Engineering practice · Air conditioning
Article 07 / 10
A 5 kW fan coil: why it may fail to cool a room with a 4 kW load
The calculated sensible heat gain in the room is 4 kW. The cost estimate specifies a 5 kW fan coil. There even appears to be spare capacity.
But first, we need to establish what that 5 kW means: total or sensible cooling capacity, at what water temperature and at which fan speed.
Without the selection conditions, spare capacity may exist only on paper.
Total cooling and temperature reduction are not the same
Consider an illustrative fan coil with a total cooling capacity of 5 kW and a sensible heat ratio of 70% at a particular operating condition.
This is insufficient for a room with a sensible load of 4 kW: the shortfall is 0,5 kW. The rest of the total capacity is associated with dehumidification and cannot automatically be counted towards the sensible heat balance.
This is why Eurovent verifies sensible and total fan coil cooling capacity separately, rather than just a single overall figure. The 5 kW and 70% used here are assumptions for our example, not the characteristics of a specific model. 1
7/12 °C is a rating condition, not a rule for every system
If a fan coil is selected for water at +7/+12 °C, its capacity cannot be assumed to remain the same at +10/+15 °C without recalculation. The temperature difference driving heat transfer between the water and air changes, so a new selection is needed.
There is no universal rule that each degree costs a fixed percentage of capacity. The result depends on the coil, flow rates and air conditions. Equipment must be reassessed using the manufacturer’s performance data; Eurovent certification also covers the performance of selection software. 1
The actual temperature at the fan coil also matters. The chiller leaving-water temperature does not prove that water reaches every terminal unit unchanged: mixing and distribution network parameters must be checked for the specific arrangement. 2
Capacity must be delivered at an acceptable noise level
A fan coil may deliver the required cooling at high speed, but not at a speed acceptable in an office or hotel room. Thermal and acoustic performance must therefore be compared at the same fan setting. Eurovent verifies performance at different speeds; external static pressure also matters for ducted models. 3
Saying ‘we will switch it to quiet mode after start-up’ is not a solution if capacity is insufficient in that mode.
What water flow rate tells us
For water at normal temperatures, the following approximation can be used:
where \(Q\) is in kW, \(\dot V\) is in m³/h and \(\Delta T\) is in K.
To transfer 5 kW with a water temperature difference of 5 K, the required flow rate is:
This is a heat balance. It does not guarantee that any coil will deliver 5 kW simply by passing this flow through it. Suitable air conditions and heat transfer are required. For glycol solutions, their specific properties must be used.
What the client should receive
In the selection, I would record sensible and total capacity, entering and leaving water conditions, air conditions, operating speed, noise and water-side pressure drop. I would also specify the valve, drainage, insulation and service access.
A fan coil is not selected by the largest number in the catalogue. It is selected for its ability to serve a specific room at an operating setting people will actually use.