As a heat network consultancy, we have a lot of experience in plant room control on both new build and legacy developments. As a result, we often see common control issues having large impacts on the efficiency and reliability of the heat supply.

These issues should be picked up at the commissioning stage via description of operation reviews and plant room demonstrations. Taking the plant room from minimum to peak load in a controlled load test is a key step to identify and rectify control issues that would otherwise only arise during occupancy.

Fortunately, a lot of the common control issues have simple solutions that can be implemented on the building management system Building Management System (BMS) panel to provide an instant boost to plant room performance. With the arrival of the heating season, there may be a number of easy alterations that can be made to reduce energy consumption and operating costs with robust controls solutions that ensure a consistent and low-cost supply to residents.

This blog piece will cover the most common plant room control issues and the simple BMS adjustments that developers and maintenance teams can make to rectify them.

1. Pumps

The electricity consumption of pumps (both shunt and network) is a significant contributor to the ongoing operating costs of heat networks. Pumps are commonly found operating ‘in hand’ or with a fixed speed setting – leading to pumps constantly running at high speeds regardless of the system load.

There are four main operating modes for pumps they are, in order of least to most efficient:

  • Hand – the pump will always operate at 100% speed
  • Fixed speed – the pump will always operate at a high speed capable of achieving peak load
  • Local differential pressure – the pump will maintain the peak differential pressure across the pump, operating at relatively high speeds at all loads
  • Index run differential pressure – the pump will operate to maintain the minimum differential pressure to serve the network at all loads

Adjusting the pump control methodology down this list is a very quick way to significantly reduce pump electricity consumption – especially if index run control is possible. It is essential when selecting a differential pressure control method that the correct set point is selected, providing further reductions in electricity consumption. Furthermore, reduction in pump speeds at low system loads can enable the closure of network bypasses, reducing return temperature and further reducing pump energy consumption.

Proposals on recent site audits have enabled pump energy reductions of over 90% by changing from a fixed speed approach to an index run approach.

Scheme

Scheme size

No. of pumps

Reduction in electricity cost (pa)

% reduction

Scheme 1

182 units

5

£3,050

67%

Scheme 2

181 units

6

£24,650

96%

Scheme 3

346 units

6

£12,720

33%

Scheme 4

164 units

8

£13,810

34%

Scheme 5

205 units

6

£7,760

80%

Scheme 6

791 units

20

£20,720

31%

Scheme 7

182 units

14

£12,490

78%

2. Boilers

Boilers are often incorrectly sequenced, leading to high boiler run hours, high gas consumption and poor consistency of heat delivery to residents.

To avoid these issues a complete boiler sequencing methodology should be implemented, utilising thermal store temperature sensors where possible, with regular rotation of the lead boiler. This will reduce wear on all boilers and significantly increase the efficiency and consistency of supply. Whilst this is likely to require a Building Management System (BMS) engineer to implement, the savings far outweigh the low initial costs.

An example of the difference between the flow temperature stability on a site with a 72 °C set point before and after BMS works to rectify boiler sequencing can be seen below. 

plant room controls

The constant cycling of the boilers seen before the works was significantly increasing the wear of the boilers – leading to early failure and increased maintenance costs. Furthermore, the large variations in flow temperature would make it difficult to reduce network flow temperatures in the future without causing issues with the supply of heating and hot water to dwellings.

3. Plate Heat Exchangers (PHEs)

Control over the operation of PHEs can have a huge impact on plant room return temperatures and, thus, generation efficiency. A well performing network with low return temperatures (c. 30-40 °C) can still have high plant room return temperatures, leading to boiler inefficiency and high pump energy consumption if PHE control is implemented poorly.

Usually, the best way to control a PHE is to control the primary flow rate with a control valve on the primary side to maintain a secondary flow temperature set point. However, on occasion the control valves are left wide open or controlled poorly, causing the PHEs to act as high flow rate bypasses within the plant room.

An example of this can be seen in the graphs below, where a 206-unit residential scheme in poplar was experiencing high primary return temperature despite a well performing secondary system. In this example the set point for the secondary flow temperature had been set to the same value as the primary flow temperature set point, thus any fluctuation in primary flow temperature caused the PHE control valve to open completely and pass very high flow rates through the PHE. The second graph shows the same system after a reduction in secondary flow temperature set point, enabling consistently low primary return temperatures representative of the network performance.

plant room controls
plant room controls

4. Building Management System (BMS) Housekeeping

Implementing a defined control strategy for each piece of equipment can only be realised if the BMS data is well structured and clearly organised.

Since issues with the controls logic can arise during original commissioning or as a result of subsequent maintenance alterations, it’s crucial that the current performance of the system is visible and can always be understood. Consequently, we would recommend the following housekeeping principles to ensure the security and visibility of BMS systems:

  • A secure BMS login should be enabled so that the operator is always aware of any subcontractor’s usage of the BMS and changes made. Any changes to the controls should also be recorded in a plant room logbook.
  • Sensor and trend labelling should be checked and verified to ensure the controls are targeting the correct set points on the correct sensors. Incorrect sensor labelling is an easy mistake to make and can lead to a variety of performance issues.
    As can be seen from the below BMS images, two sensors have been labelled as the same return temperature but do not record the same data
BMS
BMS
  • Having the ability to view live graphical analysis via the BMS interface (as above) provides visibility and enables the user to see if the system performance is stable. While a BMS screen provides worthwhile visibility at low cost, a better option would be to have a remote connection to the BMS for long term management and data logging, making any unexpected changes to the system easy to spot and diagnose.
  • Having the controls checked by a BMS engineer can provide a lot of insight into performance issues if your system has little visibility. We have often found that pump and boiler set points have been increased from their original values as an easy response to providing constant heat in response to a complaint. However, this can significantly increase the energy consumption and cost of delivering heat to residents.

Our recommended solution for new build projects

Addressing simple BMS changes such as the above can have a huge impact on energy consumption, carbon emissions, heat costs and resident satisfaction.

Issues such as the above can be avoided entirely by proper design and commissioning at the build stage. From personal experience, we have found that a full DesOps review and plant room demonstration (including load tests) are crucial to ensure compliance with the design intent and maximising the efficiency of the installed equipment.

If you have any other control tips or examples where controls changes had a big impact on system performance, please comment below or drop us a message to discuss.