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Why does commercial air conditioning smell?

Updated 23 October 2026 · SEO Dons Editorial

What does each smell usually mean?

The smell is the first clue, and the pattern narrows it: one unit or all of them, constant or only at start-up, in the room or at the intake. The table matches common smells to likely causes. It is a guide to what to report, not a diagnosis, and the checks in the third column are only those that need no tools and no access to refrigerant or electrics.

SmellLikely causeWhat site staff can note or checkWho deals with it
Damp, musty or mouldyDirt and moisture on filters and coils, or standing condensate in the tray or drainDirty visible filters or grilles, ceiling stains or drips, and whether the smell is strongest at start-upA competent contractor: coil, tray and drain cleaning
Burning, hot or electricalElectrical fault, overheating or a failing motorNothing beyond stopping the unit and isolating itA competent electrician or air conditioning contractor
Gas-like or chemicalHitachi lists gas and refrigerant leakage among the causes of such smellsDo not investigate; note where it is strongestAn F-gas certified company
Traffic, exhaust, cooking or wasteAir drawn in from near a road, car park, kitchen extract or waste areaWhat sits near the fresh-air intake, and whether complaints follow wind or delivery timesFacilities manager with the designer or contractor
Drains or sewageCondensate line connected to a drain without a working trap or siphonWhether the smell is strongest at the drain connectionContractor or plumber
Stale or stuffyToo little fresh air reaching the spaceWhether fresh-air inlets are blocked or closedVentilation engineer

Why does air conditioning smell damp or musty?

Manufacturers describe one chain. Hitachi, writing about its room units, says dirt trapped in the filters affects airflow, stops moisture escaping and allows bacteria to grow. It says the condensate pan can fill with slimy debris that stops condensation flushing away, and that a clogged drain lets condensation build up and cause a stagnant water smell. Daikin says running with dirty filters can leave an unpleasant smell. In ducted systems, BESA says fungal spores and bacteria thrive in damp, poorly maintained ducts, contributing to odours.

That also explains the start-up pattern. Hitachi says the musty smell can stay around and become more noticeable when the unit is switched on.

There is a legal frame. The Workplace Regulations say mechanical ventilation systems must be maintained, including cleaned as appropriate, and HSE says to maintain them in line with manufacturers' instructions. CIBSE suggests a weekly visual check of coils, fans, insulation and condensate drain pans for standing water. What is cleaned and how often is in the air conditioning cleaning guide.

What should you do if it smells of burning?

  1. Stop the unit and switch off its supply at the isolator or circuit breaker, if it is safe to reach. If there is smoke or any sign of fire, follow the building's fire procedure first.
  2. Keep it off until a competent person has found the cause. Daikin's guidance for its room units is to stop operation, turn off the circuit breaker and contact the place of purchase.
  3. Report where and when the smell began to the maintenance contractor or a competent electrician. Hitachi says burning and gas-like smells may need prompt professional intervention, with causes ranging from refrigerant leakage to overheating and electrical malfunction.

Why does the air smell of traffic, cooking or chemicals?

If the smell is in the supplied air rather than coming from the unit, look at the intake. CIBSE says intakes should not be sited in contaminated areas or near where contaminated discharged air is exhausted, and that where contaminants are vapours, fumes and gases, re-circulation is not recommended. HSE says to make sure clean outdoor air is actually supplied rather than assuming it is clean, and that air from heavy traffic or smoke should be filtered. Recirculated air in air-conditioning systems should be adequately filtered.

Kitchens add a second route. BESA says food particles and proteins in extract ductwork encourage microbial growth and unpleasant smells if not removed. The guides to commercial kitchen extraction and grease extract cleaning cover that side.

Why does it smell of drains?

Look at the condensate line. Hitachi says the indoor drain that channels condensation water can cause unpleasant smells where it connects to a general drain, and that fitting a siphon to the tube before the main drain connection prevents it. Ask the contractor to confirm that the line has a working trap or siphon. Hitachi also warns that chemical drain cleaners can deteriorate the pipe and advises contacting an authorised service centre to unclog the line.

Does a bad smell mean legionella?

No. A smell is not a test. HSE says legionella sampling should be tested by a UKAS-accredited laboratory, and that monitoring depends on the system and the risk assessment. A musty smell is a prompt to clean and drain, first of all. Where legionella does become a question, the guide to air conditioning and legionella explains where split and VRF systems sit and what L8 asks of the duty holder.

Which fixes are not for site staff?

  • Refrigerant. GOV.UK says it is against the law to work with F gas without the correct certification, and individuals need their own qualifications too. If a leak is found, it must be repaired as soon as possible and the test repeated within a month.
  • Coils and internal parts. LG says cleaning fins and coils means taking the front section apart, which can expose sharp surfaces and electrical components, and recommends a professional service.
  • Chemicals. Daikin says not to use a commercial cleaning agent inside the unit, because it may harm the resin parts of the heat exchanger or other internal components.
  • Electrics. Isolate and leave to a competent person.

A reader who wants a contractor to inspect a smelling system can send an enquiry through the site's form. What a visit covers is on the air conditioning service page.

Sources

Common questions

Why does the unit smell of cigarette smoke or food from the room?

Daikin says an indoor unit can absorb the odours of the room, furniture or cigarette smoke and discharge them with the airflow, and recommends calling a technician to clean the indoor unit. In kitchens and smoking areas, ask the contractor whether the filter media and coil need cleaning.

Why is the smell worse when the system starts up?

Hitachi says a musty smell from dirty filters can stay around and become more noticeable when the unit is turned on. Smells that appear only at start-up usually point to filters, the coil or the tray, so report that pattern to the contractor.

Could the smell be a refrigerant leak?

Hitachi lists gas and refrigerant leakage among the causes of burning or gas-like smells. Leak checks and repairs must be done by certified people, and GOV.UK says a leak must be repaired as soon as possible and the test repeated within a month.

Is a burning smell always an emergency?

Treat it as urgent until a competent person says otherwise. Hitachi says burning smells may need prompt professional intervention, and Daikin's guidance for its room units is to stop operation, turn off the circuit breaker and contact the supplier.

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