Air conditioning
smells
Air conditioning that smells musty for the first half minute and then settles has something growing on the indoor coil. It is the commonest complaint about a system that is otherwise working, and it is a cleaning job, not a breakdown.
- 89 pounds plus VAT to diagnose, waived if we do the repair
- Any make and any age, including other installers' systems
- We will tell you when a repair is not worth doing
Prefer to talk? 020 3393 1331
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Air conditioning we have fitted
Our own jobs, photographed on site. Real rooms, real outdoor positions, nothing from a catalogue.
Air conditioning
Outdoor unit on wall brackets
Above brick, isolator and cabling run tidy
Air conditioning
Condenser going up on straps
Lifted into a wall position, no scaffold
Air conditioning
Wall unit, commissioned
Set to 24 degrees and handed over
Air conditioning
Black wall unit in a sitting room
Chosen to sit with the room, not against it
Air conditioning
Ceiling cassette
Recessed flush into the ceiling
Air conditioning
Samsung Wind-Free
Cooling without a draught on the room
Air conditioning
Rooftop commissioning
Pressure tested and charged on gauges
Air conditioning
Outdoor unit at ground level
On paving beside the house, easy to service
Air conditioning
Black wall unit, running
Quiet enough for a bedroom
Air conditioning
One-way ceiling cassette
Only the grille shows
Air conditioning
Outdoor unit on paving
Against render, clear of the boundary
Air conditioning
Condenser high on a wall
On brackets where the ground was not an option
Air conditioning that smells musty, damp or of mildew when it starts almost always has biological growth on the indoor coil and in the condensate tray. A deep clean of a heavily soiled indoor unit is 160 pounds plus VAT, and an annual service at 95 pounds plus VAT stops it coming back.
Wash the filters first, because that is free. If the smell is still there the next day, it is behind the filters, on the coil, where a householder cannot reach.
Read the full guide: 8 sections, about 4 minutes
Two things to do first, because they are free
Wash the filters
Open the front of the indoor unit, slide the filters out, wash them in warm water and let them dry completely before they go back. A filter that has not been touched for a year holds dust, cooking grease and whatever else was in the room air, and it can smell on its own.
Notice when it smells, because the timing tells us the cause
A musty smell that arrives within half a minute of switching on and then fades is growth on the coil. A smell that is there the whole time the unit runs is more often the filters or the drain. A burning or hot plastic smell is different: switch the unit off at the isolator and call.
What each smell usually is
| What you smell | Most likely cause | Typical cost |
|---|---|---|
| Musty, damp or mildew when it starts, then it fades | Biological growth on the indoor coil and blower wheel | £160 deep clean |
| Stale all the time, with weaker airflow than it used to have | Blocked filters and a dusty coil | Free to wash the filters, or a £95 service |
| Stagnant water, sometimes with a drip from the indoor unit | Slime in the condensate tray and a partly blocked drain | £120 to £220 |
| A drain smell that comes and goes with the weather | The condensate pipe joined to a waste pipe with no working trap | Quoted after inspection |
| Burning or hot plastic | An electrical fault. Switch it off and do not run it | £89 diagnosis, repair quoted |
Prices include labour. VAT is extra. If the fault should still be under manufacturer warranty we will tell you and help you claim rather than charging you for it. Every figure on this site is an estimate built from the quotations we issued in 2026 and our own installation experience. It is not a quotation. The price you are given after the free survey is the price you pay.
Why an indoor unit starts to smell in its second or third summer
In cooling mode the indoor coil runs below the dew point of the room, so it is wet whenever the unit is on. It also sits in the dark behind a plastic cover and catches the fine dust the filters let through. Wet, dark and fed is all that mould and bacteria need.
Nothing happens for the first season. By the second or third the growth has covered the fins and the blower wheel behind them, and each time the fan starts it pushes the air that has been sitting against that surface into the room. That is why the smell is strongest at switch-on and fades as fresh air moves through.
It is not a sign of a cheap unit or a bad installation. It happens to any make, wherever a system has run for two or three summers without the coil being cleaned.
