AHU Cleaning — Coils, Drain Trays, Fans & Internal Surfaces

The air handling unit is the heart of a mechanical ventilation system. It conditions, filters and moves the air that every supply diffuser ultimately delivers to the occupied space. It is also, by design, the point in the system where air slows down, changes temperature and passes over wet surfaces — exactly the conditions in which dust, biofilm and microbial growth accumulate fastest.

Cleaning the downstream ductwork without addressing the AHU is rarely a complete project. A fouled cooling coil seeds odour and microbial contamination back into freshly cleaned ducts within weeks. A blocked drain tray standing in stagnant condensate is a recognised hygiene risk in any building. AHU cleaning is therefore the foundation of any credible air handling unit hygiene programme.

Coils accumulate compacted dust on the leading face and biofilm on the wet underside of cooling coils. Both reduce heat transfer, increase fan pressure drop and create a foothold for microbial growth. Our engineers clean coils using low-pressure rinsing and approved coil-cleaning chemistry appropriate to the coil type and fin density, recovering effluent to the condensate drain and protecting adjacent insulation and electrical components.

For heavily fouled coils, the work typically restores measurable pressure drop and visibly opens the gaps between fins. Pre- and post-clean photography is taken from both faces of the coil.

Drain trays are flushed, scrubbed, sanitised and checked for standing water and corrosion. The condensate trap is cleared and tested. Where the tray has degraded, we record it in the report and recommend remedial work. Fans — scrolls, impellers and motor housings — are isolated, inspected and cleaned. Impeller fouling is a common cause of vibration and energy loss in older units.

Filters are replaced as part of the visit where the existing filters are loaded or end-of-life, with frame seals and bypass paths checked. Internal surfaces — panels, insulation faces, attenuators and supply plenums — are cleaned with HEPA-extracted methods. Where internal insulation is degraded, damaged or shedding fibre, we record the condition and flag it for repair rather than masking it with surface cleaning.

AHUs serving healthcare, food production and sensitive office environments need contamination control beyond a routine clean. Where the brief calls for it, we work alongside the building's M&E team to plan isolations, segregate clean and dirty zones, bag and remove waste under controlled conditions and re-commission the unit cleanly. For healthcare-adjacent ventilation, our cleaning methodology is compatible with HTM 03-01 expectations, though the formal classification of a clinical space remains the responsibility of the duty holder and the relevant authorising engineer.

Every AHU project closes with photographic evidence and a written report. Combined with a routine ventilation system inspection cycle, this gives the building a clear, defensible AHU hygiene record over time.

Most AHU cleans require the unit to be isolated for several hours. We routinely plan work for evenings, weekends or scheduled shutdown windows, and stage the cleaning across multiple AHUs so that the building is never without ventilation for longer than necessary. For estates with several plant rooms, we can deliver a rolling annual programme so that no single visit causes major disruption to occupants.

For a written AHU cleaning quotation, request a quote, book an inspection or contact us. For broader ductwork hygiene, see our duct cleaning, HVAC duct cleaning and duct hygiene services.