VRF or Chilled Water for a Commercial Building?

Two different system architectures

A VRF system circulates refrigerant from outdoor units to indoor units throughout the building. Heat pump systems provide either heating or cooling to all indoor units on a circuit at any one time, while heat recovery systems can heat some zones and cool others simultaneously, moving heat between them. Ventilation is handled separately, usually by air handling units with heat recovery.

A chilled water system produces cooling centrally in chillers and distributes it by pumps to fan coil units, air handling units or chilled beams. Heating comes from boilers, heat pumps or a heat network through a separate low temperature hot water circuit, or through the same pipework in two-pipe changeover systems.

Scale, plant space and risers

VRF is modular. Outdoor units can be sized floor by floor or zone by zone, which suits small and medium buildings, phased occupation and multi-let floors. Limits on pipe length and on height difference between outdoor and indoor units, together with the number of systems a large building needs, can make roof space and refrigerant pipework hard to manage at scale. Refrigerant pipework also has to be coordinated with ceiling voids and fire-stopped wherever it crosses a compartment wall or floor.

Chilled water plant benefits from diversity across the whole building, because central plant is sized for the peak of the combined load rather than the sum of individual zone peaks. It needs a plant room or roof area for chillers and pumps, and larger risers for water pipework, but distribution through large and tall buildings is well established.

Refrigerant in occupied spaces

VRF places refrigerant pipework and indoor units in occupied rooms. If a leak occurs, the refrigerant concentrates in the room where it escapes, so the charge of each system must be checked against the volume of the smallest room it serves, following the applicable refrigerant safety standard, such as EN 378 in Europe. Where limits would be exceeded, systems are divided, rooms are served differently or leak detection and other safety measures are added.

Lower global warming potential refrigerants, including mildly flammable A2L types such as R32, add further requirements on charge and room size. In a chilled water system, refrigerant stays in the chillers, in a plant room or outdoors, and only water enters occupied spaces, which brings its own considerations for leaks and condensate.

Efficiency and control

Inverter-driven VRF systems perform well at part load, and heat recovery systems use heat rejected from cooling zones to warm others, which suits buildings with simultaneous demands. Control is inherently zonal, and energy use can be apportioned to tenancies through the system's own monitoring or through sub-metering.

Chilled water efficiency depends heavily on plant selection and operation: chilled water temperatures, variable flow pumping, chiller sequencing and free cooling. Higher chilled water temperatures improve chiller efficiency and extend free-cooling hours, and central air handling gives more direct control of latent loads. Chilled water plant also connects readily to heat pumps and thermal storage as a building's energy strategy develops.

Maintenance, replacement and future change

VRF indoor and outdoor units are generally replaced within the same product range, and refrigerant changes over the life of the building can affect whether existing pipework can be reused. Replacement usually proceeds system by system, which allows it to be phased floor by floor.

In chilled water systems, distribution pipework with well-maintained water treatment usually outlasts the chillers, so central plant can be replaced in stages without disturbing terminal units. Fan coil units on flexible connections can be relocated at fit-out, although work on the water system needs careful isolation and drain-down.

Both approaches depend on the quality of maintenance: VRF on leak checking, filter cleaning and condensate care at every indoor unit, and chilled water on water treatment, strainers and valve maintenance.

Making the comparison

Compare the options on the same scope, including ventilation plant, controls, plant space and the replacement strategy, not only the cooling equipment. The following factors usually decide the outcome.

• Building size and load density
• Tenancy and occupancy pattern
• Simultaneous heating and cooling demand
• Plant, roof and riser space
• Ceiling void depth
• Refrigerant charge in the smallest rooms
• Ventilation strategy
• Maintenance capability
• Future replacement approach