Why is my air conditioning not heating even though it is turned on?

The system is on.
The display looks normal.
The room stays cold.

This situation causes a lot of confusion. Many people assume the unit has failed. In reality, most cases come down to a small number of predictable issues linked to settings, operating conditions, or how air conditioning behaves in heating mode.

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On does not always mean heating

Turning the system on only supplies power.

It does not confirm that heating is active.

Air conditioning needs three things to deliver heat
Power
Heating mode
A temperature demand higher than the room temperature

If any one of these is missing, heating will not start.

Check the operating mode first

The most common reason heating does not start is the wrong mode.

If the system is in cooling, auto, or fan only mode, it may run without producing heat. Many systems remain in the last mode used during summer unless changed manually.

Always confirm the display shows HEAT or a sun symbol before checking anything else.

The temperature must be higher than the room

Heating only starts when there is demand.

If the room temperature is already close to your set temperature, the system may run quietly without producing warm air. This often happens when the temperature is set too low or has already been reached earlier in the day.

Increase the temperature slightly and wait several minutes to see if heating begins.

Why heating does not start immediately

Air conditioning systems protect internal components.

When heating mode is selected, the system may pause before delivering warm air. During this time, airflow may be minimal or neutral. This allows the internal coil to warm safely.

This delay is normal and usually lasts a few minutes.

Defrost cycles can stop heating temporarily

In cold weather, outdoor units collect frost.

To manage this, the system enters a defrost cycle. During defrost, heating inside the building pauses while heat is redirected to the outdoor unit. Indoor airflow may slow, stop, or feel cool.

Once defrost finishes, heating resumes automatically.

This behaviour is normal during winter.

Fan speed can make heating feel ineffective

Fan speed affects how heat spreads.

Low fan speed can make the room feel cold because warm air stays near the unit. Medium or higher fan speeds distribute heat more evenly across the space.

If the system is heating but the room feels cold, increase the fan speed for 10 minutes and reassess.

Why raising the temperature does not fix everything

Increasing the temperature does not force heat instantly.

Air conditioning operates within a fixed output range. A higher temperature setting increases run time, not heat intensity. If heating does not start at all, the issue is not the temperature setting.

Focus on mode, demand, and airflow first.

Common reasons heating does not start

These causes appear frequently during winter service visits.

Cooling mode left active
Auto mode switching back to cooling
Fan only mode selected
Temperature set too close to room temperature
Timers disabling heating
Multiple controllers set differently
Windows or doors open nearby

Checking these often resolves the issue quickly.

When the issue goes beyond settings

If heating mode is active and demand is present but no heat arrives after 15 minutes, the issue may not be user related.

Possible causes include
Low outdoor temperatures affecting output
System sized mainly for cooling
Sensor faults
Restricted airflow from dirty filters
Installation issues affecting airflow direction

These require inspection.

When to seek professional support

Contact a specialist if
Heating mode is active with no warmth after 15 minutes
The system shuts down repeatedly
Error codes appear
The outdoor unit does not operate
Airflow is absent

Before calling, note the model number, controller type, 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 support 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.

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