Engineering practice · Air conditioning
Article 02 / 10
A 60 kW VRF outdoor unit: why this does not yet mean 60 kW in the rooms
The specification lists a 60 kW outdoor unit with 78 kW of indoor units connected. The client may interpret this in two ways: either the system is overloaded, or the manufacturer allows an extra 18 kW to be obtained.
Neither conclusion follows from these figures alone. Connected nominal capacity, simultaneous load and available capacity must be distinguished.
What the connection ratio means
For our illustrative example:
This is the ratio of the nominal capacities of the indoor units to that of the outdoor unit. It does not mean that the outdoor unit can deliver 78 kW to all rooms simultaneously.
The permitted ratio and selection rules depend on the particular series. Design temperatures, pipework and other capacity corrections are checked separately. These are applied using the manufacturer’s method, not by arbitrarily multiplying universal factors. 1
Why the total indoor unit capacity can be higher
Consider illustrative rooms with a total installed nominal capacity of 78 kW. Suppose the maximum simultaneous demand across all zones is 50 kW: the individual room peaks do not coincide.
If the available outdoor system capacity at design conditions is taken as 54 kW, the balance appears sufficient: 54 kW against 50 kW.
But if a change in room use increases simultaneous demand to 62 kW, a shortfall arises:
Additional indoor units will not compensate for it. All values in this example are assumed for illustration; they are not specifications for a particular model.
The limits of useful load diversity
A reduced simultaneous load should not be assumed simply because offices face different façades. After an overnight shutdown, rooms may all require intensive cooling or heating at the same time.
I would therefore check at least two scenarios: steady-state operation and recovery after a shutdown. The technical brief must specify the time by which rooms must reach the agreed conditions.
A well-planned start-up schedule can reduce the simultaneous peak. But it does not create additional cooling capacity or remedy a shortfall that lasts throughout the working day.
60 kW of cooling is not 60 kW of electricity
Let us assume another calculation condition: the system delivers 54 kW of cooling at an EER of 3,6 W/W within defined accounting boundaries.
This is the electrical input only for the stated operating condition, not the equipment’s maximum consumption. The power supply, cables and protection are selected using the documentation for the specific system. Seasonal consumption requires the duration and characteristics of different operating conditions, not a single EER. 1
What the selection process should deliver
I would expect more than a list of models. Room loads, their simultaneous peak, the actual pipework layout and confirmation of available capacity are needed. Winter operation must be assessed separately if VRF is used as a heating source.
Such a document explains the reasons for the selection. It also provides a basis for subsequent checks: did the problem arise from a change in load, a fault or an original design limitation?
Indoor units distribute the system’s capacity. They do not increase it simply by adding nominal kilowatts.