Why does my air conditioning smell strange when heating is on?

You switch to heating.
A smell appears.
You worry something is wrong.

This concern comes up every winter. In most homes, unusual smells during heating are normal and temporary. In a smaller number of cases, the smell points to a setting issue or a problem that needs attention.

Why smells appear when heating starts

Heating mode behaves differently to cooling.

When heating starts after a long break, internal components warm up. Dust, moisture, and residues that settled during summer get heated for the first time.

This produces a noticeable smell.

In most cases, it fades quickly.

The most common harmless smells

These smells are normal during early winter use.

A dusty smell
A warm air smell
A slight dry or musty scent

They usually appear during the first few heating cycles of the season. They should fade within 10 to 30 minutes of operation.

Why the smell is stronger the first time

Systems often sit unused for months.

Dust settles on coils and internal surfaces. When heating starts, that dust warms and releases a smell. Once it burns off, it does not usually return.

This is similar to switching on radiators for the first time in winter.

Moisture inside the unit can cause odours

Moisture builds up naturally inside air conditioning units.

During heating, warm air passing over damp surfaces can release musty smells. This is more noticeable if the system has not run recently or if the room stays cool for long periods.

Running the system steadily helps dry internal components.

Fan only mode can make smells linger

Using fan only mode after heating can spread odours.

Fan only mode circulates air without warming or drying internal parts. This can keep smells present longer than necessary.

If a smell appears, continue heating mode rather than switching to fan only.

Why smells sometimes return briefly

Short returns of smell can happen.

This often occurs after
The system has been off overnight
The first heating cycle of the day
Cold and damp weather

These brief smells should fade quickly once the system stabilises.

Smells that should not be ignored

Some smells need attention.

A strong burning smell
A sharp electrical smell
A plastic or chemical smell
A smell that worsens over time

These are not normal. They may indicate electrical issues, overheating components, or material failure.

If these occur, turn the system off and seek professional support.

Why filters are not usually the cause

Many people assume filters cause smells.

In reality, filters affect airflow more than odour. Dirty filters can worsen musty smells by trapping moisture, but they are rarely the main cause of new smells during heating start up.

Smells usually come from internal coils and surfaces.

Room conditions that make smells more noticeable

Some environments amplify odours.

Poor ventilation
High humidity
Cool rooms
Low airflow settings

These conditions allow smells to linger rather than disperse.

Medium fan speed helps clear odours faster.

Common user actions that make smells worse

These behaviours appear often.

Switching the system off as soon as a smell appears
Using fan only mode instead of heating
Judging smells within the first few minutes
Running the system at very low fan speed

Each one prolongs the issue.

When smells point to a maintenance issue

Persistent smells may indicate a problem.

Possible causes include
Excess moisture build up
Restricted airflow
Internal contamination
Drainage issues

These require inspection rather than setting changes.

When to seek professional support

Contact a specialist if
A strong smell lasts beyond 30 minutes
Smells worsen over time
Electrical or burning smells appear
The system shuts down unexpectedly

Before calling, note when the smell occurs, how long it lasts, which mode is active, and whether it appears every time heating starts.

This helps diagnosis.

Related guidance

Further residential air conditioning guidance is available at
https://www.climateworks.co.uk/residential-air-conditioning

References

UK Government guidance on heat pump operation
https://www.gov.uk/guidance/heat-pumps-how-they-work

Building Regulations Approved Document L
https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l

ClimateWorks residential service observations from installations across Hampshire, Berkshire, Surrey, and Oxfordshire between 2022 and 2025

Author

Dr Julian Carter
Technical and Compliance Director
ClimateWorks

Dr Julian Carter has over 20 years of experience in building services engineering, air conditioning system design, and regulatory compliance. He advises on residential and commercial projects across the UK, covering system selection, installation standards, commissioning, and real world performance.

As Technical and Compliance Director at ClimateWorks, he oversees technical governance, installer training, fault diagnosis, and customer education. His work focuses on reducing user related issues, improving system reliability, and ensuring air conditioning systems perform consistently throughout their service life.

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