Evaporative cooling or air conditioning for a business?
Updated 23 October 2026 · SEO Dons Editorial
Evaporative cooling and air conditioning both lower the temperature of a space, but they work differently and fail differently. The choice turns on how humid the air is, whether the building can ventilate openly, and how tightly the temperature must be held.
How does each one cool a building?
HSE describes evaporative coolers as passing hot air over water-saturated pads, so that evaporation reduces the air temperature, giving a moderate reduction in temperature and an increase in humidity. CIBSE Journal separates two types. Direct evaporative cooling adds moisture straight into the airstream and uses a wetted medium, typically inside an air handling unit. Indirect cooling uses a secondary airstream and a heat exchanger, so no moisture is added to the supply air, but it is less efficient and suits buildings that need humidity control, such as laboratories.
The government's inspection guidance covers air conditioning systems where refrigeration provides the cooling. A refrigeration circuit is not tied to the outside air's wet-bulb temperature in the way evaporation is.
Two naming traps. A portable room "air cooler" is the same pad-and-fan idea at small scale, so HSE's description applies to it. And HSE uses "adiabatic" for hybrid dry/wet coolers, which serve the circuit that rejects heat, not the room air; the cooling towers versus dry coolers guide covers those. If a quote says adiabatic, ask which job it does.
What limits evaporative cooling in the UK?
The Met Office says wet bulb temperature is the lowest temperature that can be achieved through evaporative cooling, and that the greater the gap between dry bulb and wet bulb, the drier the air. CIBSE adds that humid climates limit effectiveness and that, even in suitable climates, evaporative cooling usually needs some mechanical cooling alongside it. In its modelling for London, only a small proportion of hours fell in the zone where evaporative cooling is ineffective. The hottest days are not always the driest: the Met Office notes that warm weather with an airflow from a moist source can be particularly humid.
CIBSE says the saturation efficiency of the wetted media determines how close to saturation the air becomes. Supply temperature is therefore the dry-bulb temperature minus the effectiveness multiplied by the dry-bulb to wet-bulb gap. The days below are illustrations, not UK records, and assume 80% effectiveness.
| Illustrative day | Dry-bulb (°C) | Wet-bulb (°C) | Supply air at 80% (°C) | Drop (°C) |
|---|---|---|---|---|
| Hot and dry | 30 | 19 | 21.2 | 8.8 |
| Warm and mixed | 26 | 19 | 20.4 | 5.6 |
| Hot and humid | 30 | 24 | 25.2 | 4.8 |
On the humid day the same cooler gives under 5 degrees of cooling and delivers wetter air. A proposal should state the wet-bulb temperature it was designed for.
How do running cost and water use compare?
This section stays in kWh and litres, because tariffs vary. Refrigerant air conditioning uses electricity equal to the cooling delivered divided by its seasonal efficiency ratio. For example, 100,000 kWh of cooling a year needs 33,333 kWh at an assumed ratio of 3, 25,000 kWh at 4, and 20,000 kWh at 5. The ratios are assumptions; use the data sheet.
An evaporative unit's electricity goes mainly on fans and a pump. CIBSE says the extra fan power needed to push air through the media must be counted, and that in its 2017 modelling of an office with a central variable air volume system, direct evaporative cooling gave a 28% saving over the baseline in London, while in Hong Kong the added fan power made it more energy intensive than the baseline. That was an office with air handling units, not a warehouse, so read it as a sign that climate decides the result.
Water is the other meter. CIBSE says engineers should consider the water consumption of any evaporative scheme and that many examples will probably use more water. Ask for litres per hour at design conditions.
What ventilation does evaporative cooling need?
Direct evaporation puts the water it uses into the air (CIBSE) and raises humidity (HSE). It follows that moist air must have a way out: doors, louvres or relief vents. In a sealed room the humidity would build. CIBSE also notes that the media can over-cool the air unless a bypass damper or similar control is used, and that indirect systems can use exhaust air as the secondary stream. Ask for a drawing that shows the air path in and out.
What legionella and inspection duties apply?
HSE says any water system is potentially a reasonably foreseeable legionella risk if it has certain factors, including a means of creating or spreading breathable droplets, water that is stored or recirculated, and a temperature between 20 and 45°C. For a cooling system other than a tower or evaporative condenser, HSE says other measures must be as equally effective. A pad cooler with a recirculating sump should therefore be in the water risk assessment; see can air conditioning spread legionella and the cooling tower legionella requirements.
Inspection is less clear. The 2012 energy performance regulations define an air-conditioning system as the components needed for air treatment in which the temperature is controlled or can be lowered, with no mention of refrigeration. The government's guidance says it is intended to cover systems where refrigeration provides cooling. A large evaporative system sits in that gap, so ask an accredited energy assessor; the TM44 guide explains the 12 kW trigger.
When does each one win?
| Situation | Better fit | Reason |
|---|---|---|
| Large ventilated workshop or warehouse, tolerant staff | Evaporative cooling or HVLS fans | A moderate temperature drop may be enough, and air can leave easily |
| Packing station, control room or office needing a set temperature | Air conditioning for that space | Not limited by wet-bulb temperature |
| Stock or process with a temperature or humidity limit | Refrigeration or air conditioning | Evaporative cooling adds humidity |
| Humid spells with fixed comfort targets | Air conditioning | The evaporative drop shrinks as wet-bulb rises |
The warehouse air conditioning page explains why cooling a whole shed is rarely the right brief. Readers who want to compare options for their own building can send an enquiry through the form on this page, and air conditioning costs sets out what drives the refrigerant side.
Sources
All pages checked 22 October 2026.
- HSE: Thermal comfort; Other systems that can create a risk of legionella (updated 26 September 2024); Evaporative cooling systems (updated 14 January 2026); Dry/wet cooling systems (updated 30 September 2024), checked 22 October 2026.
- Met Office: What are wet bulb temperatures (7 August 2025); Why has it been so humid recently? (15 September 2026), checked 22 October 2026.
- CIBSE Journal: Making the case for evaporative cooling (June 2017), checked 22 October 2026.
- GOV.UK: A guide to air conditioning inspections in buildings; legislation.gov.uk: Energy Performance of Buildings (England and Wales) Regulations 2012, regulation 2, checked 22 October 2026.
Common questions
Is an air cooler the same as an air conditioner?
No. An evaporative air cooler passes air over wet pads, which HSE says gives a moderate reduction in air temperature and increases humidity. An air conditioner uses a refrigeration circuit. The first suits ventilated spaces and tolerant users, the second suits spaces that need a set temperature.
Does evaporative cooling work on humid days in the UK?
Less well. The Met Office says wet bulb temperature is the lowest temperature evaporative cooling can reach, and CIBSE says humid conditions limit its effectiveness. The cooler still lowers the temperature a little, but the gap between dry-bulb and wet-bulb shrinks, and the air it delivers is wetter.
Does an evaporative cooler need a legionella risk assessment?
The duty holder has to assess water systems on the premises. HSE's tests include water that is stored or recirculated and a means of creating breathable droplets, which a recirculating pad cooler can meet. Ask the supplier for the design details so the assessment can be completed.
Is adiabatic cooling the same as evaporative cooling?
HSE uses 'adiabatic' for hybrid dry/wet coolers, which serve the circuit that rejects heat rather than the room air. A quote that uses the word could mean either, so ask what the unit cools and where the water goes.
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