Why does my air conditioning run but never reach the set temperature?

The system is running.
Air is coming out.
The room never feels right.

This issue frustrates many users in winter and summer. The system looks busy, yet the temperature never settles where you set it. In most cases, the cause is predictable and linked to how air conditioning controls heat, how rooms lose heat, and how settings interact.

How air conditioning decides when to stop

Air conditioning works to balance demand.

It measures room temperature.
It compares that to your target.
It adjusts output within a fixed range.

If the room keeps losing heat faster than the system can replace it, the unit will continue running without ever reaching the target.

Running does not mean failing.
It means chasing a moving target.

Why winter makes this problem more obvious

In winter, heat loss increases.

Cold external walls
Open plan layouts
Large windows
High ceilings

These pull heat out of the room constantly. Even a correctly sized system may struggle to fully overcome this loss during colder periods.

The system keeps working.
The room keeps losing heat.

The role of the temperature setting

The set temperature is a goal, not a command.

If the room temperature stays just below the target, the system may run gently for long periods. This can feel like it is not doing enough, even though it is operating normally.

Raising the temperature higher does not speed things up.
It only increases run time.

Why mode selection affects temperature control

Mode selection matters more than many users realise.

If the system is in auto mode, it may reduce heating or switch behaviour when sensors detect small temperature changes. This interrupts progress toward the target temperature.

Heat mode provides more stable temperature control than auto mode.

Fan speed can slow temperature recovery

Fan speed affects how heat spreads.

Low fan speed keeps warm air near the unit.
Cold areas remain cold.
Sensors may detect warmth too early.

Medium fan speed usually allows the room to equalise faster and helps the system reach the target temperature.

Standing near the unit gives the wrong impression

Airflow near the unit does not reflect room temperature.

Warm air moving across skin can feel cool. This makes users believe heating is weak, even when the room temperature is rising slowly.

Always judge comfort from the centre of the room, not directly under the unit.

Common user actions that prevent reaching temperature

These behaviours appear often during winter support visits.

Changing the temperature repeatedly
Switching modes during operation
Turning the system on and off
Using auto mode in unstable conditions
Leaving doors or windows open

Each one interrupts the control process.

When room size and system size matter

Some systems are sized mainly for cooling.

In these cases, heating output may be lower than expected in winter. The system will run continuously without fully reaching the target temperature during colder days.

This is not a fault.
It is a design limitation.

External temperatures affect performance

Air conditioning systems lose heating capacity as outdoor temperatures drop.

On colder days, the system may still provide heat but struggle to raise room temperature beyond a certain point. This is normal behaviour for air source systems.

Performance improves as outdoor temperatures rise.

When running constantly signals a real issue

Continuous running can point to a problem if comfort never improves.

Possible causes include
Restricted airflow from dirty filters
Incorrect sensor readings
Poor installation positioning
Excessive heat loss from the building

These require assessment.

When to seek professional support

Contact a specialist if
The system runs constantly with no improvement
Heating mode is confirmed and demand is present
Error codes appear
The system cycles aggressively

Before calling, note the set temperature, room temperature, outdoor temperature, and how long the issue has been happening.

This helps speed 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 performance data 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 long term 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.

‍