This post explains why an air conditioning system can feel cold even when set to 21°C. It focuses on how operating modes affect heating, why temperature alone does not control output, and how correcting a single setting usually restores heat within minutes.
You set your air conditioning to 21°C.
You expect the room to warm up.
Cold air comes out instead.
This is one of the most common winter problems for people who already use air conditioning daily. It often leads to unnecessary call outs, frustration, and the assumption that something has failed. In most cases, the system is working exactly as designed. The issue sits in how the system is being used.
Your air conditioning does not respond to temperature alone.
It responds to the operating mode you select, the current room temperature, and the target temperature you set. The operating mode tells the system what outcome you want. The temperature only defines the limit it works towards.
If the mode is wrong, the system will deliver the wrong result even if the temperature looks sensible.
Many people assume that setting 21°C always means warmth. That assumption causes most winter complaints.
If your system remains in cooling mode, it will attempt to cool the space down to 21°C. If the room temperature is higher than this, cold air will be delivered. Even when the room temperature sits close to the target, the system may still circulate cooler air during operating cycles.
The number on the screen looks right.
The instruction behind it is not.
Most modern air conditioning systems include multiple operating modes.
The most common are cooling, heating, auto, fan only, and dry. Cooling mode removes heat from the room. Heating mode adds heat to the room.
If cooling mode remains active from summer, the system will continue to cool even during winter. This explains why many homes feel cold despite a 21°C setting.
Look at your remote control or wall controller display.
Cooling mode is usually shown by a snowflake symbol, the word COOL, or a blue indicator. Heating mode is usually shown by a sun symbol, the word HEAT, or a red or orange indicator.
If you see a snowflake, the system is cooling. It will not heat the room until the mode is changed.
Switching modes usually takes less than 1 minute.
Press the MODE button.
Scroll through the available modes.
Select HEAT or the sun symbol.
Set your temperature again.
Wait 3 to 5 minutes.
Most systems delay heating start up to protect internal components. During this time, airflow may feel neutral or slightly cool. This is normal behaviour.
Heating mode behaves differently to cooling.
When heating is selected, the system may pause, slow the fan, or delay airflow. This prevents cold air being blown into the room while the internal coil warms to a safe temperature.
Avoid changing settings repeatedly during this phase. Allow the system time to stabilise and reach normal operation.
Auto mode sounds useful but frequently creates confusion.
In auto mode, the system decides whether to heat or cool based on internal sensors. This can result in unwanted cooling when sunlight warms part of the room, outdoor temperatures change quickly, or the system overshoots slightly.
For predictable winter performance, heating mode provides more consistent results than auto.
Raising the temperature does not make air conditioning heat faster.
These systems operate within a fixed output range. Setting 25°C instead of 21°C does not increase heating speed. It only increases how long the system runs.
A realistic target temperature combined with patience produces better results.
Fan speed influences how quickly heat spreads.
Low fan speed feels gentle but distributes heat slowly. Medium or higher fan speeds move warm air faster and balance room temperature more evenly.
If the room feels cold, increase the fan speed for the first 10 minutes. Once the space warms, reduce it for quieter operation.
Air conditioning heats moving air. Radiators heat surfaces first.
This creates a different sensation. With air conditioning, heat spreads faster and air movement is noticeable. With radiators, warmth builds slowly through walls and objects.
The system is functioning correctly. The difference lies in how heat is delivered.
These issues appear repeatedly during winter call outs.
Cooling mode left on from summer
Auto mode used instead of heating mode
Fan only mode selected accidentally
Timers still active from warmer months
Multiple controllers set differently
Windows or doors left open nearby
Checking these settings often resolves the problem immediately.
Sometimes the mode is correct and the room still feels cold.
Possible causes include systems sized mainly for cooling, low outdoor temperatures affecting output, high ceilings, open plan layouts, poor insulation, or restricted airflow from dirty filters.
These situations require a site assessment rather than a settings change.
Contact a specialist if heating mode remains active but the room never warms, the system shuts down repeatedly, error codes appear, no airflow is present, or the outdoor unit does not operate during heating.
Before calling, note the model number, controller type, what the display shows, and how long the issue has been occurring. This speeds up diagnosis.
If you are unsure whether your system is set up correctly for year round use, this guide links closely with residential air conditioning installation and support information available at
https://www.climateworks.co.uk/residential-air-conditioning
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 support records from installations across Hampshire, Berkshire, Surrey, and Oxfordshire between 2022 and 2025
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 has advised 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 system design 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 as intended throughout their service life.
The trusted name in air conditioning, refrigeration, and ventilation. Rely on us for exceptional service you can count on. Fully accredited for quality and safety.
ClimateWorks UK Limited, 11 Vickers House, Basingstoke RG24 9NP
Registered in England & Wales: 14878167
Copyright 2025 - ClimateWorks
.png){kind=small}
