Sizing of Heat Transfer Stations

A heat transfer station (HTS) transfers the network heat to the building-side heating circuits and domestic hot water preparation — typically via a plate heat exchanger with approach temperatures of 2—5 K. Dimensioning is based on the heat load for space heating and DHW, taking into account diversity factors that can reduce the DHW peak load to just 10—18 % of the sum of individual loads for 100 dwelling units. An undersized station causes supply shortfalls on cold days, while an oversized station increases investment costs and return temperatures.

Structure and components

A typical house transfer station consists of the following core components:

• Primary-side connection with shut-off valves, strainer and differential pressure controller
• Heat exchanger for the hydraulic separation between network and building (for indirect connection)
• Control valve with actuator for controlling the primary-side flow rate
• Heat meter for consumption metering
• Secondary-side components: circulation pump, safety group, expansion vessel

Hydraulic separation via a plate heat exchanger is standard in most networks, since it protects the network from pressurization issues and allows different operating pressures on the primary and secondary sides.

Domestic hot water preparation

In many cases, the domestic hot water (DHW) preparation determines the peak output of the transfer station and therefore its dimensioning. Two fundamental systems are distinguished:

Indirect preparation with storage tank

A DHW storage tank (typically 200 to 500 litres for single-family houses, 500 to 2000 litres for multi-family houses) is charged via a heat exchanger by the network. The required charging power depends on the chosen charging time and the storage volume:

$\dot{Q}_{\text{DHW}} = \frac{V_{\text{st}} \cdot \rho \cdot c_p \cdot \Delta T}{t_{\text{charge}}}$

with the storage volume $V_{\text{st}}$, the density $\rho$, the specific heat capacity $c_p$, the temperature difference $\Delta T$ (typically 45 K when heating from 10 to 55 °C) and the charging time $t_{\text{charge}}$.

The advantage of a storage system lies in its lower connection power: the storage tank can be charged over a longer period (e.g. 2 to 4 hours), so that the network peak output is lower. A disadvantage is the legionella risk at storage temperatures below 60 °C, which requires regular thermal disinfection.

Direct preparation (instantaneous principle)

In a fresh water station, the drinking water is heated in a flow-through mode via a plate heat exchanger. No hot drinking water is stored, which practically eliminates the legionella risk. However, this variant requires a significantly higher peak output, since the entire tap demand must be met in real time.

For a single-family house with a peak tap demand of about 15 l/min at 45 °C and a cold water temperature of 10 °C, this yields:

$\dot{Q}_{\text{peak}} = \dot{V} \cdot \rho \cdot c_p \cdot (T_{\text{hot}} - T_{\text{cold}}) = 0{,}25 \; \text{l/s} \cdot 4{,}18 \; \text{kJ/(kg}\cdot\text{K)} \cdot 35 \; \text{K} \approx 37 \; \text{kW}$

This value shows why diversity factors are so important in network sizing — not all buildings tap at full heat load simultaneously.

Diversity factors

The diversity factor $f_{\text{div}}$ describes the ratio of the actually simultaneously occurring output to the sum of all installed connection loads:

$\dot{Q}_{\text{network}} = f_{\text{div}} \cdot \sum_{i=1}^{n} \dot{Q}_{\text{connection},i}$

Diversity decreases as the number of consumers increases, since individual demand peaks occur at different times. In practice, a distinction is made between space heating diversity and DHW diversity:

Space heating diversity

Diversity for space heating is comparatively high, since all buildings heat simultaneously at low outdoor temperatures. Typical values:

DHW diversity

DHW diversity is significantly lower, since tapping events are short and statistically distributed:

These values are based on DIN 4708 and on empirical data from the operation of existing networks. For buildings with storage-tank systems, DHW diversity at network level largely disappears, since the storage tanks can be charged asynchronously.

Sizing for different building types

Single-family house (new build, KfW-55 standard)

• Heat load: 4 to 6 kW
• DHW output (instantaneous): 30 to 40 kW
• DHW output (storage, 300 l): 8 to 12 kW
• Typical connection load: 15 to 20 kW (with storage)

Multi-family house (20 dwelling units, existing building)

• Heat load: 80 to 120 kW
• DHW output (central fresh water station): approx. 120 kW
• DHW diversity factor: approx. 0.35
• Typical connection load: 120 to 160 kW

Commercial building (office, 2000 m$^2$)

• Heat load: 60 to 100 kW
• DHW output: low (5 to 10 kW)
• Typical connection load: 70 to 110 kW

Supply temperature and approach temperature

The required primary-side supply temperature depends on the secondary-side requirements plus the approach temperature of the heat exchanger. The approach temperature $\Delta T_{\text{approach}}$ describes the minimum temperature difference between the primary and secondary sides:

$T_{\text{VL,primary}} \geq T_{\text{VL,secondary}} + \Delta T_{\text{approach}}$

Typical approach temperatures for plate heat exchangers in transfer stations lie between 2 and 5 K. For DHW preparation to 55 °C with an approach temperature of 3 K, a required primary supply temperature of at least 58 °C therefore results.

Likewise, the return temperature significantly influences network efficiency. Low return temperatures (below 40 °C) reduce heat losses and increase the usable temperature spread. The secondary-side return temperature is undercut on the primary side by the approach temperature:

$T_{\text{RL,primary}} = T_{\text{RL,secondary}} - \Delta T_{\text{approach}}$

Buffer storage on the network side

In some network concepts, a central or decentralised thermal energy storage tank is used on the primary side to smooth out load peaks. The dimensioning depends on the expected peak output and the desired bridging time:

$V_{\text{buffer}} = \frac{\dot{Q}_{\text{peak}} \cdot t_{\text{buffer}}}{\rho \cdot c_p \cdot \Delta T_{\text{buffer}}}$

For a peak output of 500 kW, a bridging time of 30 minutes and a usable spread of 30 K, a storage volume of about 7200 litres results.

Conclusion

The heat transfer station is a central element of every district heating network and deserves particular care in its design. The choice between storage-based and instantaneous DHW preparation significantly influences the connection load and thus the entire network dimensioning. Diversity factors reduce the required network output considerably compared to the sum of the individual connections and should be applied realistically during planning. In VICUS Districts, transfer stations can be represented in the network model directly with their performance data and their influence on the overall hydraulics can be simulated. Low approach temperatures and return temperatures are key factors for an efficient network — but they must be weighed against the investment costs for larger heat exchangers.

Further reading: Heating Curves — temperature control at the building side of the transfer station, Network Temperatures in District Heating Networks — supply temperatures that determine transfer station design, Pressure Loss Calculation in District Heating Networks — hydraulic design including station pressure losses, Prosumers in District Heating Networks — bidirectional transfer stations for feed-in and consumption.

References and Standards

• AGFW FW 515 — Control and Regulation of District Heating Networks
• DIN EN 12828 — Heating systems in buildings — Design for water-based heating systems
• Nussbaumer, T.; Thalmann, S. (2016): Planungshandbuch Fernwärme. EnergieSchweiz / Swiss Federal Office of Energy.