Why does my air conditioning feel warm one minute and cold the next?

The air feels warm.
Moments later it feels cool.
Comfort never settles.

This behaviour feels like the system cannot make up its mind. In most homes, this happens because of how air conditioning reacts to sensors, airflow, and settings rather than because of a fault.

Air movement changes how temperature feels

Moving air affects how your body senses temperature.

Warm air moving quickly across skin can feel cooler.
Slower air feels warmer even at the same temperature.

This means airflow changes can make the system feel inconsistent even when heat output stays steady.

Fan speed exaggerates temperature swings

Fan speed plays a major role.

High fan speed moves air rapidly and increases heat loss from your body.
Low fan speed reduces air movement and makes warmth feel stronger.

When fan speed changes automatically or manually, the sensation of warmth changes even if the room temperature does not.

Auto mode creates shifting behaviour

Auto mode allows the system to adjust output and behaviour.

Small changes near the sensor can cause heating to reduce briefly. When heating reduces, airflow may continue, making air feel cooler.

This cycle repeats.

Auto mode responds logically.
Comfort becomes inconsistent.

Sensor location drives sudden changes

Temperature sensors sit near the indoor unit.

When warm air reaches the sensor quickly, the system may reduce heating. As heat spreads and cool air returns to the sensor, heating increases again.

This creates a warm cool warm cycle that feels unstable.

Defrost cycles interrupt heat delivery

In winter, outdoor units periodically defrost.

During defrost
Indoor heating pauses
Airflow may slow or continue
Air can feel cool

This temporary interruption often causes short periods of cool air before warmth returns.

This is normal behaviour.

Low fan speed causes uneven room temperatures

Low fan speed warms air near the unit first.

The sensor detects warmth and reduces heating. Cooler air from other parts of the room then moves back toward the unit.

The system responds again.

This back and forth creates fluctuating comfort.

Airflow direction affects consistency

Airflow that points straight down or up can create hot and cold zones.

Warm air collects near ceilings.
Cool air stays lower.

As airflow shifts, different air layers reach you, changing how the system feels.

Correct airflow direction improves stability.

Common setting combinations that cause swings

These appear frequently during winter visits.

Auto mode combined with high fan speed
Low fan speed with high ceilings
Timers adjusting output during the day
Multiple controllers sending commands

Each one creates variation.

How to stabilise comfort

A simple approach reduces swings.

Select heating mode manually.
Use medium fan speed.
Set a sensible temperature.
Avoid auto mode.
Allow 15 minutes for stabilisation.

This removes most causes of warm cool cycling.

When fluctuations are not a settings issue

If swings continue despite stable settings, other factors may be involved.

Possible causes include
Sensor placement issues
System sized mainly for cooling
Large heat loss from the space

These require assessment.

When to seek professional support

Contact a specialist if
Heating cycles rapidly with no stability
Error codes appear
The system shuts down unexpectedly
The outdoor unit does not operate

Before calling, note the mode, fan speed, temperature, timing of changes, and outdoor temperature.

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

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

ClimateWorks residential usage records 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.

‍