A 10 kW ducted air conditioner: why rated capacity does not always deliver cooling

A 10 kW ducted air conditioner was selected for the room. After installation, the air at the grille is cold, but the room does not reach the required temperature.

This does not yet prove that the air conditioner is undersized or incorrectly charged. What matters is not only how cold the air is, but also how much of it passes through the system.

The ducted unit and ductwork must be selected together

The indoor unit fan must overcome the resistance of the supply and return ductwork, grilles and additional components. Available external static pressure is a separate specification. Manufacturers may also provide airflow settings or automatic airflow adjustment during commissioning. ¹

Suppose the duct system requires 120 Pa at the design airflow. If the selected unit cannot deliver this operating point, a ‘10 kW’ label will not help the fan move the required volume of air.

The actual airflow in this situation is determined from the unit and system characteristics. A universal capacity penalty for ‘long ducts’ should not be assumed without calculation.

Cold air does not mean sufficient heat removal

Let us estimate the sensible cooling capacity of the airflow:

Q_s≈0,335× L×Δ T/1000

Here, \(L\) is in m³/h, \(\Delta T\) is the air temperature difference before and after treatment, and 0,335 is a rounded coefficient for air under normal conditions.

At an airflow of 2000 m³/h and a temperature difference of 10 °C, the sensible cooling capacity is about 6,7 kW. If, in another calculation scenario, airflow is only 1200 m³/h at the same temperature difference, the result is approximately 4,0 kW.

In a real system, reducing airflow will change both the coil temperature and compressor operation. These examples illustrate the airflow heat balance, not the precise performance of a specific air conditioner. The interaction between a DX coil, airflow and the outdoor unit is discussed in Trane’s engineering materials. ²

Total and sensible capacity differ

Some cooling capacity may be used to condense moisture. Therefore, 10 kW of total capacity does not mean 10 kW of sensible cooling. Selection requires matching sensible and latent loads to the corresponding equipment capabilities. ³

An illustrative example: total capacity is 10 kW, with a sensible heat ratio of 0,75 at the specified operating conditions. This leaves 7,5 kW available for the sensible load. If the room requires 9 kW of sensible cooling, these operating conditions are insufficient, even though the total figure appears higher.

Air needs a return path

An individual office may have a supply grille but an inadequate return air path when the door is closed. In another situation, supply and return points may be positioned so that part of the airflow does not properly serve the occupied zone. Air jet distribution and mixing must be checked against the room layout. ⁴

We would begin diagnosis with airflow measurements at the grilles, pressure, filter condition and a comparison of temperatures and humidity. Only then would we conclude whether there is a problem with the refrigeration circuit.

Controls can change the operating mode. But they cannot increase a duct’s cross-sectional area or create a missing return air path.

A ducted air conditioner delivers cooling through the air. If the air distribution system does not do its job, the rated kilowatts remain just a number in the catalogue.