the supply temperature remains at the maximum value even at low demand
• Unfavorable for heat pumps and condensing boilers

Constant operation is primarily used for process heat or in networks with predominantly weather-independent consumers.

Control Variable: Supply Temperature

The supply temperature of the network is typically controlled based on an outdoor temperature averaged over 12 to 25 hours. This prevents:

• Excessively rapid temperature changes in the network due to short-term weather fluctuations
• Excessive supply temperatures during brief cold spells
• Unnecessary control activity of the generation plants

The currently measured outdoor temperature should under no circumstances be used directly as a control variable, as this leads to unstable network operation.

Influence on Heat Losses and Efficiency

The network operating mode has a direct influence on the efficiency of the overall system:

The heat losses of a thermal network depend linearly on the difference between network temperature and ambient temperature. A temperature reduction of the average network temperature by 10 K typically reduces heat losses by 10 — 15%.

Seasonal Operating Adjustments

Many networks use seasonal adjustments to their operating mode:

• Winter operation: Sliding-constant operation with maximum temperature
• Summer operation: Reduced constant supply temperature, only for hot water and minimum heat losses

Summer operation typically requires only 15 — 25% of the winter volume flow rate. Some networks use a separate, smaller summer pump for this purpose, which achieves better efficiency for this load range.

Conclusion

The choice of network operating mode is a fundamental planning decision that affects heat losses, generation efficiency, and customer satisfaction. Sliding-constant operation has proven to be the standard solution for most thermal networks. For low-temperature networks with heat pumps, sliding operation offers the highest efficiency. The effects of different operating modes on the overall system can be studied in detail with simulation software such as VICUS Districts.

Further reading: Network Temperatures — the temperature level of the network determines the selectable operating mode, Pump Switching and Control — the pump control must be adapted to the chosen operating mode, Network Control — control strategies for implementing the operating mode during operation.

References and Standards

• AGFW FW 440 — Hydraulic Calculation of Hot Water District Heating Networks
• Lund, H. et al. (2014): 4th Generation District Heating (4GDH). Energy, 68, pp. 1—11.