Why does my air conditioning struggle to heat large or open plan rooms?

You turn the system on.
Heating is active.
One area warms. The rest stays cold.

This issue appears often in larger homes and open plan spaces. The system usually works as designed. The room layout works against it.

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Air conditioning heats air, not rooms evenly

Air conditioning delivers heat through moving air.

In small rooms, that air reaches every corner quickly.
In large or open plan spaces, warm air spreads unevenly.

Heat builds near the unit first.
Further areas lag behind.

This creates the impression that heating is weak.

Open plan layouts lose heat faster

Open plan spaces expose more surface area.

External walls
Large glazing
Long air paths

Heat escapes faster than it can build. The system must replace lost heat before raising overall temperature.

This slows visible progress.

High ceilings trap heat above you

Warm air rises.

In rooms with high ceilings, heat collects above head height. The system heats the space, but much of that warmth sits where you do not feel it.

The room temperature may rise.
Comfort does not.

One unit may not cover the whole space

Some systems are sized mainly for cooling.

In heating mode, output may not be enough to serve a large footprint. One indoor unit struggles to move warm air across long distances or around corners.

This is a coverage issue, not a fault.

Airflow direction matters more in large rooms

Airflow that works in small rooms often fails in big spaces.

If warm air blows straight down or straight up, it does not travel far. Adjusting airflow to push heat across the room improves distribution.

Air direction matters more than temperature.

Fan speed affects reach, not output

Fan speed controls how far warm air travels.

Low fan speed
Feels quiet
Limits reach

Medium fan speed
Moves heat further
Balances temperature better

In large rooms, medium fan speed usually improves comfort without needing higher temperatures.

Auto mode makes large spaces harder to heat

Auto mode reacts to local sensors.

In large rooms, one area warms first. The sensor detects this and reduces heating while the rest of the room stays cold.

Manual heating mode avoids this problem.

Cold surfaces slow warm up in big rooms

Large rooms contain more cold mass.

Floors
Walls
Furniture

These absorb heat before air temperature rises. This effect is stronger in winter and more noticeable in open plan spaces.

Time matters more than settings.

Common habits that worsen large room heating

These appear often in open plan homes.

Low fan speed used continuously
Frequent temperature changes
Switching the system off too early
Judging comfort near the unit

Each one prevents heat from spreading.

What helps heating work better in large spaces

A practical approach works best.

Select heating mode manually.
Set a realistic temperature.
Use medium fan speed initially.
Adjust airflow to push heat across the room.
Allow at least 30 minutes in cold weather.

This supports even heat distribution.

When layout needs a different solution

Some spaces exceed what one unit can handle.

Very large open plan areas
High ceilings throughout
Multiple exposed walls

In these cases, additional indoor units or different system layouts may be needed for consistent heating.

When to seek professional support

Contact a specialist if
Heating never reaches distant areas
The system runs constantly with little effect
Comfort improves only near the unit
Settings are confirmed correct

Before calling, note room size, ceiling height, unit position, and outdoor temperature.

This helps assessment.

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 performance
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 installation observations 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 improving real world performance and reducing user related comfort issues.

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