Air handling units

An air handling unit (AHU) moves and, where specified, treats air before it enters or leaves a building. Its casing brings the selected fans, filters and treatment sections together. Ducts distribute the air; room terminals deliver or remove it. The unit is one part of the ventilation system.

A standalone fan moves air; a complete AHU combines the functions needed for a particular duty. A fan-coil mainly treats room air and does not, by itself, establish the required outdoor-air supply.

Supply/extract describes air paths. Modular/compact describes construction. Recovery describes energy transfer. Hygienic describes execution. DX describes the cooling or heat-pump circuit. These are intersecting criteria: a modular supply-and-extract unit may also be hygienic and use DX cooling.

For a fair comparison, fix airflow, external pressure, entering air state, required leaving state, filter duty and acoustic limits. Then compare verified performance, electrical input, installed size, service access and lifecycle cost. A larger nominal airflow or one peak efficiency value is not an overall ranking.

A supply unit draws in outdoor air and delivers it after the specified filtration and treatment. The building still needs an extract or relief path. Adding supply air without checking the return path can change room pressure and prevent the intended airflow.

Distinguish a unit handling only outdoor air from one mixing outdoor and recirculated air. The outdoor-air requirement, winter heating duty and frost protection must be checked for the actual intake conditions.

• Detailed guide: Supply air handling units

An extract unit removes air from rooms or processes. Replacement air must enter through a designed supply or transfer path. Filters, materials and fan execution depend on the extracted contaminants and temperature.

Ordinary general-ventilation equipment is not automatically suitable for grease, corrosive fumes, explosive atmospheres or smoke extraction. Those duties require a separately specified system and evidence for the exact application.

• Detailed guide: Extract air handling units

A supply-and-extract unit combines two air paths in one coordinated assembly. The flows are selected to meet ventilation and room-pressure requirements; equal nominal flows do not by themselves establish a balanced building.

The assembly may include heat recovery, but having two fans does not prove that it does. Check air-path separation, intake and discharge positions, leakage, control interaction and access to both sides.

• Detailed guide: Supply-and-extract air handling units

Modular AHUs combine functional sections to suit a duty and layout. This allows the air treatment, casing cross-section and access arrangement to be selected together. Modules may be delivered separately or joined before delivery.

A functional section is not necessarily a transport block. Confirm the shipping splits, lifting points, site joints, overall dimensions and clear withdrawal space for the largest replaceable component.

• Detailed guide: Modular air handling units

A compact unit puts a defined combination of functions into a relatively small enclosure. It can be supply-only, extract-only or two-stream with recovery. A box fan is one possible assembly; it is not the definition of every compact AHU.

Compare the complete installed envelope, including ducts, drainage and filter access. A smaller casing may increase internal air velocity, pressure loss and sound; suitability depends on the measured selection data.

• Detailed guide: Compact air handling units

Location is a separate classification from function. Indoor units need a suitable plant room and drainage. Outdoor units need weather-resistant construction, protected openings, a suitable base and freeze protection. Roof placement also requires structural and access coordination.

Placement has three independent attributes: location indoors or outdoors; mounting on a floor/base or by a permitted suspended arrangement; and, for two air paths, stacked or side-by-side construction. A location does not determine the mounting or air-path arrangement. Confirm the permitted orientation, load-bearing supports, drain position, weather protection and access for the actual assembly.

Door hardware provides a grip, retains the service panel and helps maintain access to the casing. A pull handle, a latch or clamp and a hinge perform different tasks; their compatibility depends on the actual door construction.

Check the panel thickness, mounting pattern, opening direction, gasket contact, service clearance and materials for the operating environment. Loads, sealing and any safety function require confirmation for the complete assembly.

These are separate construction examples. The 3D motion explains the mechanism; it does not establish loads, tightening torque, permitted travel or compatibility with a particular unit.

• Service access and maintenance
• Confirm the supplied hardware

Heat recovery uses a temperature or moisture difference between extracted and incoming air to reduce treatment demand. A plate device exchanges energy across separating surfaces; a rotor transfers it through a rotating matrix; a run-around system uses two coils and a pumped liquid loop. Moisture transfer depends on the selected technology.

