Air Handling Units (AHUs)

An air handling unit conditions and moves air for a zone, a floor or a whole building. Inside its casing are fans, filters, heating and cooling coils, often heat recovery, sometimes humidification, dampers for outside, return and exhaust air, sound attenuators and a drain pan under the cooling coil. Units sit in plant rooms, on roofs or in ceiling voids, and serve the ductwork and terminal units downstream.

Units vary from large site-built units in office plant rooms serving several floors, through packaged rooftop units on retail and industrial buildings (often with gas heating or direct expansion cooling built in), to dedicated outdoor air units supplying ventilation to fan coils or chilled beams. Hospitals, laboratories and cleanrooms have units built to stricter hygiene and filtration standards.

What shortens life:

• Neglected filters, which let dust foul coils and fans and raise fan energy.
• Blocked drains and standing condensate, which corrode drain pans and casings and grow microbes.
• Coastal and industrial air on rooftop casings and coils.
• Freezing in cold climates where frost protection fails.
• Poor access inside units, so parts are never cleaned or inspected.
• Running 24 hours when schedules have been lost or overridden.

Hygiene failures

Cooling coils produce condensate. If the drain pan does not drain because it is flat, corroded, or the trap is dry or the wrong depth for the fan pressure, water stands in the pan and grows bacteria and fungi. Wet fibrous insulation inside casings and attenuators does the same. Air carries spores and odours downstream. This is the commonest cause of musty smells from air conditioning and a recognised indoor air quality hazard.

Coil fouling and failure

Dust bypassing filters coats coil fins and cuts heat transfer; the coil then cannot cool or dehumidify fully and the chilled water system sees less temperature difference (low delta T). Coils corrode at fins and tube joints, and chilled water coils leak; heating and cooling coils burst when frozen.

Fan and drive wear

Bearings wear and run hot; belts slip and shed; pulleys wear; fans lose balance and vibrate. Older forward-curved and belt-driven fans use far more energy than modern direct-drive plug fans with electronically commutated motors.

Damper and outside air faults

Outside air dampers seized shut starve the building of fresh air, raising carbon dioxide and complaints; seized open, they waste energy and can freeze coils. Economy cycle failures lose free cooling. Minimum outside air settings drift as controls change.

Casing corrosion and leakage

Panels corrode from condensation and drain pan leaks; seals harden; rooftop units let in rain. Leakage into a unit from the plant room or out of the unit wastes energy and bypasses filters.

Heat recovery faults

Wheels lose seals and leak exhaust air into supply; drives fail and wheels stop; plate heat exchangers foul. Recovery efficiency falls without obvious signs.

Humidifier risks

Steam humidifiers scale; adiabatic (spray or pad) humidifiers can grow Legionella and other bacteria if water quality and maintenance lapse.

Contaminated intakes

Outside air intakes near cooling towers, car park exhausts, kitchen exhausts, loading docks or flues draw contaminants into the building.

At the unit

• Nameplate data: manufacturer, year, airflow, fan type.
• Water on the plant room floor; corrosion at the base of casings; rust stains below rooftop units.
• Drain pan condition: standing water, slime, rust; trap present and full.
• Filters overloaded, missing, or bypassed around frames.
• Coil fins dirty, flattened or corroded.
• Noise and vibration; belt dust; hot bearings.
• Dampers and actuators: position against what the controls show.
• Internal insulation wet, shedding or mouldy.

In the building

• Musty smells when air conditioning starts.
• Stuffiness, high carbon dioxide and complaints in meetings rooms.
• Dust deposits around supply grilles.
• Temperature and humidity complaints.

In the records

• Maintenance logs: filter changes, belt changes, coil cleaning.
• Hygiene inspection reports.
• Indoor air quality monitoring.
• BMS trends: fan status, damper position, supply temperature.
• Energy consumption.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A mechanical services engineer leads, with a hygiene specialist where microbial issues are suspected.

• Unit inspection with panels open: casing, fans, motors, belts, filters, coils, drain pans and traps, humidifiers, heat recovery, dampers and actuators, attenuators.
• Airflow measurement: supply, return and outside air, against design and code minimums.
• Coil performance: air and water temperatures and flows; pressure drop.
• Fan performance: airflow, pressure, power, vibration.
• Hygiene inspection: visual, swabs and dust samples to AS/NZS 3666.2, BESA TR/19 or ASHRAE 180 practice.
• Indoor air quality: carbon dioxide, particulates, temperature and humidity in served spaces.
• Intake review: distance to towers, exhausts, flues and docks.
• Controls review: schedules, setpoints, economy cycle, outside air control.

The report should give the condition of each unit, airflow and outside air against requirements, hygiene status, and whether to repair, refurbish in place, or replace.

• Hygiene remediation: clean coils, drain pans and casings; replace wet insulation with closed-cell or faced material; fix drain falls and traps.
• Filter upgrade: better grades and sealed frames to stop bypass.
• Fan upgrade: replace belt-driven fans with direct-drive plug fans and electronically commutated motors, often a large energy saving.
• Coil replacement where corroded or leaking; coatings for coastal sites.
• Damper and actuator replacement and outside air control with carbon dioxide sensors.
• Heat recovery repair or addition.
• Refurbishment in place: new internals within the existing casing, where the casing is sound and access is difficult.
• Replacement of the whole unit, sized to current loads and ventilation needs.
• Intake relocation where contamination is the problem.

Inspection, airflow measurement and hygiene testing are lump sums. Component replacements (fans, motors, coils, dampers, humidifiers) are itemised per unit. Refurbishment in place is a lump sum per unit and avoids the cost of getting a new unit into a plant room. Replacement is a lump sum per unit including access (cranes for rooftop units, partial dismantling or wall openings for plant rooms), electrical and controls integration, and commissioning. Hygiene cleaning and filter changes recur.

Replacement and refurbishment are lifecycle; outside air and hygiene rectification can be code compliance; repairs are repairs and maintenance. Fan upgrades to direct-drive EC fans often pay back through energy savings and can sit in an energy line.

• Australia. The National Construction Code calls up AS 1668.2 for ventilation; AS/NZS 3666.1 and 3666.2 set design, installation and maintenance of air-handling systems for microbial control, and in several states maintenance of air-handling systems is an essential safety measure. NABERS Indoor Environment rates indoor air quality.
• United Kingdom. Approved Document F sets ventilation; BS EN 16798-3 covers non-residential ventilation performance; BESA TR/19 sets ventilation hygiene; air-conditioning systems over 12 kW need energy inspection at least every five years; healthcare follows HTM 03-01.
• United States. ASHRAE 62.1 sets ventilation rates, ASHRAE 180 inspection and maintenance practice, ASHRAE 241 control of infectious aerosols, and ASHRAE 170 healthcare ventilation; the International Mechanical Code applies.

Healthcare, laboratory and cleanroom units follow additional standards for filtration and hygiene in all three countries.

• Maintenance logs: filter, belt and coil cleaning records.
• Hygiene inspection reports and any microbial sampling.
• Airflow and outside air test and balance reports.
• Indoor air quality monitoring (CO₂, particulates).
• Installation dates and nameplate data for each unit.
• BMS trends for fans, dampers and supply temperatures.
• Energy consumption data.
• Drawings showing intake locations relative to exhausts and towers.