Why Your Split System Air Conditioner Smells - And What to Do About It

Why does my split system air conditioner smell?

If your split system smells when you turn it on - especially on cooling - you’re not imagining things. It’s one of the most common summer complaints we hear from Melbourne homeowners.

Most air conditioner smells come down to moisture, bacteria, or drainage issues inside the indoor unit.

Here are the main culprits.

1. Mould and bacteria on the indoor coil

When your air conditioner cools, it pulls moisture out of the air. That moisture sits on the evaporator coil and drip tray. Over time, dust + moisture = mould.

This usually causes:

Musty or damp smells
A sour or vinegary smell when cooling starts
The smell fading after a few minutes, then returning

This is by far the most common cause.

2. Blocked or dirty drain line

If the condensate drain is partially blocked, stagnant water builds up inside the unit. That water goes stale fast.

Signs include:

Vinegar or dirty-sock smells
Occasional dripping inside
Smell strongest right after startup

A blocked drain won’t fix itself. It only gets worse.

3. Dirty air filters (the easy one)

Clogged filters trap dust, skin cells, pet hair, and moisture. Once bacteria gets involved, the smell follows.

If you haven’t cleaned your filters in the last 2–3 months, start there. It’s basic maintenance, but it matters.

4. Electrical or plastic smells (less common, more serious)

A burning or chemical smell is not normal.

This can indicate:

Overheating components
Wiring issues
Failing control boards

If the smell is sharp, electrical, or persistent - turn the unit off and get it checked.

Can I fix the smell myself?

You can reduce the smell, but most DIY fixes are temporary.

Things that help:

Wash and dry the filters
Run the unit on fan mode for 20–30 minutes to dry it out
Lightly clean visible dust around vents

What usually doesn’t work long-term:

Spraying random cleaners into the unit
Masking smells with fragrances
Ignoring the drain line

If mould is already on the coil, it needs a proper clean.

When should you book a professional clean?

You should get the unit professionally cleaned if:

The smell keeps coming back
It’s worse on cooling than heating
The unit hasn’t been serviced in years
You notice reduced airflow or efficiency

A proper clean involves the indoor coil, drain tray, drain line, and fan barrel - not just the filters.

The takeaway

Smelly split systems are common, fixable, and usually preventable. Left alone, they get worse, not better.

If your air conditioner smells when cooling, it’s telling you something. Ignoring it just means breathing it in all summer.