The name "Air Handling Unit" (AHU) stems from its original purpose: ventilation, which is the exchange of stale air for fresh air. Furthermore, the device was intended to be "central," meaning all air treatment processes took place in one location.
As client needs evolved and approaches to heating system design shifted, AHUs began to take over functions from other HVAC segments, particularly heating and cooling systems.
Today, while AHUs retain their original name, they are much more complex and are often directly referred to as "Air Conditioning Units." Modern AHUs are primarily responsible for ventilation, but also for heat recovery from exhaust air, air circulation, heating and cooling, humidification and dehumidification, and filtration. In specific cases, these units can also work with other installations, such as local exhaust systems or technological ventilation.
To fully understand the principles governing AHUs, it is essential to learn the basic parameters used to describe them. One of the most important is the Heating Capacity of the unit. In an AHU, air is heated in several stages: first, during heat recovery, where outdoor air is preheated by heat recovered from the exhaust air. Next, the air may mix with recirculated air, resulting in a total air stream with a higher temperature. The final element is the heater (coil) or the heat pump exchanger, which heats the air to its final state—and this is the element worth focusing on.
The output of a heater depends on its physical capabilities, but it is designed so that the supply air temperature meets specific assumptions. This brings us to a critical point in AHU design: the unit's actual function may depend on how the heater is selected.
a) Ventilation Function: If the AHU supplies air at a temperature equal to the exhaust air temperature, the unit is selected primarily for ventilation. In this case, the heater only covers the ventilation loss that could not be recovered from the exhaust air.
b) Heating Function: If the unit supplies air at a temperature higher than the exhaust air temperature, it is selected for space heating. We then refer to this as Air Heating, and the AHU is the device utilized for this purpose.
In both cases, a heater (e.g., electric) is installed to warm the air, but a different power output is applied.
Example: If a heater receives an air stream of 5000 m³/h at 15°C, and the exhaust air from the room is at 20°C:
For the ventilation function:
The heater output will be approx. 8.4 kW. The AHU will supply air at 20°C; however, the room will not be heated, and the unit's net heating capacity for the room will be 0 kW.
For the heating function:
If we want to supply air at 30°C, the heater output must be approx. 25.1 kW. In this case, the room will be heated with a net capacity of approx. 16.8 kW.
Heating Capacity is NOT the same as Heater Output!
This brings us to a vital conclusion:
The heating capacity of the AHU (delivered to the room) is not equal to the nominal output of the internal heater. In the industry, one often encounters colloquial inquiries like: "I need a unit with 17 kW of heating." In most cases, this results in selecting a unit with a heater of that nominal power, which—as we have shown in our example—would be insufficient for actual space heating.
Understanding how an AHU operates and, above all, understanding the client’s needs and intentions is the key to avoiding this error and selecting the right device.
If you are unsure what the heating capacity of the AHU or the heater output in your project should be, please contact us. Ratherm, as an experienced manufacturer, offers technical support, full calculations, and professional AHU selection services.