Compare performance at the same airflows and conditions, plus leakage or carry-over (extract air retained in the rotor and transported into the supply path), pressure losses, pump/fan energy, bypass and frost strategy. A high recovery percentage is not the same as annual energy savings or adequate supply temperature.

• Detailed guide: Heat-recovery air handling units

Hygienic execution addresses cleanability, drainage, access, suitable internal surfaces, sealed filter mounting and controlled leakage. It is a construction and verification requirement that can apply to different airflow and treatment arrangements.

Define the room risk, pressure relationships, filtration stages and cleaning procedure. Fine-particle or high-efficiency filtration does not make a complete installation suitable for a cleanroom or clinical process by itself. The project needs the required performance evidence and commissioning results.

• Detailed guide: Hygienic air handling units

DX means direct expansion: refrigerant evaporates in the air-cooling coil and absorbs heat. In a reversible heat-pump system, a coil can also heat air when refrigerant condenses in it. A water coil instead receives cooled or heated liquid from a separate plant.

A DX AHU must be matched to its refrigeration equipment and controls. Check refrigerant, airflow range, coil circuits, capacity modulation, outdoor limits, defrost interaction and permitted pipe lengths/elevations. Nominal cooling capacity alone does not establish compatibility.

• Detailed guide: Direct-expansion air handling units

Hot water, glycol mixtures, refrigerant, electricity or steam may provide treatment energy where the selected section supports them. Specify fluid type, concentration, temperatures and flow separately from coil materials. Glycol changes heat transfer and pressure loss as well as freeze resistance.

Relative humidity expresses how close air is to saturation at its current temperature; moisture content expresses how much water vapour it carries. Heating alone lowers relative humidity without removing water. Cooling below the dew point (the temperature at which water vapour begins to condense for the stated moisture content and pressure) removes water as condensate. Particle filters do not remove every gas or odour; specify the contaminant and the corresponding treatment.

Offices, schools, retail and accommodation need ventilation matched to occupancy and operating hours. Production spaces add process emissions and heat loads. Pools add high moisture and corrosive conditions. Laboratories, healthcare and clean areas add pressure, contamination and verification requirements.

A shared AHU name does not make these duties interchangeable. Establish whether air may be recirculated, whether energy recovery creates a contamination concern and which areas need separate systems. Fire, explosion, process exhaust and critical hygiene requirements need project-specific design.

Begin with room use, occupancy, pollutant and moisture sources, design weather and the required indoor conditions. Determine outdoor-air needs, supply/extract balance, pressure relationships and operating modes before choosing a casing size.

Coordinate duct resistance, sound paths, water or refrigerant services, electric power, drainage, structure, fire strategy and control responsibilities. The result is a system specification and a defined AHU duty; a catalogue selection is only one part of that design.

Specify supply, outdoor and extract airflows in m³/h or m³/s, the design air conditions and the required external static pressure in Pa at those flows. State the inlet/outlet connection boundaries for each air path.

Account separately for losses in internal filters, coils and other sections. Check the fan duty point against both the network and internal components, with the stated filter condition. The selection sheet must connect this duty to the chosen configuration, performance and limits.

External static pressure

The static pressure difference required for the network outside the AHU at a stated airflow and defined unit connection boundaries; internal unit losses are accounted for separately.

• Check the document set
• Compare the supply scope
• Verify the selected duty during commissioning

Airflow is volume per time: divide m³/h by 3600 to obtain m³/s. For an illustrative sensible-air heat balance, Q = ρ × cp × V × ΔT: Q is heat rate in kW; ρ is air density in kg/m³; cp is specific heat in kJ/(kg·K); V is flow in m³/s; ΔT is temperature change in K. Use density for the stated air condition.

This balance excludes condensation, humidification, leakage and detailed coil performance. Moisture processes require an enthalpy and water balance; enthalpy is the heat-content measure used for moist air. Final selection also needs pressure losses, fan curves, sound, part-load and frost checks. The shared tool calculates only the constant-property sensible air balance; it does not provide equipment sizing or a project design output.

