Pump Sizing in District Heating Networks

Pump sizing determines whether a district heating network can be run efficiently and economically. The flow rate follows from the network heat output and the temperature spread, while the pump head follows from the pressure losses along the most critical supply path. In a well-designed network, pump electricity consumption stays at 1—2 % of the transported heat. With unfavourable dimensioning, especially in low-temperature networks with small temperature spreads, it can climb to 5—10 % and put the economics of the whole project at risk.

Online calculator: sizing the network pump

This calculator is also available as a standalone page: district heating pump sizing calculator.

Enter the network heat output, the temperature spread and the pressure losses of the most critical supply path — the calculator determines the design operating point (flow rate and pump head), the hydraulic and electrical pump power, and estimates the annual electricity demand via equivalent full-load hours. If you also enter the annual heat delivered, it checks the pump electricity share against the 1—2 % guide value. The article below explains each calculation step.

Central vs. decentralised pump concepts

Central pump concept

In the central concept, one or more main circulation pumps sit in the heating plant and generate the differential pressure for the entire network. At the transfer stations, differential pressure controllers or control valves set the flow rate to each consumer.

Maintenance and monitoring stay in one place, the large pumps run at high efficiency, and the hydraulic structure is simple. The downsides are that the high differential pressure at consumers close to the plant has to be throttled away, and that the central pump must overcome every pressure loss in the network.

Decentralised pump concept

Here a circulation pump is installed at every heat transfer station, and each one overcomes only the local pressure loss plus its share of the network pressure loss. The network itself carries little or no centrally generated differential pressure. This concept is typical for passive cold district heating networks, where the brine circulation pumps built into the heat pumps provide the circulation.

There are no throttling losses, since each pump works only against the pressure loss it actually faces, which suits networks with widely varying distances to the plant. Set against this are the many individual pumps at lower efficiency, the greater maintenance effort, and the check valves needed to prevent backflow.

Hybrid concepts

In practice, hybrid approaches are common. A central pump generates a base differential pressure that covers most of the network losses, and additional booster pumps support the supply at distant consumers. This can lift the overall energy efficiency above a purely central concept.

Dimensioning: flow rate and pump head

Flow rate

The nominal flow rate of the network pump follows from the total heat load demand and the temperature spread between supply and return:

$\dot{V} = \frac{\dot{Q}_{\text{network}}}{\rho \cdot c_p \cdot (T_{\text{VL}} - T_{\text{RL}})}$

For a conventional network with 2 MW output and a spread of 30 K, this yields:

$\dot{V} = \frac{2000 \; \text{kW}}{1000 \; \text{kg/m}^3 \cdot 4.18 \; \text{kJ/(kg}\cdot\text{K)} \cdot 30 \; \text{K}} \approx 16 \; \text{l/s} \approx 57.5 \; \text{m}^3/\text{h}$

By comparison, a cold district heating network with the same output but only a 3 K spread needs a flow rate ten times higher, about 160 l/s. This is why pressure loss optimisation matters so much in low-temperature networks.

Pump head

The required pump head $H$ is composed of:

$H = \Delta p_{\text{network}} + \Delta p_{\text{generator}} + \Delta p_{\text{HTS}} + \Delta p_{\text{fittings}}$

Here, $\Delta p_{\text{network}}$ is the pressure loss of the most critical supply path (index point), meaning the hydraulically most unfavourable consumer, typically the one furthest away or the one with the largest pressure losses along its route.

For the pipe pressure losses, an approximate relationship is:

$\Delta p_{\text{pipe}} = R \cdot L$

with the specific pressure loss $R$ in Pa/m and the pipe length $L$ in m. Common design values for $R$ are:

Minimum pressure at the pump inlet (NPSH)

Besides the pump head, the pressure on the suction side has to be verified. The governing criterion is the NPSH condition: the NPSH available from the system must exceed the NPSH required by the pump by at least 0.5 m of water column. If that reserve is undercut, the water evaporates inside the suction port and the pump cavitates — even when the absolute pressure at every other point of the network is sufficient.

The available NPSH follows from the absolute pressure at the pump inlet minus the saturation pressure at the operating temperature. The pump inlet is therefore subject to the same lower limit as the high point of the network: saturation pressure plus 0.5 bar against evaporation and cavitation (broad professional consensus) and around 1.0 bar for control tolerance and transients (a planning recommendation), 1.5 bar in total. The derivation and the numbers are given in Meeting Operating Pressure Limits in Heat Networks.