Why a spray can makes it worse
Deodorising sprays and foam cleaners sold for air conditioners put scent through the coil and leave a residue behind. The scent lasts a week or two. The residue stays on the fins, holds more dust, and gives the growth something else to live on, so the smell returns stronger than before.
A proper clean is mechanical. The covers come off, the coil is cleaned with the right chemistry and rinsed, the blower wheel is taken out and washed, and the condensate tray and drain are flushed so the unit is not sitting over standing water. That is what the 160 pound deep clean is.
What keeps it from coming back
An annual service. The 95 pound service cleans the filters and the indoor coil and flushes the drain, which removes the growth before it is established enough to smell. On the same visit each further unit is 55 pounds plus VAT.
Between services, wash the filters every couple of months in the cooling season. Many units also have a dry or self-clean setting that runs the fan after cooling stops, so the coil is not left wet. If yours has one, the manual will show it, and it is worth leaving on.
What happens when you call us
- You describe the symptom. Half the time that is enough for us to tell you the likely cause and the likely cost before anyone travels.
- We attend and diagnose properly, with instruments rather than by guessing. That is 89 pounds plus VAT.
- You get the cause, the price and an honest answer on whether it is worth repairing, before anything is touched.
- If you go ahead on the same visit, the diagnosis charge comes off.
Other faults
Not cooling
Running but blowing warm air, and the five things that cause it.
Not coolingLeaking water
Water down the wall from the indoor unit. Nearly always the drain.
Leaking waterMaking a noise
What each noise means, and what to do when a neighbour complains.
Making a noiseShowing a code
Verified Daikin and Mitsubishi fault codes, and what each family costs.
Showing a codeRelated
All air conditioning repairs
Every symptom, cause and price in one table, plus when replacing beats repairing.
RepairsAnnual servicing
95 pounds plus VAT. The one thing that prevents most of the faults on this page.
Book a serviceReplacement cost
What a new system costs, for when the repair stops making sense.
See prices
Your estimate
£120to£700
Plus VAT, and the 89 pound diagnosis comes off if we do the work.
Send this on WhatsAppWhat the repair is likely to cost
Diagnosis is 89 pounds plus VAT and it is waived entirely if you go ahead with the repair. What it costs after that depends on what it turns out to be.
Last step: where do we send your written price?
Every figure on this site is an estimate built from the quotations we issued in 2026 and our own installation experience. It is not a quotation. The price you are given after the free survey is the price you pay.
Questions people ask about this
Why does my air conditioning smell musty when I turn it on?
Because there is mould and bacteria growing on the indoor coil and the blower wheel, and the first air through the unit carries the smell into the room. It fades after half a minute or so because fresh air has replaced the air that was sitting inside.
Washing the filters helps a little. Clearing it properly means cleaning the coil and the blower wheel, which is behind the covers.
How much does it cost to get rid of the smell?
A deep clean of a heavily soiled indoor unit is 160 pounds plus VAT: chemical coil clean, blower wheel removed and washed, tray and drain flushed. If it is caught early, the 95 pound annual service is usually enough.
For several indoor units, tell us how many and what make they are and we will give you a figure for the visit before we come out.
Do you clean systems you did not install?
Yes. Any make and any age, including systems another company fitted. Tell us the make and roughly how old it is when you get in touch, so the engineer arrives knowing how that unit comes apart.
Is the smell bad for us?
We are engineers, not doctors. What we can tell you is what it is: mould and bacteria on a wet surface, with room air being blown across it. If anyone in the house has asthma or allergies, that is a reason to have it cleaned sooner rather than leave it for another season.
Can I clean the coil myself?
The filters, yes, and you should. The coil and the blower wheel we would leave alone. The fins bend at a touch, the electronics sit beside them, and water or foam in the wrong place turns a cleaning job into a repair.
It is also the part where shop-bought sprays do the harm described above.
It smells of burning. Is that the same thing?
No. A burning or hot plastic smell is electrical: a fan motor, a connection or a circuit board. Switch the unit off at its isolator and do not run it again until it has been looked at. A diagnosis is 89 pounds plus VAT and it comes off the repair.
Tell us the symptom
Describe what it is doing and we will tell you the likely cause and a realistic price before we come out. Diagnosis is 89 pounds plus VAT and it comes off the repair.
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