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8. Chillers – Monsters on the roof

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Chillers often waste considerable of amounts energy, both directly and through their interaction with heating. Yet in most cases this is not identified or measured and substantial inefficiencies go unaddressed – whilst the chillers themselves are out of sight and out of mind on the roof or somewhere equally hidden.
In this issue of Intelligent Energy Insights we explain some of the reasons why chillers waste energy and what you can do to make yours more efficient.


It covers the following topics:

  • Chillers
  • Oversizing and part load performance
  • Isolation and sequencing
  • Maintenance

You might also like to consult the related Insights:

Air Conditioning Split Systems
Is your BMS Wasting Energy?
Managing Energy for Comfort

Click here to see all Intelligent Energy Insight topics

John Treble, Managing Director


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Many buildings require cooling, particularly if they are very large, accommodate energy intensive processes or have a requirement for humidity control. A common cooling medium is chilled water, usually provided from “packaged” chillers which are often located on the roof of the building. This may result in them being almost forgotten. These chillers, which may have cooling capacities of 1,000 kW or more (equivalent to an electrical input of over 250 kW), incorporate compressors, evaporators and condensers and quite often the primary circulating pump, all within a single package. Most sites have more than one chiller in the same way that they have multiple boilers, and many of the resulting problems are very similar.

Oversizing and part load performance

In the vast majority of cases, packaged chillers are installed as part of the ventilation and air-conditioning system, and in the UK the demands for cooling are seasonal. In the winter months there should be no requirement for cooling and some organisations not only switch off their chillers throughout the winter, but actually drain-down the chilled water systems as well. This is recommended as it can save a fortune just in the reduced use of energy for frost protection, aside from other benefits. However, the variable nature of the cooling load means that chillers really need to be able to operate effectively at part load. The requirement for this is exacerbated by the fact that most chiller installations are massively over-sized for their duty. Very often a single chiller will provide sufficient cooling in all conditions – with two additional units merely complicating the control process. It is therefore very important that the chiller has the capacity to operate at a variety of loads, and this will usually be through the incorporation of a number of separate compressors. However, some big chillers will have only two or three, and in this case part load performance is likely to be poor. The result will be frequent short-cycling of the compressors, which is both inefficient and detrimental to the longevity of the plant.

Isolation and sequencing

Chillers are often “plumbed” in parallel, usually in a “reverse return” configuration designed to equalise the flow through them. However, even in a correctly sized multiple-chiller installation, there will not be a requirement for more than one chiller to operate for the vast majority of the time, and this results in the problem of “dilution”. In a three-chiller system only one third of the circulation is actually being cooled. The installation of automatic isolating valves is a pre-requisite for optimal operation, and there is no better time to fit them than during the winter. The BMS must then be programmed to sequence the chillers according to the cooling load. On many sites, all chillers bar one could be manually isolated for much of the year, and the set point for the remaining chiller may be increased by several degrees, resulting in much more efficient chiller operation.


A comprehensive maintenance regime is essential to the preservation and optimisation of assets such as chillers that are both expensive to buy and operate. Refrigeration plant is extremely robust and, as it is often considerably over-sized, it will continue to do the job even when it is actually in an awful condition. Any money saved through skimping on maintenance will, in the longer term, be lost many times over through additional expenditure on energy, replacement compressors and other components, and eventually new plant. In particular, any refrigerant leaks that occur should be dealt with immediately as the energy use of this plant will increase dramatically if a circuit is undercharged. The corresponding compressor may end up running continuously, achieving very little except extended running hours and considerable electricity use.

Your Independent RISK FREE Solutions

The Green Consultancy will be pleased to review all of the above issues as part of an EPBD Compliance Plus Air Conditioning Inspection or one of our more general services to identify energy efficiency opportunities.

For an investigation of all equipment controlled by your BMS you might like to consider a BMS Health Check or a BMS Audit.

For more information click the above links or call John Treble on 01761 176300, or email John@GreenConsultancy.com

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Air Conditioning Inspections for Berkshire Healthcare NHS Foundation Trust

“I found the process from receiving Air Conditioning Inspection quotations, to receiving the final reports, was managed very effectively by the Green Consultancy. Once our order was sent out, my further input was just how I like it – minimal.”

Nigel Lloyd, Senior Estates Manager

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Air Conditioning Inspection for EWAB Engineering

"Your service was quick, efficient and operated by friendly, polite and knowledgeable staff. Some of the recommendations highlighted in the report may be implemented across the business."

Robert Bradburn, Quality Representative & Build Engineer

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Air Conditioning Inspections for PPP-Infrastructure Management (Compass)

“The whole process of Inspections worked well across all sites and communication was maintained whenever any potential problems occurred. We are currently reviewing all of the recommendations to look at what will be implemented.”

Steve Gayter, Operations