Turning on your air conditioning should mean instant relief from the heat, not an unpleasant smell filling the room. If you’ve noticed a strange odour coming from your AC unit, there are several possible causes, which can normally be attributed to:
In this blog, we’ll look at the common reasons your air conditioning smells, how to fix it and answer the common questions you may have.
Here’s a quick guide to the most common air conditioning smells, what tends to cause them, and how urgent each one is.
| Smell | Likely Cause | What to do | Urgency |
| Musty/damp | Mould from a blocked drain or dirty filters | Clean/replace filters, clear the condensate drain | Low – routine maintenance |
| Wet dog | Moisture + bacteria on the coils | Deep-clean coils, replace filters, reduce humidity | Low – medium |
| Rotten eggs/sulphur | Dead pest decomposing or a possible gas leak | Rule out gas first, then organise a professional duct/unit clean | High – check for gas immediately |
| Urine/ammonia | Mould breakdown or pest infestation | Professional inspection, cleaning, and mould remediation | Medium |
| Burning/acrid | Electrical or mechanical fault | Shut the unit down, call an engineer | High – fire risk |
| Stale | Dust/organic debris in clogged filters | Clean or replace filters | Low |
There can be many reasons why your air conditioning smells. What it smells like may be the clue as to why it smells in the first place. We have many people coming to us asking why their air conditioning smells like eggs, or why their air conditioning smells like wet dogs, and the reasons typically stem from condensation, dust and debris, or faults with the air conditioning unit.
Air conditioning units produce condensation internally as they cool the air. This is because the refrigerant inside the pipes gets cold. Water in the air then condenses, and droplets form on the coils within the system.
In most cases, these droplets should drain away, but in some cases, the route is blocked or broken. This leaves the moisture where it is, enabling the damp environment to be a home for mould growth and dirty water. When you turn the air conditioning on, this mould and old water contribute to the musty smell from the air conditioning unit.
Any piece of equipment can develop a fault. Continuous use naturally leads to wear and tear. In the case of air conditioning, this could be parts overheating or breaking, for example. When this happens, you’ll notice a burning smell coming from the unit. Due to the potential fire risk and harm that could be caused, it’s best to have a strong burning smell from your air conditioning checked right away. If you notice yours starts to exhibit this issue, shut the system down right away.
Air conditioning units naturally gather dust and pollen as they operate. This means that over time, the filters in the units can become clogged unless cleaned periodically. If left uncleaned the air con unit will gradually be unable to function properly and start to accumulate trapped particles inside. These could vary from the dust and pollen we have already mentioned to other organic matter that may start to decay. As this process takes hold, you could notice a stale smell coming from the air conditioner when you turn it on.
Several smells could indicate faulty air conditioning, and it’s worth noting that some could indicate a more severe problem than others. From air conditioning that smells like eggs to air conditioning that smells like urine, let’s take a look.
Damp or mouldy smells from your air conditioning are normally due to a lack of general maintenance. There could be blocked filters or drainage, and both will result in trapped moisture, which helps to speed up mould growth and lead to unpleasant smells in the home. Mould can be dangerous to health, so this issue should be tackled as soon as possible.
If your air conditioning smells like a wet dog, it is usually a sign of moisture and bacteria build-up on the coils within the unit. As damp lingers on the coils and mixes with the bacteria that gather there, you’ll notice a very distinctive musty, animal-like odour every time you use the system. It’s similar to the damp and mouldy smell we mentioned earlier, but with a bit more “kick”. It can normally be fixed by cleaning or replacing the filters, but if the smell persists, the coils may require specialist cleaning from a professional.
If your air conditioning smells like eggs or, more specifically, rotten eggs or sulphur, it’s not one to ignore. The first thing is to rule out a gas leak. Natural gas has a sulphur-like scent added so that leaks are easy to detect, so if the smell is strong, treat it as a possible leak. Leave the property and call the National Gas Emergency service on 0800 111 999. Once gas has been ruled out, the most likely cause is a small animal that has found its way into the unit and died there, with the smell circulating every time the unit is turned on. An air conditioning engineer can locate the pest and clean up the affected parts.
If your air conditioning smells like urine, often described as a sharp or ammonia-like smell, it generally comes down to one of two causes: mould or pests. As certain moulds grow and break down inside a damp system, they can give off an ammonia-like odour that’s easily mistaken for urine.
The other likely explanation is a pest problem, where droppings, urine or even a dead animal can create the smell.
Whether pests or mould, both should be treated right away; mould spores and pest waste can quickly affect the air quality in the home. Hiring an engineer to identify the cause and clean the system can solve the issue quickly, safely and professionally.
A burning or acrid smell is one to take very seriously. A faint smell that fades within a minute or two is usually just dust burning off the internal parts. A strong, sharp or persistent burning smell could mean electrical or mechanical faults such as overheating components or faulty wiring. This is a genuine fire risk, and the system should be shut down and an engineer called.
The best way to stop your air conditioning from smelling is with regular maintenance. Keeping the filters clean, ensuring the drainage is clear, and the coils are free of mould and bacteria will help keep your unit functioning at its best.
In most cases, if the smell persists after the filters have been cleaned, it may be best to call an engineer.
Some jobs shouldn’t be tackled by anyone other than a trained professional. In fact, many issues need professional attention right away.
Calling an experienced engineer for air conditioning repairs and maintenance in Sussex is often the best solution for removing unwanted smells. You benefit from trained professionals who diagnose and rectify issues safely, leaving your air conditioning unit in its best condition. At Sussex Air Conditioning & Heat Pumps, we install and maintain units across the county. Contact us to book an engineer or find out more about our domestic air conditioning services.
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