A small portion of the energy required to operate a heating network is used to circulate the heat transfer medium. This energy expenditure is referred to as pump work. This page explains how pump work is calculated in nPro and what assumptions are made.

Centralized and decentralized pump concepts

Pump work refers to the annual energy expenditure needed to circulate the heat transfer medium through the heating network. The water circulation can be achieved in two ways:

central circulation pump, and
decentralized circulation pumps.

The first method uses a central circulation pump located in the energy center. This method is commonly used in larger heating networks due to its higher efficiency and better controllability. The second method achieves circulation of the heat transfer medium using decentralized pumps installed in the buildings. This method is particularly common in 5GDHC networks, where the circulation pumps of the decentralized heat pumps can be utilized.

Assumptions for calculating pump work

The calculation approach is based on the method described on page 134 ff. of the District Heating Planning Manual by EnergieSchweiz/Federal Office of Energy BFE. In this method, the nominal pump capacity is multiplied by the full operating hours of the heat consumers. The full operating hours of the heat consumers are determined by dividing the total heat demand by the maximum heat output. The nominal pump capacity is calculated based on the maximum pressure loss the pump must overcome and the maximum flow rate. The following formula is used:

\[\text{Maximum pump capacity} = \text{Total pressure loss} \times \text{Flow rate}\]

The pump work over the entire year is then calculated using the following formula:

\[\text{Pump work} = \frac{\text{Maximum pump capacity} \times \text{Full load hours}}{\text{Pump efficiency}}\]

A hydraulic and electrical pump efficiency value of 75 % is recommended in the planning manual, and this value is also used in nPro.

Pump work for central circulation pump

For a central circulation pump, it is assumed that all pump work is provided by a single pump located in the energy center. The determining factor for maximum pump capacity is exclusively the pressure loss in the path of the critical network point (from the energy center through the supply line, the heat transfer station, and the return line). For calculating full load hours, the heat or cooling demand of all consumers connected to the network must be considered and divided by the maximum required output at the energy center. The resulting full load hours can then be multiplied by the maximum calculated pump capacity to determine the annual energy demand for pump work.

Pump work for decentralized circulation pumps

When calculating pump work for a heating network with decentralized circulation pumps, the pump work for each connected building must be calculated individually and then summed up across all buildings. The maximum pressure loss between the energy center and each connected building is calculated, which includes pressure losses in the energy center, pipes, fittings, and bends. Based on these calculations, the maximum pump capacity for each building can be determined. Subsequently, as with the central circulation pump, the full load hours for each building are calculated. These are multiplied by the maximum pump capacities and summed across all buildings. The result is the annual electricity demand for network circulation pumps.

What is the critical network point?

The critical network point represents the critical location in a heating network where the maximum pressure loss occurs. This point is crucial for calculating pump work, as it determines the performance of the central pump.

Relative electricity demand

The relative electricity demand of the network pump indicates the ratio of pump work to the total delivered heat or cooling energy. It is typically between 1 and 2 %. The AGFW worksheet FW 401-1 (German guideline) specifies a value of 1.5 %.