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By Grant Laidlaw

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.

Hi Aisha, thanks for the question, the acronym AHU stands for air handling unit.

An air handling unit, also called an AHU, is part of an air-conditioning system which contains the necessary equipment for conditioning and moving the air (fans), cooling coils or evaporators, air filters, heater and humidifiers all placed in an accessible box like structure in or outside a building. The AHU is the part of the system the handles the air, both fresh air and recirculated air, thus the name.

They are usually found on the roof of buildings, in ceilings, in plant rooms or specially designed rooms – plenum spaces in a building using ducting and sometimes ceiling voids, the air is circulated to reach and return from each of the rooms of the building in question.

While not all AHUs have all of the components mentioned, when looking at one we normally find:

• A sealed, insulated box type structure, accessible through access hatches with electrical, controls, air, water drain and chilled water or refrigerant connections. In some cases, hot water or steam connections may be present. Sound deadening materials are often applied.
• Air intakes: AHUs collect air from outside (fresh air), which is distributed throughout the building; and mixed return indoor air. Typically we always maintain a minimum of 10% fresh air to ensure good air quality, maintain a positive static pressure, and maintain acceptable levels of oxygenation. Some units only use 100% fresh air.
• Fans: these increase the air pressure to pump the air to from the AHU into the ducts that distribute the air as required throughout the building.
• Filters: The air is ‘cleaned’ depending on the air purity requirements, we may find primary, secondary HEPA, ultraviolet lighting etc to deal with airborne contaminants such as solid air borne particles, bacteria, viruses, odours and other air pollutants.
• Humidifiers, often bottle type steam generators or water sprays, are used to adjust relative humidity levels.
• Heaters, normally elements are used in South Africa but sometimes we may find steam or hot water coils.
• Heat recovery wheels: recover heat from air leaving the building for reuse.
• Evaporators: in direct expansion systems directly using the refrigerant to absorb heat. Sometimes even the reverse cycle for heating is utilised.
• Condensate pan with drain and P-trap to collect any condensate produced whilst cooling the air.
• Plenums: empty spaces in which the air flow is homogenised (mixed). Fresh air is mixed with return air. Mixing processes are performed to obtain the desired air temperature and humidity or to maintain energy-efficient performance.
• Control systems: control systems can vary from simple control systems to advanced control systems that use the latest technologies such programmable logic controllers. Usually controllers are used to control the temperatures and humidity of the supply air to the zones in the building. The system may control the damper systems in the AHU. Moreover, advanced control systems can even control the fan rotational speed and thus the rate of air supplied to the building based on the required parameters.

In addition to managing the proper ventilation of the interior of a building with outside air and the correct air flow, the AHU performs other functions:

• Filtration and control of the quality of the air that will reach the interior, thanks to the filters, and depending on the retention of these filters, the air will be filtered.
• Control of the air temperature that regulates the air-conditioning system so that the temperature in the interior of the building is according to requirements.
• Relative humidity monitoring and control for greater indoor comfort.

AHU are intended for use, for example, where the flow of people is high and with many people at the same time and whose natural ventilation is limited: hotel dining rooms, function rooms, restaurants and convention halls.

AHUs are also suitable option for those spaces with very high hygiene requirements: laboratories, clean rooms or operating theatres, among others. An AHU can also be used to ventilate places where air conditioning is provided.

Types of Air Handling units

• Horizontal and vertical units: Vertical units typically require less floor area.
• Rooftop (outdoor) and indoor units: There are specific AHUs made for usage outside. They resist corrosion and are typically positioned on the roof. However, the majority of AHUs are indoor units.
• Draw-through and blow-through units: The AHU is either a blow-through or draw-through mechanism depending on where the fan is located.
• Outdoor air (makeup air) or mixing units: While mixing units (as the name suggests) can utilise both recirculating air and outside air, makeup air units use only outdoor air, used for example in hospital theatres.
• Single-zone and multi-zone units: For dual-duct systems, a multi-zone AHU is more frequently employed.
• Custom-built units: Standard units are lower in cost and quicker to deliver. More versatility is available with custom-built AHUs, which may also be made to be the most energy-efficient option.

Now Aisha, moving on to the second part of your question, servicing.

All safety and usage of personal protection equipment apply. Routine AHU cleaning and maintenance is imperative.

The following consists of most of the servicing items, not all systems will have all of the components mentioned:

• Examine electrical switchgear and safeties including lockouts
• Examine control panel and boxes including labelling
• Check contactors/starters/relays
• Check operating amps
• Tighten all electrical connections
• Check control set points
• Examine / lubricate air dampers, actuators
• Examine filter section
• Change filters if necessary
• Maintain filter frames, seals and filter clips
• Examine heating coil / clean if necessary
• Clean heating coil
• Examine drain line and pan / clean
• Examine cooling coil, clean if necessary
• Examine supply air fan, check for vibration
• Clean fans and housing
• Lubricate fan’s shaft bearings if applicable
• Check belts, pulleys and alignment and tension on supply fan drive
• Examine all motors
• Lubricate all fan motor bearings if applicable
• Check fan(s) rotation
• Examine heating section (if applicable)
• Examine heater safeties
• Check ducting seals / collars and access door seals hinges, locks.
• Check corrosion
• Examine other accessories (if applicable)
• Complete logbooks

As always, follow the manufacture recommendations and do not perform maintenance tasks you are not experienced with or trained on.

Common problems encountered with AHUs include:

• Fan motor malfunction: Faulty fan motors can lead to inadequate air circulation, reduced airflow and compromised temperature control.
• Coil fouling: This involves the accumulation of dirt, dust and debris on the evaporator and condenser coils. This can decrease heat transfer efficiency, resulting in poor performance.
• Leakage and ductwork issues: Damaged or poorly sealed ductwork can lead to air leaks, decreased efficiency, and inefficient air distribution.
• Belt and pulley problems: Worn-out or misaligned belts and pulleys can cause excessive noise, reduced airflow, and increased energy consumption.
• Control system failures: Faulty sensors, malfunctioning thermostats, or issues with the control panel can disrupt the AHU’s operation.
• Air filter issues: Clogged or dirty air filters can obstruct airflow,
  leading to reduced system efficiency and poor indoor air quality.
• Blocked drains.

The frequency of maintenance for air handling units depends on several factors, including your specific system requirements, manufacturer guidelines and environmental conditions. As a general rule, AHUs should undergo maintenance at least twice a year. This preventive maintenance schedule helps ensure optimal performance, energy efficiency and prolongs the lifespan of the unit. The ideal maintenance frequency is based on specific requirements and usage patterns.

Aisha, I hope that this increases your understanding of air handling units.

Thanks to everybody for the overwhelming response. I receive an average of over sixty questions per month and cannot publish all of them. But keep them coming, as I may answer you directly. Looking forward to hearing from you.

Grant Laidlaw

References

• SETA training
• ASHRAE
• ACRA

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