We use cookies to give you the best possible experience on our website. By continuing to browse the site you are agreeing to our use of cookies. Cookie policy

Chillers – Monsters on the roof

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.

This article explains 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 read these related articles:

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


3450_10-img-165x227-10 - - becky

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

A Green Consultancy Investment Grade Energy Efficiency Audit will identify and prioritise all cost-effective energy saving opportunities in cooling and all other aspects of your energy consumption. For an investigation of all equipment controlled by your BMS you might like to consider a BMS Health Check or a BMS Audit.

Put us to the test NOW! To discuss your requirements please call John Treble on 01761 419081 or email John@GreenConsultancy.com

To see all other articles please click here

4_southampton-solent-logo-165x227 - Peter Hawtin - peterh

Investment Grade Energy Audit of New Building for Southampton Solent University

“Enthusiastic and knowledgeable energy auditor, flexible in his approach and report produced well.”

Martin Birt, Estates Energy Manager

Thames Valley Police

Investment Grade Energy Audits and Condition Surveys for Thames Valley Police

“As the result of a rigorous tendering process, Thames Valley Police commissioned The Green Consultancy (TGC) to carry out Investment Grade Energy Audits of three large police stations.

We were impressed with TGC’s approach to the brief, the service, the professionalism shown in developing the project with us, and the audits carried out.

In fact their work is "light years ahead” of any energy consultancy that we have previously experienced. Their forensic

continued ...

Thames Valley Police

continued ...

analysis of data coupled with their Energy Efficiency Methodology identified many major cost-saving opportunities – in particular, relating to poor BMS control and the massive over-sizing of plant by the M&E designers of the buildings.

After a meeting of our estates team with TGC to discuss the outcomes, we now have a robust energy-saving strategy for the buildings.

Following a further tendering exercise TGC, is now providing Condition Surveys combined with Investment Grade Energy Audits of more buildings to enable us to decide which to keep and how best to minimise costs and carbon emissions.

Neil Wickham, Energy Manager

East Staffs Borough Council

Energy Audit for East Staffordshire Borough Council

“I was extremely impressed and satisfied by the thoroughness and the sound scientific approach of the work that The Green Consultancy has undertaken for us. I look forward to continuing to work with them on implementing their suggestions and have no hesitation in recommending their services.”

Matt Bancroft, Brewhouse Technical Manager

3450_b26-165x227-hadlow-01 - - becky

Energy Efficiency Audit for Hadlow College

“This was our second energy audit so the obvious ‘quick wins’ had already been identified and implemented where financially viable. The consultant understood our constraints and demonstrated a practical and knowledgeable approach to energy saving. We were impressed and grateful that he was able to quantify further valuable projects, amounting to a further 25%cost savings, and recommend ones that were suitable for the latest round of SALIX loans, which was very timely.”

Sue Brimlow, Business & Sustainability Co-ordinator

Small Woods

Energy Audit for Small Woods Association

"We have benefited greatly from The Green Consultancy’s expertise and flexibility. We are still in the early stages of working through the recommendations, we are however confident that we will have better functioning energy systems as a result of your support."

Ian Baker, Chief Executive Officer

3450_nhs-box-165x227-nhs-01 - - becky

Energy Efficiency Audit  for Nottingham University Hospitals

“Thank you very much for the Energy Efficiency Audit Report.  I am very pleased with the very high standard of work achieved.  It is the best energy report I’ve seen – and I’ve seen a large number of them.”

Neil Alcock, Environmental Manager

3450_b29-165x227-khartw-01 - - becky

Energy Efficiency Audit for K Hartwall

"The Green Consultancy investigated the potential for reducing of our energy consumption associated with air extraction. Variable Speed Drives were recommended, installed and are operating well. Because of the proven savings another VSD will be installed. The payback on 28% savings is less than two years so it is good financially as well as environmentally."

Stuart Richardson, Quality Manager

3450_3450_dmgt - - becky

Energy work for Daily Mail and General Trust

“The work undertaken for DMGT demonstrates the very high technical aptitude of The Green Consultancy within the area of energy management.”

Dr Rick Stunt, Group Paper Director

See case history of our work for DMGT