Chillers & Chilled-Water Systems: capacity, cost and efficiency

What a commercial chiller is, and how chilled water works

A chiller is a central cooling machine. Rather than piping refrigerant to dozens of indoor units the way VRF air conditioning does, a chiller cools water and pumps that chilled water around the building to air handling units, fan-coils and process equipment. It is the difference between a distributed refrigerant network and a central plant with a hydraulic distribution system, and that distinction drives everything about where chillers win.

Chillers come in two broad families. Air-cooled chillers reject their heat to the outside air through fans and are the common choice for UK commercial buildings because they need no cooling tower and carry no Legionella water-treatment duty. Water-cooled chillers reject heat through a cooling tower or dry cooler and can be more efficient on large, dense loads — but they bring an ACOP L8 water-hygiene regime with them. The right choice depends on the load, the site and the plant space, and a proper design weighs efficiency against the maintenance and compliance burden of each.

Where a chiller wins, and where VRF wins instead

The single most useful thing to understand about chillers is when they are the right answer and when they are not. VRF is modular, efficient at part-load, and ideal for buildings with many separately-controlled comfort zones up to roughly 150–200 kW of cooling. A chiller earns its place above that threshold, or wherever chilled water is simply the better distribution medium:

• Large offices and campuses where the total cooling load and the distances involved make a central plant and chilled-water pipework more practical than long refrigerant runs.
• Hospitals with year-round, life-critical cooling and ventilation demand across large floor plates.
• Data suites and server rooms with a flat 24/7 load, where close-control cooling and high resilience matter more than zone comfort.
• Manufacturing and process loads where the cooling has to serve equipment, not just people, and often runs at duties and temperatures no comfort system would reach.

Many buildings use both technologies deliberately: VRF on the office floors for zone comfort, and a chiller for a data suite or a process load in the same building. The right answer is usually the one that matches each load, not a single technology imposed everywhere. If your building is dominated by comfort zones under 150 kW, start with our VRF page instead.

Sizing and economics

Chillers span an enormous range — from around 100 kW for a medium office to 2 MW and beyond for a hospital, data centre or process plant — and are usually specified with multiple chillers in parallel so the plant can turn down to match low loads and provide resilience if one unit is offline. Air-cooled plant sits on a roof plant deck or ground compound of roughly 20–200 sqm.

Indicative project values run from around £80,000 for a single mid-size air-cooled chiller and associated pipework up to £1.5 million or more for a large multi-chiller plant with pumps, headers, buffer vessels, controls and craneage. The cost is driven by the total duty, the number and type of chillers, the distribution pipework, plant-room and structural constraints, refrigerant choice and any electrical supply work. Because a chiller project is a major capital item with a long life, the whole-life running cost matters as much as the install price — we model both from a survey. There are honest indicative ranges for every system type on our cost guide, and the capital-allowance routes that fund the work on our grants and funding page.

The running-cost and efficiency case — part-load is where the money is

Chiller performance is quoted as SEER for cooling and, crucially, as ESEER (European Seasonal Energy Efficiency Ratio), which weights efficiency across a range of part-load conditions rather than at full duty. This is the number to focus on, because a chiller almost never runs at 100 per cent duty. A building spends the overwhelming majority of its hours at part-load, and the running cost is set by how efficiently the plant behaves at 25, 50 and 75 per cent — not by its full-load headline.

Two design features do most of the heavy lifting. Free-cooling uses low outside-air temperatures to cool the water directly or partially, bypassing or unloading the compressors for large parts of the UK year — a significant saving on any building with a year-round cooling demand, such as a data suite. Turndown and sequencing controls stage multiple chillers so each runs in its efficient band rather than one machine grinding at low load. Specifying free-cooling and good part-load control is usually a bigger prize than chasing the headline efficiency figure, and it is exactly the kind of detail a box-swap quote skips.

The refrigerant, F-Gas and L8 compliance angle

Large chillers hold a substantial refrigerant charge, so they almost always fall under F-gas leak-check duties at the six-monthly (50 tonnes CO2-equivalent) or even quarterly (500 tonnes) frequency, well beyond the annual duty that catches smaller systems. Those leak checks, and the refrigerant records that go with them, must be carried out by an F-Gas registered company (REFCOM, the Quidos F-Gas Register or Bureau Veritas), and the Environment Agency enforces the regime. Handling them is a core part of a PPM contract.

Refrigerant choice is now a design decision in its own right. New chillers move away from high-GWP R410A toward low-GWP R32, R454B and R1234ze, and increasingly R290 (propane, GWP 3) on smaller duties, which sits outside the F-gas quota entirely but brings DSEAR siting requirements because it is flammable. BS EN 378 governs system safety. Specifying low-GWP now avoids stranding the plant as the phase-down bites.

There is one compliance duty unique to this system type: Legionella and ACOP L8. Water-cooled chillers with cooling towers, and any wet system, carry a legal water-hygiene regime — risk assessment, treatment, monitoring and record-keeping — that air-cooled chillers avoid entirely. It is one of the genuine trade-offs when choosing between air-cooled and water-cooled plant, and it is a recurring cost most owners underestimate. External plant noise is assessed to BS 4142, with internal plant-room targets around 35 dB(A).

