What do the different air conditioning modes actually do?

You see heat.
You see cool.
You see fan and dry.

You press them.
The system changes behaviour.
The result often surprises you.

Most comfort issues come from using the wrong mode for the job. Each control function has a specific purpose. Using the wrong one at the wrong time creates cold rooms, stale air, or slow heating.

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Heating mode explained

Heating mode adds heat to the room.

The system extracts heat from outdoor air and moves it indoors. It is designed for space heating, not quick blasts of warmth.

Heating mode works best when
You want steady warmth
Outdoor temperatures are cool or cold
You plan to leave the system running

Warm air may not feel immediate. Comfort builds as the room structure warms.

Heating mode should be your default in winter.

Cooling mode explained

Cooling mode removes heat from the room.

It lowers air temperature and removes moisture as a by product. Cooling mode is designed for warm weather comfort, not air movement alone.

Cooling mode works best when
The room feels hot
Outdoor temperatures are warm
You want temperature reduction

Using cooling mode in winter causes cold air complaints, even when the temperature setting looks sensible.

Fan only mode explained

Fan only mode moves air without heating or cooling it.

The system circulates existing room air. No heat is added. No heat is removed.

Fan only mode works best when
You want air movement
The room temperature already feels comfortable
You want to circulate stale air

In winter, fan only mode usually makes rooms feel colder.

Dehumidifying mode explained

Dehumidifying mode focuses on moisture reduction.

The system runs in short cooling cycles with low fan speed to remove moisture from the air. Temperature control is secondary.

Dehumidifying mode works best when
The room feels clammy
Humidity is high
Temperatures are mild

Dehumidifying mode is not designed for heating. In winter, it often feels cold and uncomfortable.

Why dehumidifying feels cold

Dehumidifying relies on cooling.

Even though the temperature setting may stay the same, the system cools air slightly to remove moisture. The fan runs slowly, which exaggerates the cold feeling.

This is normal behaviour.

Dry mode should not be used for winter comfort.

Auto mode combines all functions

Auto mode lets the system choose between heating and cooling.

It reacts to sensor readings near the indoor unit. It does not consider comfort evenly across the room.

Auto mode works best in
Small rooms
Stable temperatures
Minimal sunlight variation

In winter, auto mode often causes unwanted cooling.

Why temperature alone does not control behaviour

The temperature sets a target.
The mode sets the action.

If the mode is wrong, the temperature number does not matter. This is the most common misunderstanding with air conditioning controls.

Always choose the correct mode before adjusting temperature.

Common mode mistakes that cause comfort issues

These appear frequently during support visits.

Using fan only mode expecting heat
Using dehumidifying mode in winter
Leaving cooling mode active from summer
Relying on auto mode for heating
Changing temperature without checking mode

Each one causes predictable problems.

How to choose the right mode day to day

A simple rule helps.

Cold outside and you want warmth
Use heating mode

Hot outside and you want cooling
Use cooling mode

Room feels stuffy but temperature is fine
Use fan only briefly

Room feels clammy in mild weather
Use dehumidifying mode

Avoid auto mode unless conditions are very stable.

Why switching modes too often causes problems

Each mode change resets the system.

Protection delays restart
Heating pauses
Airflow changes

Frequent switching stops the system from stabilising. Comfort drops instead of improving.

Pick the correct mode and allow time.

When mode choice is not the issue

If the correct mode is selected and comfort does not improve, the cause may sit elsewhere.

Possible reasons include
Room heat loss
System sizing limits
Airflow restrictions
Sensor placement

These need assessment rather than further mode changes.

When to seek professional support

Contact a specialist if
The system behaves incorrectly in the correct mode
Modes change without input
Error codes appear
Heating or cooling never stabilises

Before calling, note the mode selected, temperature, fan speed, and outdoor conditions.

This speeds up 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

UK Government guidance on indoor air quality
https://www.gov.uk/government/publications/ventilation-and-indoor-air-quality

ClimateWorks residential usage 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 comfort issues and ensuring air conditioning systems perform as intended throughout the year.

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