Pressure maintenance in district heating networks

Tasks of pressure maintenance in district heating networks

Pressure maintenance fulfils three tasks in district heating networks. It keeps the pressure above a minimum level at every point of the network to prevent negative pressure, air ingress, evaporation and cavitation at pumps and valves. It absorbs the temperature-related change in volume of the heating water. And it holds a water reserve that compensates for small water losses during operation.

There are two basic types, which are used as alternatives:

• Static pressure maintenance with a membrane expansion vessel: The pressure is generated by a pre-charged gas cushion. The system works without auxiliary energy and is inexpensive, but the pressure rises with the fill level. Typically used in smaller local heating networks up to approx. 1 MW.
• Dynamic pressure maintenance with a pressurisation unit: Pumps or compressors keep the pressure almost constant. Nearly the entire vessel volume can be used, and make-up and degassing are often integrated. This is the standard solution in larger district heating networks.

Calculation in nPro

The calculation is based on EN 12828 and VDI 4708 Part 1 as well as on approaches commonly used for district heating networks. All pressures are gauge pressures.

Expansion volume

The expansion volume results from the change in density of the heat transfer medium between the lowest temperature and the highest operating temperature. The fill temperature of 10 °C is used as the lowest temperature, or the lower network temperature in cold networks. The density is calculated as a function of temperature for water or the selected water-glycol mixture:

with $V_{S}$ and $V_{R}$ as the water content of the supply and return lines, $\rho_{min}$ as the density at the lowest temperature and $\rho(t)$ as the density at the respective maximum temperature.

As is common practice in district heating, the supply line expands up to the maximum supply temperature and the return line only up to the maximum return temperature. This gives more realistic results than the approach used for building heating systems, where the entire volume is heated to the highest temperature. The water content used is the volume of the pipe network from the network design.

In addition, a water reserve of 0.5 % of the water content is provided, with a minimum of 3 litres:

with $V_A$ as the total water content of the system.

Nominal volume of pressurisation unit and expansion vessel

With a pressurisation unit, the pressure is independent of the fill level. The nominal volume is therefore only 10 % larger than the volume of water to be absorbed:

with $V_n$ as the nominal volume, $V_e$ as the expansion volume and $V_V$ as the water reserve.

In a membrane expansion vessel, the gas cushion is compressed as the fill level rises, so only part of the vessel can be used:

The pre-charge pressure $p_0$ in bar corresponds to the minimum pressure of the pressure maintenance set in the project. The final pressure $p_e$ is determined for a standard vessel with a maximum operating pressure of 6 bar, with $p_{SV}$ as the set pressure of the safety valve:

If the pressure band is insufficient due to a high pre-charge pressure, a 10 bar vessel is used instead. In addition, the pressure must not exceed 90 % of the pipe pressure rating at any point in the network. The expansion vessel is only shown for networks with a peak load of up to approx. 1 MW; above that, a pressurisation unit is the usual solution.

Surcharge for storage and generators

Buffer storage tanks, heat generators and other plant components are often not yet defined at the network planning stage, but frequently account for the largest share of the water content. nPro therefore provides a surcharge per m³ of additional water content in addition to the nominal volume for the pipe network. It is calculated conservatively up to the highest network temperature. Example: 72 L + 34 L/m³ with 20 m³ of buffer storage results in 72 L + 20 · 34 L = 752 L.

Pressure maintenance in district heating and cold district heating

How much the water expands depends primarily on the network temperature. Table 1 summarises typical values per m³ of water content for different network types (pre-charge pressure 3 bar, expansion vessel with 6 bar).

In cold district heating networks, the water hardly expands at all. The nominal volume is therefore determined mainly by the water reserve. nPro automatically takes two special features into account: if network temperatures fall below the fill temperature in winter, the lower temperature is used as the starting point. And because the return line can be warmer than the supply line in summer in networks with cooling, the surcharge for storage is based on the highest temperature of both lines. You can find more about the fundamentals under cold district heating and about the design under sizing of 5GDHC networks.

In conventional district heating, on the other hand, the expansion determines the result. Due to the pressure factor, an expansion vessel is about two and a half times as large as the vessel of a pressurisation unit.

Pressure rating of the pipes

nPro determines the smallest standard pressure rating from the highest pressure of the hydraulic pressure calculation in the design case, separately for each pipe type selected in the pipe dimensioning. In mixed networks, for example with steel main lines and flexible plastic house connections, the highest pressure rating is shown. Transient pressures such as pressure surges are not taken into account.

Validation of the pressure maintenance calculation

For validation, an example network was recalculated with the design tool Reflex Solutions Pro by Reflex Winkelmann GmbH (as of October 2026) and compared with the results from nPro. Boundary conditions: water content of the pipe network 2,600 L, maximum supply temperature 75 °C, maximum return temperature 55 °C, pre-charge pressure 3.0 bar, safety valve set pressure 6 bar, water reserve 0.5 %, generator capacity 400 kW. In Reflex Solutions Pro, the design criterion “mean system temperature” was selected, which is closest to the approach used in nPro with separate consideration of supply and return.

The deviations are below 5 %. The pressure factor of the expansion vessel is virtually identical; the differences in volume result from the tabulated expansion coefficients in Reflex Solutions Pro compared with the density function used in nPro. For this network, Reflex Solutions Pro selects a Reflex N 200 vessel (static pressure maintenance) or a Variomat pressurisation unit with a VG 200 basic vessel. This illustrates a point that is important for small networks: nPro outputs the calculated minimum volume, while available vessels are rounded up to the next size. Depending on the manufacturer, basic vessels for pressurisation units start at approx. 200 L.

Further comparisons of nPro with established planning tools can be found under validation.

Sources