The electrification and MEES tie-in

Chillers are an all-electric load already, so on the electrification ladder the job is to keep that load efficient rather than to switch fuel. Low-GWP refrigerant, free-cooling and part-load control are what stop a big cooling plant becoming a big electricity bill. On buildings that will also electrify heat, a chiller can be specified as a reversible or heat-recovery machine so that heat rejected from cooling is captured for hot water or heating elsewhere — the same logic that makes heat-recovery VRF efficient, at central-plant scale, and a natural bridge to commercial heat pumps.

Efficient central cooling also protects an EPC. Because HVAC dominates a commercial building’s modelled energy use, a chiller upgrade with free-cooling and good controls lifts the rating that governs MEES lettability — EPC E required to let since 1 April 2023, with EPC B proposed by 2031 for larger privately-rented non-domestic buildings, subject to secondary legislation (confirm the current position on gov.uk). You can read the capital allowances and full expensing guidance that funds the work on GOV.UK.

Resilience and phasing — the design questions unique to central plant

Because a chiller is central plant rather than a distributed network, a single failure can take a large part of a building offline — which is why resilience is a design decision from the outset, not an afterthought. Critical loads such as hospital wings and data suites are typically served by an N+1 configuration, where an extra chiller is installed so the full duty is still available if one machine is down for service or fails. Sequencing controls then rotate the running order so the machines wear evenly and each spends its time in an efficient part-load band.

Replacement carries its own distinctive challenges. A chiller changeover usually needs craneage, structural checks on the plant deck, and careful management of the chilled-water system — flushing, treating and re-balancing the pipework so the new plant sees clean water and the correct flow rates. On a live 24/7 load the whole thing is a programming exercise: temporary rental cooling keeps the critical load served while the permanent plant is swapped, and the changeover is staged so cooling is never fully lost. None of this applies to a distributed VRF retrofit, and it is why a chiller project needs a specialist who plans the logistics as carefully as the thermodynamics.

Objections we hear, answered honestly

“A new central chiller sounds like a huge capital hit — is there a cheaper route?” Sometimes. Where an existing chiller is sound but tired, a compressor or controls upgrade, a free-cooling retrofit or a low-GWP refrigerant conversion can extend its life for a fraction of a replacement. Where the plant is genuinely end-of-life or the duty has changed, replacement is the honest answer. We survey first and give you the whole-life cost of each option.

“Should we go air-cooled or water-cooled?” For most UK commercial buildings, air-cooled — because it avoids the cooling tower, the L8 Legionella regime and the associated water treatment. Water-cooled earns its place on very large, dense loads where the efficiency gain outweighs the added maintenance and compliance. We size and compare both against your actual load.

“Our data suite can never go offline — how do you replace the chiller?” With resilience designed in: N+1 chiller configurations, staged changeovers with temporary cooling, and sequencing that keeps the critical load cooled throughout. Replacing plant on a live 24/7 load is a programming problem as much as an engineering one, and we plan it around your uptime.

Comfort cooling and process cooling are not the same job

One thing worth drawing out on chillers specifically is the difference between comfort cooling and process cooling, because a single building often needs both and they pull the design in different directions. Comfort cooling keeps people comfortable and can tolerate a degree or two of drift, so it can be modulated and set back when spaces are empty. Process cooling — a data suite, a manufacturing line, a cold store or a laboratory — has to hold a tight temperature around the clock regardless of occupancy, and a failure is not a comfort complaint but a stopped process or a lost data hall. That is why we frequently split the load: a chiller sized and controlled for the flat, critical process demand, with VRF or separate comfort cooling handling the variable people load. Trying to serve both from one loosely-specified plant tends to compromise the critical duty and waste energy on the comfort side.

Frequently asked questions

What is the difference between a chiller and VRF?

A chiller cools water and pumps it around the building to AHUs and fan-coils, scaling far beyond VRF for large offices, hospitals, data suites and process loads. VRF pipes refrigerant directly to many indoor units and suits buildings up to roughly 150–200 kW of comfort cooling. Chillers win on large central and process loads; many buildings use both.

What size building needs a chiller rather than VRF?

Broadly, above 150–200 kW of cooling, or where long pipe runs, high loads or a mix of comfort and process cooling make chilled water the better distribution medium. Below that, VRF is usually more efficient and more economical. We size on your actual heat gains, not a rule of thumb.

Do water-cooled chillers need Legionella controls?

Yes. Any wet system with a cooling tower carries ACOP L8 Legionella duties — risk assessment, water treatment, monitoring and records. Air-cooled chillers avoid this entirely, which is one reason they are the common UK choice. It is a genuine trade-off we weigh in the design.

How efficient is a modern chiller?

Modern chillers are quoted on ESEER, which weights efficiency across part-load conditions where the plant actually spends its time. Free-cooling and multi-chiller sequencing lift real-world efficiency well above the full-load headline. Always compare ESEER, not just full-load SEER.

How long does a chiller installation take?

A single chiller changeover can be a few weeks including craneage and commissioning; a large multi-chiller plant room can run to several months. On live 24/7 loads such as data suites we stage the work with temporary cooling and N+1 resilience so the critical load stays cooled throughout.

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We install and refurbish commercial chillers and chilled-water systems across the UK, including London, Birmingham and Leeds. For a chiller design, a free-cooling retrofit or a resilience-led replacement, request a survey and quote, or read the most common commercial HVAC questions.