A selected AHU becomes a manufactured configuration through a confirmed section schedule, casing and material specification, fan/coil/filter selections, controls scope and drawings. Production may use standard modules while the assembled duty remains project-specific.

Agree factory checks, transport splits, identification and supplied accessories before release to manufacture. An educational description of production is not evidence that a supplier manufactures a proposed configuration or includes every accessory.

1. 1. Before startingBefore starting, inspect assembly, cleanliness, filters, fan freedom, connections, drainage and control wiring.
2. 2. Airflow setup and measurementSet and measure airflows and balance the connected network; record the conditions and instruments used.
3. 3. Controls and protectionDemonstrate temperature control, alarms, interlocks, damper sequences, frost protection and required building-system signals.
4. 4. Comparison with the agreed dutyCompare measured airflow, power, pressure, sound and leaving-air conditions with the agreed duty.
5. 5. Settings and test recordsRetain settings and test records as the operating baseline.

• Review the agreed selection duty
• Check the documents and commissioning records

Operate against the commissioned airflow, temperature and schedule settings. Track fan power, drain performance, repeated alarms and the pressure difference across filters at the stated airflow. For a filter, the measured difference is used to assess flow resistance. A changed reading can also reflect changed airflow, so it does not prove fouling on its own.

Optimise schedules and demand control while preserving required ventilation and room-pressure relationships. Compare total fan, pump, heating and cooling energy across seasons. A reduced fan speed saves no useful energy if it leaves the occupied space under-ventilated.

Differential pressure

The pressure difference between two stated measurement points. Its meaning depends on the points and operating condition.

Pressure drop

The pressure decrease associated with flow resistance through a component or network; state the measurement basis and airflow.

• Review the commissioning baseline
• Plan maintenance for the operating conditions

Record when the fault occurs and compare it with commissioning data; one symptom may have several causes.

Confirm the failed component and cause before ordering work. A leaking coil, failed motor, damaged seal and control fault require different repairs. Address the cause, including freezing, corrosion, electrical faults or incorrect operating conditions.

Use compatible parts and the manufacturer’s approved repair method. Refrigerant work and safety functions need qualified personnel and the required checks. After repair, verify leakage, protection, airflow and the affected treatment duty; document the change and warranty implications.

Replacement starts from the current duty and interfaces, not the old model name alone. Survey dimensions, access, connections, service side, fan pressure, coil conditions, electrical supply and controls. Confirm whether the original building duty has changed.

Check component withdrawal and whole-unit transport separately. A replacement fan, filter or coil can alter resistance, energy use and controls. Re-select and recommission the affected system; an equal envelope does not prove equal performance.

Technical passport

Identifies the supplied unit and its rated data.

Selection sheet

Records the agreed operating duty.

Arrangement and connection drawings

Define physical interfaces.

Installation, operation and maintenance instructions

Define permitted use and service procedures.

Also establish electrical/control diagrams, conformity or test evidence required for the project, commissioning records, spare-parts references and the final supply list. Each issued document must match the unit, configuration and revision. A general example or family brochure is not that unit’s issued passport or approval.

Obtain the applicable written warranty for the exact supplier and configuration: start event, duration, covered parts/work, exclusions, required commissioning and maintenance records, and the fault-report procedure. These terms differ by contract.

Record serial identification, operating conditions, alarms, photographs and service history when reporting a fault. Do not infer coverage from a catalogue page, a generic warranty period or an unapproved repair. No warranty term is offered by this reference guide.

Compare quotations against the same duty and revision. Confirm responsibilities at every interface.

• Equipment and sparesunit; spare filters.
• Controls and regulationcontrols; sensors; valves; drives.
• Installation accessoriessupports; flexible connectors; drainage accessories.
• Freight and servicesfreight; installation; commissioning.

Ask for a complete configuration code, exclusions, price/currency and validity, delivery basis, production lead time, packing and acceptance documents. Reference coverage is not stock, an offer or a service commitment. Use the manufacturer catalogue to review actual families, then obtain a project-specific selection and commercial offer.

• Prepare the selection brief
• Plan commissioning and acceptance