How does commercial air conditioning work?

What does an air conditioner actually do?

It moves heat from one place to another. It does not make cold. The ecodesign rules define an air conditioner as a device that cools or heats indoor air "using a vapour compression cycle driven by an electric compressor". They describe the cold-generating part of a cooling product as one that lets heat be extracted from the indoor space to be cooled and transferred to a heat sink, "such as ambient air, water or ground".

In a commercial building the heat source is the occupied space and the heat sink is usually the outdoor air. The same definition covers units that add "dehumidification, air-purification, ventilation or supplemental air-heating", so a commercial system is rarely just a cooler.

What happens at each stage of the cycle?

Refrigerant circulates round a closed loop. Follow it round and the parts of any system, from a single split to a large chiller, make sense:

1. Evaporator. In an air conditioner, the indoor side heat exchanger (the evaporator) extracts heat from the air in the space. The rules also note the "condensate water that is formed on the evaporator side", which is why every system needs a drain.
2. Compressor. In the Energy Technology List's words, the compressor "compresses the refrigerant gas from the low pressure of the evaporator to a higher pressure so that it can condense in the condenser".
3. Condenser. Here the heat leaves the system. In an air-to-air air conditioner the outdoor side heat exchanger (the condenser) allows heat transfer to air. The Energy Technology List adds that condensers "liquefy the refrigerant gas discharged by the compressor".
4. Expansion valve. The refrigerant is "condensed for return to the evaporator as a high pressure liquid", and the Energy Technology List describes evaporators with "expansion valve control". The valve sits between condenser and evaporator, and the cycle starts again.

A rooftop air conditioner is the same loop in one box: the ecodesign rules define it as a unit whose evaporator, compressor and condenser are integrated into a single package.

What do you actually see on site?

The cycle shows up in a small number of physical parts. A survey or specification should name every one of them.

Part you will see	What it is	Where it sits in the cycle
Indoor unit or units	The unit or units in the space being cooled; formats are compared in cassette, ducted and wall-mounted units compared	Evaporator side
Outdoor unit	The Energy Technology List calls the outdoor and indoor units of a split system "separate heat collection and rejection units"	Condenser side
Refrigerant pipework	The insulated pipes joining the two. The Energy Technology List says split-system units are "connected together during installation by refrigerant pipework"	The loop between the stages
Condensate drain	Carries away the condensate water that forms on the evaporator side	Evaporator side
Controls	Defined in the ecodesign rules as equipment that interfaces with the end user on the values and timing of the desired indoor temperature	Sets when and how hard the cycle runs

How does the same cycle scale to multi-split, VRF and chilled water?

The physics does not change as systems get bigger. Only the arrangement does.

• Multi-split. The ecodesign rules define a multi-split air conditioner as one incorporating more than one indoor unit, with one or more refrigerant circuits, compressors and outdoor units. The indoor units "may or may not be individually controlled".
• VRF. The Energy Technology List describes one outdoor unit connected to one or more indoor units "using a common refrigerant circuit with the indoor units individually controlled". That is the feature that lets one system serve many zones.
• Chilled water. A comfort chiller runs the same cycle but cools water rather than air. Its evaporator extracts heat from a water-based cooling system and its condenser releases that heat to ambient air, water or ground. Fan coil units and air handling units then use the chilled water to cool the air in the building.

Which arrangement suits a building depends on the number of zones, the space available for plant and the volume of refrigerant the design involves. The commercial air conditioning overview compares the options, and what HVAC covers shows where air conditioning sits among the other building systems.

Why does variable capacity matter?

Buildings are rarely at their design load. The ecodesign rules classify a unit's capacity control by how far it can change the refrigerant flow rate: "fixed" if it cannot, "staged" for not more than two steps, and "variable" for three or more steps.

The efficiency test then accounts for the "effects of the degradation of the energy efficiency caused by on/off cycling". Seasonal efficiency is defined as representative "for the whole cooling season" rather than for one full-load point. That is why seasonal figures are the sensible basis for comparing units, and how to read SEER, SCOP and ratings explains the numbers.

Can the same system heat as well as cool?

Many can. A reversible air conditioner is one capable of both cooling and heating, and the Energy Technology List explains that some products provide cooling "by reversing the refrigeration flows around the product". Heating is a use of the same cycle, and stationary heat pumps are on the same F-gas leak-check list as air conditioning. This page stays with cooling.

What does the refrigerant do, and why is it regulated?

Refrigerant is the working fluid that carries the heat round the loop. It is regulated because of what happens if it escapes. The ecodesign rules measure global warming potential as how much 1 kg of the refrigerant "is estimated to contribute to global warming". Government guidance on air conditioning inspections says F-gases are "powerful greenhouse gases, with environmental impacts many times greater than that of carbon dioxide", and that the most common types, HFCs, "are often used as the refrigerant in air conditioning systems".

That is the basis of the F-gas duties. Users of stationary air-conditioning equipment are responsible for getting it checked for leaks, and that responsibility is shared between the users of the equipment and the service technicians who install, maintain or dispose of it. Equipment holding F gas equivalent to 5 to less than 50 tonnes of carbon dioxide must be checked at least once every 12 months. If a leak is found, it must be repaired as soon as possible and the test repeated within a month. The refrigerants and GWP table lists the common refrigerants rather than repeating them here.

What should a buyer check in a system description?

• Which system family is proposed, and why it suits the number of zones and the building.
• Where the outdoor unit, pipework and condensate route go, and who owns that space.
• The refrigerant type, so the leak-check duty and the cost of future top-ups are understood.
• Whether capacity control is fixed, staged or variable, and the declared seasonal efficiency.
• How the controls will zone the building and whether they link to a building management system.

Readers weighing these options can read the commercial air conditioning guide or send an enquiry through the site's form.

Sources

• Legislation.gov.uk (Crown), Commission Regulation (EU) No 206/2012, article 2, annex I and annex II, as retained in Great Britain, checked 22 October 2026.
• Legislation.gov.uk (Crown), Commission Regulation (EU) 2016/2281, article 2 and annex I, as retained in Great Britain, checked 22 October 2026.
• DESNZ, Energy Technology List: refrigeration equipment and air to air heat pumps, split, multi-split and VRF, checked 22 October 2026.
• GOV.UK (MHCLG), A guide to air conditioning inspections, checked 22 October 2026.
• GOV.UK (Environment Agency and Defra), Checking F gas equipment for leaks, checked 22 October 2026.

Common questions

Does air conditioning create cold air?

No. It extracts heat from the indoor space and transfers it to a heat sink such as the outdoor air, which is how the ecodesign rules describe the process. The air leaving the indoor unit is cooler because heat has been taken out of it.

Why does an air conditioner have a drain pipe?

Water condenses on the cold indoor coil, and the ecodesign rules refer to this as condensate water formed on the evaporator side. The drain or pump carries it away. A blocked or badly falling drain is a common reason for water appearing at an indoor unit.

Is the refrigerant the same in every system?

No. Refrigerants differ in global warming potential, and some programmes set limits: the Energy Technology List only accepts split, multi-split and VRF products that use a refrigerant with a GWP of 750 or less. Check the refrigerant named on the equipment before agreeing a specification.

Is a chiller a different technology from a split system?

It uses the same vapour compression cycle. The difference is what the evaporator cools: a split system cools room air directly, while a comfort chiller cools water that is then piped to fan coil units or air handling units.

Related guides

• How to choose energy-efficient commercial air conditioning
• The Future Buildings Standard: what Part L 2026 changes for HVAC
• Through-wall air conditioning units: when they suit a business
• Absorption chillers: when waste heat can drive cooling