Chillers and fan coil units
A chilled-water system takes a different approach from refrigerant-based AC. Rather than pumping refrigerant around the building, a central chiller cools water, which is piped to fan coil units (FCUs) or air handling units (AHUs) throughout the building, where fans blow air across the chilled-water coils to condition the space. It’s the typical choice for very large buildings.
How it works
Instead of sending refrigerant all around the building, a chilled-water system keeps the refrigerant cycle at a central chiller, which cools water. That chilled water is pumped through insulated pipes to fan coil units (FCUs) in rooms, or to larger air handling units (AHUs) serving zones, where a fan blows air across a chilled-water coil to condition the space. Heating can come from a reversible chiller, or from a separate boiler feeding the same units with hot water. Crucially, AHUs can also introduce and condition fresh air from outside — something most refrigerant-based AC doesn’t do (see ventilation vs air conditioning).
Pros and cons
Pros: water is easy to distribute over long distances, suiting very large buildings; the refrigerant charge stays concentrated at the chiller rather than spread through the building; AHUs combine conditioning with fresh-air ventilation; robust and well-suited to large, steady loads; centralised plant simplifies some servicing.
Cons: large capital cost and significant plant space (a plant room and/or roof); more moving parts to maintain — pumps, water treatment, coils; generally less efficient than VRF at part load on smaller jobs; better suited to scale than to small premises.
Best for / not ideal for
Ideal for hospitals, large offices, shopping centres and industrial buildings — anywhere with large loads, a need for fresh-air handling, or a preference for centralised plant and water distribution. Not cost-effective for small or medium premises, where VRF or split systems fit better.
Capacity and design
Chilled-water systems are engineered to the building, with the chiller(s), pump sizing, pipe network and terminal units (FCUs/AHUs) designed together — and fresh-air rates factored in where AHUs are used. This is specialist design work, built on a full commercial heat-load calculation.
What it costs
A large capital investment in plant, distribution and installation, justified at scale and where centralised plant and ventilation are wanted.
Compliance: chillers carry a refrigerant charge, so F-gas obligations apply, and larger systems may fall under TM44 inspections. Water systems also need ongoing treatment and maintenance.
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