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Air conditioning split systems

In many buildings air-conditioning or “comfort-cooling” is provided by “split” systems which can also be used for heating and humidity control. These systems are often installed and operated in an uncontrolled way with many organisations not even having a proper record of the units they have installed.

The potential for these units to be wasting a significant amount of energy is high – sometimes with cooling fighting excessive heating to make matters even worse. Due to the localised nature of the units and the lack of control over them, the amount of energy being used and wasted is frequently hidden from sight. A well thought out plan to minimise energy costs must include split systems.

This article explains what split systems are and how to reduce the energy they waste.

It covers the following topics:

  • Split Systems
  • Control and behaviour change
  • Maintenance
  • Your effort rewarded

You might also like to read these related articles:
Chillers – Monsters on the roof
Is your BMS Wasting Energy?
Managing Energy for Comfort

Split Systems

Split systems have an evaporator in a cassette usually mounted flush with a suspended ceiling and a separate external unit accommodating the compressor and condenser. In some systems the internal unit takes the form of a wall mounted “fan-coil” type unit, or is concealed above the ceiling and discharges through grilles. For all systems the principles are the same.3450_spilt-system - - becky

These units are common and numerous and can easily pass under the radar when considering energy efficiency, but even the smallest units are likely to be rated at least 3 kW and larger units can have a maximum electrical input of 12 kW with a cooling capacity of 40 kW or more. Some units are “heat-pumps” capable of heating as well as cooling. Although the compressors should not usually run continuously, there are circumstances under which they will, in which case their running costs could range from about 30p to over £1/hour, or between £3,000 and £9,000 pa in extreme conditions. For estates fitted with large numbers of these the capacity for wasting energy is substantial. To minimise this risk the following issues must be addressed.

Control and behaviour change

In many aspects of energy efficiency, control is the critical factor and this is one of the most common issues with these units. The majority have “stand-alone” controls with either a wall-mounted programmer or a portable remote control device. Many incorporate timers, although rarely set for sensible operation, and a common factor is that operators have no idea how the systems work, or even what they do. A useful starting point is to provide simple and clear instructions to the users on how to operate the units. These instructions could include an indication of running costs.

If there are a large number of units a more comprehensive approach should be considered. The precise solution will vary from site to site. In many buildings lighting is controlled on presence detectors, and controlling the operation of cassette units using the same detectors is a possibility. The potential savings from controlling cooling are often greater than for controlling the lighting. For sites which have a Building Energy Management System (BMS), imposing control over the split units using the BMS is the ideal solution. This will allow units to be both scheduled to prevent unnecessary operation and controlled on ambient temperature – for example preventing operation below 24oC.

This has the added benefit that it can often prevent simultaneous heating and cooling. Split systems are often retro-fitted where there is already a heating system in place, usually conventional radiators. These frequently operate at the same time as the cassettes, resulting not only in excessive energy use but also discomfort due to extremely low humidity - avoiding simultaneous heating and cooling with these systems should be a high priority.


Maintenance costs are obvious expenditure, whilst energy consumption is effectively invisible, and it is tempting to save money by reducing or eliminating maintenance on split systems. This will always be a false economy in the longer term, as the units will end up using additional energy, and reduced maintenance will adversely affect their longevity as well. Aside from the mechanical operation of the units, there are two really important aspects to maintenance: the cleanliness of heat exchangers and air-filters, and the detection and remediation of leaks.

Dirty heat-exchangers reduce the dissipation of heating/cooling and diminish both the effectiveness of the units and their efficiency. Refrigerant leaking from a system is usually extremely serious, as even a small reduction in the volume of gas will result in a considerable increase in energy use. If the loss is great enough, the compressor will run continuously whilst failing to deliver the cooling (or heating) required.

It is largely due to the critical nature of these aspects that air-conditioning inspections are now a legal requirement (see below), but these inspections do not in any way reduce the importance of frequent and regular maintenance as well as, if possible, more frequent checks by building operators.

Your effort rewarded

Split Systems may be a relatively invisible part of your energy consumption but they could be quietly making a significant contribution to your energy costs. Getting to grips with the energy they are wasting requires a proper inventory and the active participation of building users as well as correct control and maintenance. However the effort made will deliver a noticeable reduction in energy costs as well as a longer life for the units – well worth doing.

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

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