Free cooling chillers: how many hours each UK region offers
Updated 23 October 2026 · SEO Dons Editorial
What is a free cooling chiller?
Free cooling means using cold outdoor air to cool the chilled-water loop so the compressors can switch off or unload. One manufacturer of free-cooling units describes the sequence: when the ambient air is 1 °C below the process fluid return temperature, a three-way valve sends the return water through a free-cooling coil for partial free cooling, and as the air gets colder the chiller's load falls progressively until the air removes all the heat.
It comes in two forms: a chiller with the coil built in, or a separate free-cooling unit working in series with a chiller. The same manufacturer says a standalone system can work alongside an existing chiller, and that an integrated system needs colder air, usually in the region of 2 °C, to reach full free cooling, against 5 °C for standalone systems. Daikin Applied UK lists air-cooled free-cooling chiller ranges of 252 to 1,012 kW (scroll) and 150 to 1,950 kW (screw). CIBSE Journal adds that a mechanical cooling system will normally still be installed, because the right weather cannot be assured.
The government's Energy Technology List (ETL) sets a test for integral free cooling: the control must maximise its use for outside air dry-bulb temperatures between 2.0 and 15.0 °C. For the machines themselves, see the types of chillers guide.
What outdoor temperature does free cooling need?
It depends on the loop. The manufacturer's rule is that full free cooling arrives when the air is 5 °C below the supply temperature, so a warmer loop gets more hours. CIBSE Journal makes the same point for data centres: the warmer the facility water supply temperature, the less mechanical cooling is required and the greater the potential for free cooling. The table in the next section counts hours at four thresholds.
- At or below 2 °C: about where an integrated free-cooling chiller reaches full free cooling.
- At or below 5 °C: about where a standalone free-cooling unit reaches full free cooling.
- At or below 10 °C: partial free cooling is likely for a loop returning at about 11 °C or more.
- At or below 15 °C: the top of the ETL's control range, relevant to warmer loops.
How many free cooling hours does each UK region offer?
The table gives modelled hours in an average year, rounded to the nearest 100. They are illustrative figures from a simple method described below, using one Met Office climate station per region, and they count temperatures in whole degrees. Each region name links to that region's page.
| Region | Met Office station | January mean daily max / min (°C) | Hours at or below 2 °C | Hours at or below 5 °C | Hours at or below 10 °C | Hours at or below 15 °C |
|---|---|---|---|---|---|---|
| Scotland | Edinburgh (Royal Botanic Garden) | 7.29 / 1.67 | 1,000 | 2,300 | 5,000 | 7,300 |
| North East | Durham | 6.86 / 1.29 | 1,100 | 2,400 | 5,100 | 7,300 |
| North West | Rochdale | 6.93 / 1.50 | 1,100 | 2,400 | 5,000 | 7,200 |
| Yorkshire | Sheffield | 7.01 / 2.24 | 800 | 2,000 | 4,700 | 6,900 |
| East Midlands | Nottingham (Watnall) | 6.91 / 1.65 | 900 | 2,200 | 4,700 | 7,000 |
| West Midlands | Pershore | 7.89 / 1.68 | 1,000 | 2,200 | 4,500 | 6,600 |
| East of England | Cambridge (NIAB) | 7.65 / 1.88 | 900 | 2,000 | 4,400 | 6,600 |
| London | Heathrow | 8.42 / 2.68 | 700 | 1,600 | 3,900 | 6,200 |
| South East | Reading (University) | 8.01 / 2.13 | 900 | 1,900 | 4,300 | 6,600 |
| South West | Yeovilton | 8.61 / 1.97 | 1,000 | 2,100 | 4,400 | 6,600 |
| Wales | Cardiff (Bute Park) | 8.58 / 2.50 | 800 | 1,800 | 4,200 | 6,500 |
| Northern Ireland | Aldergrove | 7.25 / 2.11 | 800 | 2,100 | 5,000 | 7,400 |
| Ecodesign reference climate | Average climate in the Union | Not applicable | 1,500 | 2,500 | 4,300 | 6,200 |
Three patterns stand out. The coldest stations, in Scotland, the North East and the North West, see 1,000 to 1,100 hours at or below 2 °C, while Heathrow sees about 700. Free cooling is not a northern monopoly, though: every station has thousands of hours at or below 10 °C, which suits warmer loops. And the UK has fewer very cold hours than the ecodesign reference climate. The regulation for process chillers sets its test hours from a table of outdoor temperatures that it describes as average climate conditions in Europe, and summing that table gives about 1,500 hours at or below 2 °C against 700 to 1,100 here, but more hours in the 5 to 15 °C band that favour partial free cooling.
How were the hours calculated, and what are the limits?
- Take each station's 1991 to 2020 monthly mean daily maximum and minimum temperature from the Met Office.
