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HVAC smells: What causes unpleasant air-conditioning odour?

For building occupants, unpleasant odours emanating from the air-conditioning system can make life pretty uncomfortable. But what are the causes of HVAC smells, and how can they be combatted? Here we look at some of the usual culprits.


Dirty socks

A common reason for HVAC system odours is the presence of moisture and the resulting mould which creates a ‘dirty sock’ smell as it’s known. Stagnant water and moisture in the air means mould can form on evaporator coils, in drip pans and on drain lines. It has the appearance of dark, slimy sludge along with that unmistakeable earthy, musty smell.

Rotten eggs

Many air-conditioners are part of an overall HVAC system which may house a heat pump. Should this pump be fuelled by natural gas, then a ‘rotten egg’ smell indicates that there may be a gas leak. This is a serious problem and if it’s suspected you should turn off your system immediately, open windows and call in your gas company or HVAC provider.


If your system is discharging a burning or smoke-like smell then this suggests mechanical failure of a fan, compressor or electric motor. In this case, power down the unit immediately before contacting a HVAC professional.


Birds and rodents can find their way into ducts where they become trapped and die before decomposing. You should also check ducts for decaying food which may have been dropped through floor vents.

If left unattended, these odours won’t get any better, so swift action is required before the problem becomes unbearable for building occupants. You may be able to access components and ducts yourself, but if you’d prefer your air-conditioning system to be professionally examined and safely cleared of mould and other debris, look no further than Gibbons HVAC Services. As an experienced provider and installer of HVAC equipment, we know air conditioning systems inside out and are expert at identifying and remedying odour problems.

For more information, call Gibbons’ HVAC Services Manager Andrew Knight on 07850 204915 or email

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