Engineering practice · Air conditioning
Article 01 / 10
VRF, DX or chiller–fan coil: why no single system is best for every building
Asking ‘Which is better: VRF or a chiller?’ is rather like choosing equipment before defining the task. For one building, tenant independence will be decisive. For another, it may be year-round cooling, the ability to refurbish in stages or an existing water distribution system.
That is why we do not start with the brand. First, we establish how the building needs to operate. Then we select a system capable of delivering those operating modes.
First, let us clarify the terminology
DX means direct expansion: the refrigerant evaporates directly in the air cooling coil. VRF is also a DX technology, but uses a shared refrigerant circuit with capacity control and multiple indoor units. When a commercial proposal lists VRF and DX separately, DX usually refers to split systems, ducted air conditioners or outdoor units serving air handling units (AHUs). 1
In a chiller–fan coil system, chilled water or a water-based solution is generally distributed to the rooms, where water coils condition the air. The refrigerant circuit is still present: in a vapour-compression system, it remains within the chiller. 2
Hybrid solutions also exist. HVRF, for example, combines a refrigerant circuit with a water circuit serving the indoor units. The actual system configuration therefore matters more than the technology's name. 3
Floor area alone does not determine the choice
Imagine two offices with the same floor area. The first is an open-plan space with a single operating schedule. The second has separate tenants, meeting rooms, a server room and offices facing different directions.
In the first case, a shared system with relatively simple zoning may be suitable. In the second, independent operating modes, maintenance and the consequences of shutting down a single circuit all need separate consideration.
Even the same total cooling demand does not make these requirements identical. What matters is when the load occurs, in which zones, and what happens outside normal working hours.
Different temperatures do not always mean different operating modes
Being able to set the temperature in each office is not the same as being able to heat one office while cooling another. With VRF, it is important to distinguish between a standard heat pump system and a heat recovery system. 4
System configuration also matters for water-based systems: a typical two-pipe fan coil uses one water circuit, while a four-pipe unit has separate heating and cooling connections. Whether both sources are available depends on the system as a whole. 5
If this is not clarified before procurement, individual controllers will not overcome the design limitations.
What we compare in the cost estimate
The comparison must cover the complete package: equipment, distribution systems, controls, power supplies, drainage, commissioning and service access. Ventilation must also be specified separately: the name of an air conditioning system alone does not confirm that it supplies the required outdoor air. 1
To assess operational performance, I would insist on consistent baseline conditions: operating schedules, temperatures, loads and energy accounting boundaries. You cannot compare the consumption of a single outdoor unit with that of an entire chiller plant, including its pumps.
It is also best to discuss connection to analytics in advance: which parameters will be available, where historical data will be stored and which commands the manufacturer will permit.
The best system is not the winner of a general technology comparison. It is the system whose limitations have been accounted for in your particular building.