What decides the case is where the pressure maintenance is tied in. With upstream pressure maintenance it sits on the suction side ahead of the pump and sets the inlet pressure directly, so the NPSH condition is usually uncritical. With downstream pressure maintenance the tie-in is on the discharge side, which leaves the suction side one pump head below the imposed pressure level — here the inlet pressure is the critical point and has to be checked separately. Both variants are covered in Pressurization and Expansion.

Index-point control and differential pressure control

Index-point control

Index-point control (differential pressure setpoint control at the index point) is the most efficient control strategy for network pumps. A differential pressure sensor is installed at the hydraulically most unfavourable consumer, and the pump speed is regulated so that the minimum differential pressure the heat transfer station needs for proper operation is always held at that point. It is typically 0.5 to 1.0 bar, with 0.8 to 1.0 bar as the planning value; the technical connection requirements (TAB) of the network operator are binding. The value is derived in Meeting Operating Pressure Limits in Heat Networks.

$\Delta p_{\text{index point}} \geq \Delta p_{\text{HTS,min}}$

This method dynamically adapts the pump output to the actual demand and avoids the over-supply of consumers close to the plant.

Constant pressure control

Alternatively, the differential pressure can be kept constant at the pump outlet or another central point. This is simpler to implement but consumes more energy, since the setpoint has to be designed for the maximum case with all consumers at full load. At part load, the excess pressure is then destroyed at the control valves.

Variable speed control and energy savings

Modern network pumps are operated with variable-speed drives that allow stepless speed adjustment. The relationship between speed and power consumption follows the affinity laws:

$\frac{P_2}{P_1} = \left(\frac{n_2}{n_1}\right)^3$

Reducing the speed to 80 % of nominal lowers the power consumption to $0.8^3 = 0.512$, about 51 % of nominal power. Since network pumps run at part load for most of the year, this saves considerable energy compared with unregulated pumps.

Energy demand of network pumps

Typical characteristic values

The pump electricity consumption of a well-designed district heating network is typically 1 to 2 % of the annually transported heat. In poorly dimensioned networks, above all low-temperature networks with small temperature spreads, this share can rise to 5 to 10 % or more.

The electrical pump power is calculated as:

$P_{\text{el}} = \frac{\dot{V} \cdot \Delta p}{\eta_{\text{total}}}$

with the total efficiency $\eta_{\text{total}}$, the product of hydraulic efficiency, motor efficiency and drive efficiency. For large network pumps, typical total efficiencies lie between 0.60 and 0.75.

Full-load hours approach

To estimate the annual pump electricity consumption, the full-load hours approach is often used:

$W_{\text{el,a}} = P_{\text{el,nom}} \cdot f_{\text{part-load}} \cdot t_{\text{operation}}$

The part-load factor $f_{\text{part-load}}$ accounts for the fact that the pump does not run at nominal power on an annual average. Typical values:

The equivalent full-load hours result from the product $f_{\text{part-load}} \cdot t_{\text{operation}}$ and typically lie between 1500 and 3000 h/a.

Practical recommendations

• Minimise pressure losses: generous pipe dimensioning (low specific pressure loss) saves more in pump electricity over the service life than the larger pipes add in investment cost.
• Use index-point control: depending on the network, the energy saving over constant pressure control is 20 to 40 %.
• Provide redundancy: at least two pumps, one running and one on standby, secure the supply during failure or maintenance.
• Use simulation: dynamic network simulation with software such as VICUS Districts computes pressure losses and pump operating points across the whole year.

Getting pump sizing right

Pump sizing is a building block of district heating planning that is often underrated. In conventional networks with a large temperature spread, the effects stay moderate. In low-temperature networks, and cold district heating in particular, an oversized pump system or an unfavourable control strategy can undo the economics of the whole project. The keys to efficient operation with minimum pump electricity are the right pump concept, a careful hydraulic design and variable-speed pumps with index-point control. Tools such as VICUS Districts compute the pump operating points dynamically and report the annual pump electricity consumption as part of the overall balance.

Further reading: Pressure Loss Calculation in District Heating Networks — determining head loss that the pump must overcome, Thermo-Hydraulic Simulation — dynamic analysis of pump operating points over the year, Sizing of Heat Transfer Stations — system integration at the consumer side, Economic Analysis According to VDI 2067 — accounting for pump lifecycle costs in the economic assessment, Pressure Profile and Pressure Diagram — the number behind the minimum differential pressure at the index point, Meeting Operating Pressure Limits in Heat Networks — the minimum pressure at the pump inlet.

References and Standards

• DIN EN 16297 — Pumps — Rotodynamic pumps — Glandless circulators — Requirements and tests
• AGFW FW 524 — Hydraulic Calculation of Hot Water District Heating Networks
• Europump & Hydraulic Institute (2004): Variable Speed Pumping — A Guide to Successful Applications. Elsevier.