- Assume the temperature rises and falls smoothly between those two values each day.
- Widen the model for day-to-day variation. The Met Office table also gives days of air frost, which indicates how far daily temperatures swing around the monthly average, typically about 3 to 4 K in these stations.
- Count the hours below each threshold across the year, using the 365.2425-day year.
The limits are real. A station is a point, not a region, and altitude, coast and city heat all move the numbers. Monthly means smooth out cold snaps, so the counts at 2 °C are the least certain, and the figures describe past climate, not a forecast. For a design, use hourly weather data: CIBSE publishes Test Reference Year files, which represent a typical weather year for annual energy assessments, with hourly data for 28 climatic zones across the UK.
What does that mean for a real building?
Illustrative example, not a quotation. A data room has a constant 200 kW cooling load and an air-cooled chiller whose compressors draw 66.7 kW at an assumed seasonal EER of 3.0. In the hours at or below 5 °C, a free-cooling unit could in principle carry the whole load. Using the table, that is about 1,600 hours at the Heathrow station and about 2,400 at Durham. The compressor electricity avoided is then roughly 108,000 kWh a year in London and 162,000 kWh in the North East, before subtracting the fan and pump power of the free-cooling circuit, which the datasheet gives, and before counting any partial free cooling at warmer temperatures. Multiply by your own electricity rate to turn it into cost.
The result favours year-round loads. Free cooling pays when cooling is needed in the cold months, as with data rooms and many process loads, and much less for comfort cooling that runs only in summer. See data centre cooling methods, server room cooling requirements and process and glycol chillers.
For the warm side of the picture, see cooling degree days by UK region. If you are specifying plant, the commercial chillers page explains the process, and you can send an enquiry through the site's form.
Sources
Regulations and guidance are revised, so confirm the current text before relying on a figure.
- Location-specific long-term averages: Edinburgh (Royal Botanic Garden), Met Office; checked 22 October 2026.
- Location-specific long-term averages: Durham, Met Office; checked 22 October 2026.
- Location-specific long-term averages: Rochdale, Met Office; checked 22 October 2026.
- Location-specific long-term averages: Sheffield, Met Office; checked 22 October 2026.
- Location-specific long-term averages: Nottingham (Watnall), Met Office; checked 22 October 2026.
- Location-specific long-term averages: Pershore, Met Office; checked 22 October 2026.
- Location-specific long-term averages: Cambridge (NIAB), Met Office; checked 22 October 2026.
- Location-specific long-term averages: Heathrow, Met Office; checked 22 October 2026.
- Location-specific long-term averages: Reading (University), Met Office; checked 22 October 2026.
- Location-specific long-term averages: Yeovilton, Met Office; checked 22 October 2026.
- Location-specific long-term averages: Cardiff (Bute Park), Met Office; checked 22 October 2026.
- Location-specific long-term averages: Aldergrove, Met Office; checked 22 October 2026.
- Packaged Chillers, Energy Technology List, UK Government; checked 22 October 2026.
- Commission Regulation (EU) 2015/1095, annex VI, legislation.gov.uk, revised 09/01/2017; checked 22 October 2026.
- Commission Regulation (EU) 2015/1095, annex VIII, legislation.gov.uk, revised 09/01/2017; checked 22 October 2026.
- Lower Your Energy Costs With Free Cooling, ICS Cool Energy, manufacturer's own page, updated 14 February 2023; checked 22 October 2026.
- EWFT-B-C, Daikin Applied UK, manufacturer's own page; checked 22 October 2026.
- TZ-D free cooling, Daikin Applied UK, manufacturer's own page; checked 22 October 2026.
- Module 39: Cooling options for data centres, CIBSE Journal, posted April 2012; checked 22 October 2026.
- Module 254: Liquid cooling in data centre applications, CIBSE Journal, posted September 2025; checked 22 October 2026.
- Weather data, CIBSE; checked 22 October 2026.
Common questions
Does free cooling work in a UK summer?
Mostly not. A summer afternoon is usually too warm for the loop to reject heat to the air, so the compressors carry the load. Adiabatic assistance can extend the range, as CIBSE Journal describes for data centres.
Can free cooling be added to an existing chiller?
One manufacturer says a standalone free-cooling system can work alongside an existing chiller. Loop temperatures, pipework and space decide whether that suits your plant, so ask a competent engineer to check.
Do free cooling systems need glycol?
Not always. CIBSE Journal describes economisers that use water or a glycol solution, and Daikin Applied lists a version in which the glycol is confined to a closed loop. Frost risk on the outdoor circuit drives the choice.
Which UK region gets the most free cooling?
On this modelled data, Scotland, the North East and the North West are ahead at every threshold, with London and Wales at the other end. Every region still has thousands of hours at or below 10 °C, so a warm loop can use free cooling anywhere.
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